Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Jason Baron wrote: > On 02/09/2015 11:49 PM, Eric Wong wrote: > > Do you have a userland use case to share? > > I've been trying to describe the use case, maybe I haven't been doing a good > job :( Sorry, I meant if you had any public code. Anyways, I've restarted work on another project which I'll hopefully be able to share in a few weeks which might be a good public candidate for epoll performance testing. > > Did you try my suggestion of using a dedicated thread (or thread pool) > > which does nothing but loop on accept() + EPOLL_CTL_ADD? > > > > Those dedicated threads could do its own round-robin in userland to pick > > a different epollfd to call EPOLL_CTL_ADD on. > > Thanks for your suggestion! I'm not actively working on the user-space > code here, but I will pass it along. > > I would prefer though not to have to context switch the 'accept' thread > on and off the cpu every time there is a new connection. So the approach > suggested here essentially moves this dedicated thread (threads), down > into the kernel and avoids the creation of these threads entirely. For cmogstored, I stopped using TCP_DEFER_ACCEPT when using the dedicated thread. This approach offloads to epoll and ends up giving similar behavior to what used to be infinite in TCP_DEFER_ACCEPT in Linux <= 2.6.31 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 11:49 PM, Eric Wong wrote: > Jason Baron wrote: >> On 02/09/2015 05:45 PM, Andy Lutomirski wrote: >>> On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron wrote: On 02/09/2015 03:18 PM, Andy Lutomirski wrote: > On 02/09/2015 12:06 PM, Jason Baron wrote: >> Epoll file descriptors that are added to a shared wakeup source are >> always >> added in a non-exclusive manner. That means that when we have multiple >> epoll >> fds attached to a shared wakeup source they are all woken up. This can >> lead to excessive cpu usage and uneven load distribution. >> >> This patch introduces two new 'events' flags that are intended to be used >> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the >> event >> source in an exclusive manner such that the minimum number of threads are >> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, >> can >> also be added to the 'events' flag, such that we round robin around the >> set >> of waiting threads. >> >> An implementation note is that in the epoll wakeup routine, >> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a >> successful >> wakeup, only when there are current waiters. The idea is to use this >> additional >> heuristic in order minimize wakeup latencies. > I don't understand what this is intended to do. > > If an event has EPOLLONESHOT, then this only one thread should be woken > regardless, right? If not, isn't that just a bug that should be fixed? > hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. >>> Now I understand. Why are you using multiple epollfds? >>> >>> --Andy >> So the multiple epollfds is really a way to partition the set of >> events. Otherwise, I have all the threads contending on all the events >> that are being generated. So I'm not sure if that is scalable. > I wonder if EPOLLONESHOT + epoll_wait with a sufficiently large > maxevents value is sufficient for you. All events would be shared, so > they can migrate between threads(*). Each thread takes a largish set of > events on every epoll_wait call and doesn't call epoll_wait again until > it's done with the whole set it got. > > You'll hit more contention on EPOLL_CTL_MOD with shared events and a > single epoll, but I think it's a better goal to make that lock-free. Its not just EPOLL_CTL_MOD, but there's also going to be contention on epoll add and remove since there is only 1 epoll fd in this case. I'm also concerned about the balancing of the workload among threads in the single queue case. I think its quite reasonable to have user-space partition the set of events among threads as it sees fit using multiple epoll fds. However, currently this multiple epoll fd scheme does not handle events from a shared event source well. As I mentioned there is a thundering herd wakeup in this case, and the wakeups are unbalanced. In fact, we have an application that currently does EPOLL_CTL_REMOVEs followed by EPOLL_CTL_ADDs periodically against a shared wakeup source in order to re-balance the wakeup queues. This solves the balancing issues to an extent, but not the thundering herd. I'd like to move this logic down into the kernel with this patch set. > (*) Too large a maxevents will lead to head-of-line blocking, but from > what I'm inferring, you already risk that with multiple epollfds and > separate threads working on them. > > Do you have a userland use case to share? I've been trying to describe the use case, maybe I haven't been doing a good job :( >> In the use-case I'm trying to describe, I've partitioned a large set >> of the events, but there may still be some event sources that we wish >> to share among all of the threads (or even subsets of them), so as not >> to overload any one in particular. > >> More specifically, in the case of a single listen socket, its natural >> to call accept() on the thread that has been woken up, but without >> doing round robin, you quickly get into a very unbalanced load, and in >> addition you waste a lot of cpu doing unnecessary wakeups. There are >> other approaches to solve this, specifically using SO_REUSEPORT, which >> creates a separate socket per-thread and gets one back to the >> separately partitioned events case previously described. However, >> SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition >> does not have knowledge of the threads that are actively waiting as >> the epoll code does. > Did you try my suggestion of using a dedicated
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 11:49 PM, Eric Wong wrote: Jason Baron jba...@akamai.com wrote: On 02/09/2015 05:45 PM, Andy Lutomirski wrote: On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron jba...@akamai.com wrote: On 02/09/2015 03:18 PM, Andy Lutomirski wrote: On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. Now I understand. Why are you using multiple epollfds? --Andy So the multiple epollfds is really a way to partition the set of events. Otherwise, I have all the threads contending on all the events that are being generated. So I'm not sure if that is scalable. I wonder if EPOLLONESHOT + epoll_wait with a sufficiently large maxevents value is sufficient for you. All events would be shared, so they can migrate between threads(*). Each thread takes a largish set of events on every epoll_wait call and doesn't call epoll_wait again until it's done with the whole set it got. You'll hit more contention on EPOLL_CTL_MOD with shared events and a single epoll, but I think it's a better goal to make that lock-free. Its not just EPOLL_CTL_MOD, but there's also going to be contention on epoll add and remove since there is only 1 epoll fd in this case. I'm also concerned about the balancing of the workload among threads in the single queue case. I think its quite reasonable to have user-space partition the set of events among threads as it sees fit using multiple epoll fds. However, currently this multiple epoll fd scheme does not handle events from a shared event source well. As I mentioned there is a thundering herd wakeup in this case, and the wakeups are unbalanced. In fact, we have an application that currently does EPOLL_CTL_REMOVEs followed by EPOLL_CTL_ADDs periodically against a shared wakeup source in order to re-balance the wakeup queues. This solves the balancing issues to an extent, but not the thundering herd. I'd like to move this logic down into the kernel with this patch set. (*) Too large a maxevents will lead to head-of-line blocking, but from what I'm inferring, you already risk that with multiple epollfds and separate threads working on them. Do you have a userland use case to share? I've been trying to describe the use case, maybe I haven't been doing a good job :( In the use-case I'm trying to describe, I've partitioned a large set of the events, but there may still be some event sources that we wish to share among all of the threads (or even subsets of them), so as not to overload any one in particular. More specifically, in the case of a single listen socket, its natural to call accept() on the thread that has been woken up, but without doing round robin, you quickly get into a very unbalanced load, and in addition you waste a lot of cpu doing unnecessary wakeups. There are other approaches to solve this, specifically using SO_REUSEPORT, which creates a separate socket per-thread and gets one back to the separately partitioned events case previously described. However, SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition does not have knowledge of the threads that are actively waiting as the epoll code does. Did you try my suggestion of using a dedicated thread (or thread pool) which does nothing but loop on accept() + EPOLL_CTL_ADD? Those dedicated threads could do its own round-robin in userland to pick a different epollfd to call EPOLL_CTL_ADD on. Thanks for your
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Jason Baron jba...@akamai.com wrote: On 02/09/2015 11:49 PM, Eric Wong wrote: Do you have a userland use case to share? I've been trying to describe the use case, maybe I haven't been doing a good job :( Sorry, I meant if you had any public code. Anyways, I've restarted work on another project which I'll hopefully be able to share in a few weeks which might be a good public candidate for epoll performance testing. Did you try my suggestion of using a dedicated thread (or thread pool) which does nothing but loop on accept() + EPOLL_CTL_ADD? Those dedicated threads could do its own round-robin in userland to pick a different epollfd to call EPOLL_CTL_ADD on. Thanks for your suggestion! I'm not actively working on the user-space code here, but I will pass it along. I would prefer though not to have to context switch the 'accept' thread on and off the cpu every time there is a new connection. So the approach suggested here essentially moves this dedicated thread (threads), down into the kernel and avoids the creation of these threads entirely. For cmogstored, I stopped using TCP_DEFER_ACCEPT when using the dedicated thread. This approach offloads to epoll and ends up giving similar behavior to what used to be infinite in TCP_DEFER_ACCEPT in Linux = 2.6.31 -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Jason Baron wrote: > On 02/09/2015 05:45 PM, Andy Lutomirski wrote: > > On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron wrote: > >> On 02/09/2015 03:18 PM, Andy Lutomirski wrote: > >>> On 02/09/2015 12:06 PM, Jason Baron wrote: > Epoll file descriptors that are added to a shared wakeup source are > always > added in a non-exclusive manner. That means that when we have multiple > epoll > fds attached to a shared wakeup source they are all woken up. This can > lead to excessive cpu usage and uneven load distribution. > > This patch introduces two new 'events' flags that are intended to be used > with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the > event > source in an exclusive manner such that the minimum number of threads are > woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, > can > also be added to the 'events' flag, such that we round robin around the > set > of waiting threads. > > An implementation note is that in the epoll wakeup routine, > 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a > successful > wakeup, only when there are current waiters. The idea is to use this > additional > heuristic in order minimize wakeup latencies. > >>> I don't understand what this is intended to do. > >>> > >>> If an event has EPOLLONESHOT, then this only one thread should be woken > >>> regardless, right? If not, isn't that just a bug that should be fixed? > >>> > >> hmm...so with EPOLLONESHOT you basically get notified once about an event. > >> If i have multiple epoll fds (say 1 per-thread) attached to a single > >> source in EPOLLONESHOT, then all threads will potentially get woken up > >> once per event. Then, I would have to re-arm all of them. So I don't think > >> this addresses this particular usecase...what I am trying to avoid is this > >> mass wakeup or thundering herd for a shared event source. > > Now I understand. Why are you using multiple epollfds? > > > > --Andy > > So the multiple epollfds is really a way to partition the set of > events. Otherwise, I have all the threads contending on all the events > that are being generated. So I'm not sure if that is scalable. I wonder if EPOLLONESHOT + epoll_wait with a sufficiently large maxevents value is sufficient for you. All events would be shared, so they can migrate between threads(*). Each thread takes a largish set of events on every epoll_wait call and doesn't call epoll_wait again until it's done with the whole set it got. You'll hit more contention on EPOLL_CTL_MOD with shared events and a single epoll, but I think it's a better goal to make that lock-free. (*) Too large a maxevents will lead to head-of-line blocking, but from what I'm inferring, you already risk that with multiple epollfds and separate threads working on them. Do you have a userland use case to share? > In the use-case I'm trying to describe, I've partitioned a large set > of the events, but there may still be some event sources that we wish > to share among all of the threads (or even subsets of them), so as not > to overload any one in particular. > More specifically, in the case of a single listen socket, its natural > to call accept() on the thread that has been woken up, but without > doing round robin, you quickly get into a very unbalanced load, and in > addition you waste a lot of cpu doing unnecessary wakeups. There are > other approaches to solve this, specifically using SO_REUSEPORT, which > creates a separate socket per-thread and gets one back to the > separately partitioned events case previously described. However, > SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition > does not have knowledge of the threads that are actively waiting as > the epoll code does. Did you try my suggestion of using a dedicated thread (or thread pool) which does nothing but loop on accept() + EPOLL_CTL_ADD? Those dedicated threads could do its own round-robin in userland to pick a different epollfd to call EPOLL_CTL_ADD on. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 05:45 PM, Andy Lutomirski wrote: > On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron wrote: >> On 02/09/2015 03:18 PM, Andy Lutomirski wrote: >>> On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. >>> I don't understand what this is intended to do. >>> >>> If an event has EPOLLONESHOT, then this only one thread should be woken >>> regardless, right? If not, isn't that just a bug that should be fixed? >>> >> hmm...so with EPOLLONESHOT you basically get notified once about an event. >> If i have multiple epoll fds (say 1 per-thread) attached to a single source >> in EPOLLONESHOT, then all threads will potentially get woken up once per >> event. Then, I would have to re-arm all of them. So I don't think this >> addresses this particular usecase...what I am trying to avoid is this mass >> wakeup or thundering herd for a shared event source. > Now I understand. Why are you using multiple epollfds? > > --Andy So the multiple epollfds is really a way to partition the set of events. Otherwise, I have all the threads contending on all the events that are being generated. So I'm not sure if that is scalable. In the use-case I'm trying to describe, I've partitioned a large set of the events, but there may still be some event sources that we wish to share among all of the threads (or even subsets of them), so as not to overload any one in particular. More specifically, in the case of a single listen socket, its natural to call accept() on the thread that has been woken up, but without doing round robin, you quickly get into a very unbalanced load, and in addition you waste a lot of cpu doing unnecessary wakeups. There are other approaches to solve this, specifically using SO_REUSEPORT, which creates a separate socket per-thread and gets one back to the separately partitioned events case previously described. However, SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition does not have knowledge of the threads that are actively waiting as the epoll code does. Thanks, -Jason -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron wrote: > On 02/09/2015 03:18 PM, Andy Lutomirski wrote: >> On 02/09/2015 12:06 PM, Jason Baron wrote: >>> Epoll file descriptors that are added to a shared wakeup source are always >>> added in a non-exclusive manner. That means that when we have multiple epoll >>> fds attached to a shared wakeup source they are all woken up. This can >>> lead to excessive cpu usage and uneven load distribution. >>> >>> This patch introduces two new 'events' flags that are intended to be used >>> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the >>> event >>> source in an exclusive manner such that the minimum number of threads are >>> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can >>> also be added to the 'events' flag, such that we round robin around the set >>> of waiting threads. >>> >>> An implementation note is that in the epoll wakeup routine, >>> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a >>> successful >>> wakeup, only when there are current waiters. The idea is to use this >>> additional >>> heuristic in order minimize wakeup latencies. >> >> I don't understand what this is intended to do. >> >> If an event has EPOLLONESHOT, then this only one thread should be woken >> regardless, right? If not, isn't that just a bug that should be fixed? >> > > hmm...so with EPOLLONESHOT you basically get notified once about an event. If > i have multiple epoll fds (say 1 per-thread) attached to a single source in > EPOLLONESHOT, then all threads will potentially get woken up once per event. > Then, I would have to re-arm all of them. So I don't think this addresses > this particular usecase...what I am trying to avoid is this mass wakeup or > thundering herd for a shared event source. Now I understand. Why are you using multiple epollfds? --Andy > >> If an event has EPOLLET, then the considerations are similar to >> EPOLLONESHOT, right? >> > > EPOLLET is still going to cause this thundering herd. > >> If an event is a normal level-triggered non-one-shot event, then I don't >> understand how a round-robin wakeup makes any sense. It's level-triggered, >> after all. > > Yeah, so the current behavior is to wake up all of the threads. I'm trying to > add a new mode where it load balances among the threads interested in the > event. Perhaps, the test program I attached to 0/2 will show the issue better? > > Also, this originally came up in the context of a single listening socket > which was attached to multiple epoll fds each in a separate thread. With the > attached patch, I can measure a large decrease in cpu usage and better > balancing behavior among the accepting threads. > > Thanks, > > -Jason -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 03:18 PM, Andy Lutomirski wrote: > On 02/09/2015 12:06 PM, Jason Baron wrote: >> Epoll file descriptors that are added to a shared wakeup source are always >> added in a non-exclusive manner. That means that when we have multiple epoll >> fds attached to a shared wakeup source they are all woken up. This can >> lead to excessive cpu usage and uneven load distribution. >> >> This patch introduces two new 'events' flags that are intended to be used >> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event >> source in an exclusive manner such that the minimum number of threads are >> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can >> also be added to the 'events' flag, such that we round robin around the set >> of waiting threads. >> >> An implementation note is that in the epoll wakeup routine, >> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a >> successful >> wakeup, only when there are current waiters. The idea is to use this >> additional >> heuristic in order minimize wakeup latencies. > > I don't understand what this is intended to do. > > If an event has EPOLLONESHOT, then this only one thread should be woken > regardless, right? If not, isn't that just a bug that should be fixed? > hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. > If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, > right? > EPOLLET is still going to cause this thundering herd. > If an event is a normal level-triggered non-one-shot event, then I don't > understand how a round-robin wakeup makes any sense. It's level-triggered, > after all. Yeah, so the current behavior is to wake up all of the threads. I'm trying to add a new mode where it load balances among the threads interested in the event. Perhaps, the test program I attached to 0/2 will show the issue better? Also, this originally came up in the context of a single listening socket which was attached to multiple epoll fds each in a separate thread. With the attached patch, I can measure a large decrease in cpu usage and better balancing behavior among the accepting threads. Thanks, -Jason -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
[CC += linux-...@vger.kernel.org]
On Mon, Feb 9, 2015 at 9:06 PM, Jason Baron wrote:
> Epoll file descriptors that are added to a shared wakeup source are always
> added in a non-exclusive manner. That means that when we have multiple epoll
> fds attached to a shared wakeup source they are all woken up. This can
> lead to excessive cpu usage and uneven load distribution.
>
> This patch introduces two new 'events' flags that are intended to be used
> with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
> source in an exclusive manner such that the minimum number of threads are
> woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
> also be added to the 'events' flag, such that we round robin around the set
> of waiting threads.
>
> An implementation note is that in the epoll wakeup routine,
> 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
> wakeup, only when there are current waiters. The idea is to use this
> additional
> heuristic in order minimize wakeup latencies.
>
> Signed-off-by: Jason Baron
> ---
> fs/eventpoll.c | 25 -
> include/uapi/linux/eventpoll.h | 6 ++
> 2 files changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index d77f944..382c832 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -92,7 +92,8 @@
> */
>
> /* Epoll private bits inside the event mask */
> -#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
> +#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | \
> +EPOLLEXCLUSIVE | EPOLLROUNDROBIN)
>
> /* Maximum number of nesting allowed inside epoll sets */
> #define EP_MAX_NESTS 4
> @@ -1002,6 +1003,7 @@ static int ep_poll_callback(wait_queue_t *wait,
> unsigned mode, int sync, void *k
> unsigned long flags;
> struct epitem *epi = ep_item_from_wait(wait);
> struct eventpoll *ep = epi->ep;
> + int ewake = 0;
>
> if ((unsigned long)key & POLLFREE) {
> ep_pwq_from_wait(wait)->whead = NULL;
> @@ -1066,8 +1068,10 @@ static int ep_poll_callback(wait_queue_t *wait,
> unsigned mode, int sync, void *k
> * Wake up ( if active ) both the eventpoll wait list and the ->poll()
> * wait list.
> */
> - if (waitqueue_active(>wq))
> + if (waitqueue_active(>wq)) {
> + ewake = 1;
> wake_up_locked(>wq);
> + }
> if (waitqueue_active(>poll_wait))
> pwake++;
>
> @@ -1078,6 +1082,8 @@ out_unlock:
> if (pwake)
> ep_poll_safewake(>poll_wait);
>
> + if (epi->event.events & EPOLLROUNDROBIN)
> + return ewake;
> return 1;
> }
>
> @@ -1095,7 +1101,12 @@ static void ep_ptable_queue_proc(struct file *file,
> wait_queue_head_t *whead,
> init_waitqueue_func_entry(>wait, ep_poll_callback);
> pwq->whead = whead;
> pwq->base = epi;
> - add_wait_queue(whead, >wait);
> + if (epi->event.events & EPOLLROUNDROBIN)
> + add_wait_queue_rr(whead, >wait);
> + else if (epi->event.events & EPOLLEXCLUSIVE)
> + add_wait_queue_exclusive(whead, >wait);
> + else
> + add_wait_queue(whead, >wait);
> list_add_tail(>llink, >pwqlist);
> epi->nwait++;
> } else {
> @@ -1820,8 +1831,7 @@ SYSCALL_DEFINE1(epoll_create, int, size)
> SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
> struct epoll_event __user *, event)
> {
> - int error;
> - int full_check = 0;
> + int error, full_check = 0, wait_flags = 0;
> struct fd f, tf;
> struct eventpoll *ep;
> struct epitem *epi;
> @@ -1861,6 +1871,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
> if (f.file == tf.file || !is_file_epoll(f.file))
> goto error_tgt_fput;
>
> + wait_flags = epds.events & (EPOLLEXCLUSIVE | EPOLLROUNDROBIN);
> + if (wait_flags && ((op == EPOLL_CTL_MOD) || ((op == EPOLL_CTL_ADD) &&
> + ((wait_flags == EPOLLROUNDROBIN) || (is_file_epoll(tf.file))
> + goto error_tgt_fput;
> +
> /*
> * At this point it is safe to assume that the "private_data" contains
> * our own data structure.
> diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
> index bc81fb2..10260a1 100644
> --- a/include/uapi/linux/eventpoll.h
> +++ b/include/uapi/linux/eventpoll.h
> @@ -26,6 +26,12 @@
> #define EPOLL_CTL_DEL 2
> #define EPOLL_CTL_MOD 3
>
> +/* Balance wakeups for a shared event source */
> +#define EPOLLROUNDROBIN (1 << 27)
> +
> +/* Add exclusively */
> +#define EPOLLEXCLUSIVE (1 << 28)
> +
> /*
> * Request the handling of system wakeup events so as to prevent system
>
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, right? If an event is a normal level-triggered non-one-shot event, then I don't understand how a round-robin wakeup makes any sense. It's level-triggered, after all. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Epoll file descriptors that are added to a shared wakeup source are always
added in a non-exclusive manner. That means that when we have multiple epoll
fds attached to a shared wakeup source they are all woken up. This can
lead to excessive cpu usage and uneven load distribution.
This patch introduces two new 'events' flags that are intended to be used
with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
source in an exclusive manner such that the minimum number of threads are
woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
also be added to the 'events' flag, such that we round robin around the set
of waiting threads.
An implementation note is that in the epoll wakeup routine,
'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
wakeup, only when there are current waiters. The idea is to use this additional
heuristic in order minimize wakeup latencies.
Signed-off-by: Jason Baron
---
fs/eventpoll.c | 25 -
include/uapi/linux/eventpoll.h | 6 ++
2 files changed, 26 insertions(+), 5 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d77f944..382c832 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -92,7 +92,8 @@
*/
/* Epoll private bits inside the event mask */
-#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
+#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | \
+EPOLLEXCLUSIVE | EPOLLROUNDROBIN)
/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4
@@ -1002,6 +1003,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned
mode, int sync, void *k
unsigned long flags;
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi->ep;
+ int ewake = 0;
if ((unsigned long)key & POLLFREE) {
ep_pwq_from_wait(wait)->whead = NULL;
@@ -1066,8 +1068,10 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned
mode, int sync, void *k
* Wake up ( if active ) both the eventpoll wait list and the ->poll()
* wait list.
*/
- if (waitqueue_active(>wq))
+ if (waitqueue_active(>wq)) {
+ ewake = 1;
wake_up_locked(>wq);
+ }
if (waitqueue_active(>poll_wait))
pwake++;
@@ -1078,6 +1082,8 @@ out_unlock:
if (pwake)
ep_poll_safewake(>poll_wait);
+ if (epi->event.events & EPOLLROUNDROBIN)
+ return ewake;
return 1;
}
@@ -1095,7 +1101,12 @@ static void ep_ptable_queue_proc(struct file *file,
wait_queue_head_t *whead,
init_waitqueue_func_entry(>wait, ep_poll_callback);
pwq->whead = whead;
pwq->base = epi;
- add_wait_queue(whead, >wait);
+ if (epi->event.events & EPOLLROUNDROBIN)
+ add_wait_queue_rr(whead, >wait);
+ else if (epi->event.events & EPOLLEXCLUSIVE)
+ add_wait_queue_exclusive(whead, >wait);
+ else
+ add_wait_queue(whead, >wait);
list_add_tail(>llink, >pwqlist);
epi->nwait++;
} else {
@@ -1820,8 +1831,7 @@ SYSCALL_DEFINE1(epoll_create, int, size)
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
struct epoll_event __user *, event)
{
- int error;
- int full_check = 0;
+ int error, full_check = 0, wait_flags = 0;
struct fd f, tf;
struct eventpoll *ep;
struct epitem *epi;
@@ -1861,6 +1871,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
if (f.file == tf.file || !is_file_epoll(f.file))
goto error_tgt_fput;
+ wait_flags = epds.events & (EPOLLEXCLUSIVE | EPOLLROUNDROBIN);
+ if (wait_flags && ((op == EPOLL_CTL_MOD) || ((op == EPOLL_CTL_ADD) &&
+ ((wait_flags == EPOLLROUNDROBIN) || (is_file_epoll(tf.file))
+ goto error_tgt_fput;
+
/*
* At this point it is safe to assume that the "private_data" contains
* our own data structure.
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index bc81fb2..10260a1 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -26,6 +26,12 @@
#define EPOLL_CTL_DEL 2
#define EPOLL_CTL_MOD 3
+/* Balance wakeups for a shared event source */
+#define EPOLLROUNDROBIN (1 << 27)
+
+/* Add exclusively */
+#define EPOLLEXCLUSIVE (1 << 28)
+
/*
* Request the handling of system wakeup events so as to prevent system
suspends
* from happening while those events are being processed.
--
1.8.2.rc2
--
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More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, right? If an event is a normal level-triggered non-one-shot event, then I don't understand how a round-robin wakeup makes any sense. It's level-triggered, after all. --Andy -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
[PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Epoll file descriptors that are added to a shared wakeup source are always
added in a non-exclusive manner. That means that when we have multiple epoll
fds attached to a shared wakeup source they are all woken up. This can
lead to excessive cpu usage and uneven load distribution.
This patch introduces two new 'events' flags that are intended to be used
with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
source in an exclusive manner such that the minimum number of threads are
woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
also be added to the 'events' flag, such that we round robin around the set
of waiting threads.
An implementation note is that in the epoll wakeup routine,
'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
wakeup, only when there are current waiters. The idea is to use this additional
heuristic in order minimize wakeup latencies.
Signed-off-by: Jason Baron jba...@akamai.com
---
fs/eventpoll.c | 25 -
include/uapi/linux/eventpoll.h | 6 ++
2 files changed, 26 insertions(+), 5 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d77f944..382c832 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -92,7 +92,8 @@
*/
/* Epoll private bits inside the event mask */
-#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
+#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | \
+EPOLLEXCLUSIVE | EPOLLROUNDROBIN)
/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4
@@ -1002,6 +1003,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned
mode, int sync, void *k
unsigned long flags;
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi-ep;
+ int ewake = 0;
if ((unsigned long)key POLLFREE) {
ep_pwq_from_wait(wait)-whead = NULL;
@@ -1066,8 +1068,10 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned
mode, int sync, void *k
* Wake up ( if active ) both the eventpoll wait list and the -poll()
* wait list.
*/
- if (waitqueue_active(ep-wq))
+ if (waitqueue_active(ep-wq)) {
+ ewake = 1;
wake_up_locked(ep-wq);
+ }
if (waitqueue_active(ep-poll_wait))
pwake++;
@@ -1078,6 +1082,8 @@ out_unlock:
if (pwake)
ep_poll_safewake(ep-poll_wait);
+ if (epi-event.events EPOLLROUNDROBIN)
+ return ewake;
return 1;
}
@@ -1095,7 +1101,12 @@ static void ep_ptable_queue_proc(struct file *file,
wait_queue_head_t *whead,
init_waitqueue_func_entry(pwq-wait, ep_poll_callback);
pwq-whead = whead;
pwq-base = epi;
- add_wait_queue(whead, pwq-wait);
+ if (epi-event.events EPOLLROUNDROBIN)
+ add_wait_queue_rr(whead, pwq-wait);
+ else if (epi-event.events EPOLLEXCLUSIVE)
+ add_wait_queue_exclusive(whead, pwq-wait);
+ else
+ add_wait_queue(whead, pwq-wait);
list_add_tail(pwq-llink, epi-pwqlist);
epi-nwait++;
} else {
@@ -1820,8 +1831,7 @@ SYSCALL_DEFINE1(epoll_create, int, size)
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
struct epoll_event __user *, event)
{
- int error;
- int full_check = 0;
+ int error, full_check = 0, wait_flags = 0;
struct fd f, tf;
struct eventpoll *ep;
struct epitem *epi;
@@ -1861,6 +1871,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
if (f.file == tf.file || !is_file_epoll(f.file))
goto error_tgt_fput;
+ wait_flags = epds.events (EPOLLEXCLUSIVE | EPOLLROUNDROBIN);
+ if (wait_flags ((op == EPOLL_CTL_MOD) || ((op == EPOLL_CTL_ADD)
+ ((wait_flags == EPOLLROUNDROBIN) || (is_file_epoll(tf.file))
+ goto error_tgt_fput;
+
/*
* At this point it is safe to assume that the private_data contains
* our own data structure.
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index bc81fb2..10260a1 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -26,6 +26,12 @@
#define EPOLL_CTL_DEL 2
#define EPOLL_CTL_MOD 3
+/* Balance wakeups for a shared event source */
+#define EPOLLROUNDROBIN (1 27)
+
+/* Add exclusively */
+#define EPOLLEXCLUSIVE (1 28)
+
/*
* Request the handling of system wakeup events so as to prevent system
suspends
* from happening while those events are being processed.
--
1.8.2.rc2
--
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the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
[CC += linux-...@vger.kernel.org]
On Mon, Feb 9, 2015 at 9:06 PM, Jason Baron jba...@akamai.com wrote:
Epoll file descriptors that are added to a shared wakeup source are always
added in a non-exclusive manner. That means that when we have multiple epoll
fds attached to a shared wakeup source they are all woken up. This can
lead to excessive cpu usage and uneven load distribution.
This patch introduces two new 'events' flags that are intended to be used
with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event
source in an exclusive manner such that the minimum number of threads are
woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can
also be added to the 'events' flag, such that we round robin around the set
of waiting threads.
An implementation note is that in the epoll wakeup routine,
'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful
wakeup, only when there are current waiters. The idea is to use this
additional
heuristic in order minimize wakeup latencies.
Signed-off-by: Jason Baron jba...@akamai.com
---
fs/eventpoll.c | 25 -
include/uapi/linux/eventpoll.h | 6 ++
2 files changed, 26 insertions(+), 5 deletions(-)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d77f944..382c832 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -92,7 +92,8 @@
*/
/* Epoll private bits inside the event mask */
-#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET)
+#define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | \
+EPOLLEXCLUSIVE | EPOLLROUNDROBIN)
/* Maximum number of nesting allowed inside epoll sets */
#define EP_MAX_NESTS 4
@@ -1002,6 +1003,7 @@ static int ep_poll_callback(wait_queue_t *wait,
unsigned mode, int sync, void *k
unsigned long flags;
struct epitem *epi = ep_item_from_wait(wait);
struct eventpoll *ep = epi-ep;
+ int ewake = 0;
if ((unsigned long)key POLLFREE) {
ep_pwq_from_wait(wait)-whead = NULL;
@@ -1066,8 +1068,10 @@ static int ep_poll_callback(wait_queue_t *wait,
unsigned mode, int sync, void *k
* Wake up ( if active ) both the eventpoll wait list and the -poll()
* wait list.
*/
- if (waitqueue_active(ep-wq))
+ if (waitqueue_active(ep-wq)) {
+ ewake = 1;
wake_up_locked(ep-wq);
+ }
if (waitqueue_active(ep-poll_wait))
pwake++;
@@ -1078,6 +1082,8 @@ out_unlock:
if (pwake)
ep_poll_safewake(ep-poll_wait);
+ if (epi-event.events EPOLLROUNDROBIN)
+ return ewake;
return 1;
}
@@ -1095,7 +1101,12 @@ static void ep_ptable_queue_proc(struct file *file,
wait_queue_head_t *whead,
init_waitqueue_func_entry(pwq-wait, ep_poll_callback);
pwq-whead = whead;
pwq-base = epi;
- add_wait_queue(whead, pwq-wait);
+ if (epi-event.events EPOLLROUNDROBIN)
+ add_wait_queue_rr(whead, pwq-wait);
+ else if (epi-event.events EPOLLEXCLUSIVE)
+ add_wait_queue_exclusive(whead, pwq-wait);
+ else
+ add_wait_queue(whead, pwq-wait);
list_add_tail(pwq-llink, epi-pwqlist);
epi-nwait++;
} else {
@@ -1820,8 +1831,7 @@ SYSCALL_DEFINE1(epoll_create, int, size)
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
struct epoll_event __user *, event)
{
- int error;
- int full_check = 0;
+ int error, full_check = 0, wait_flags = 0;
struct fd f, tf;
struct eventpoll *ep;
struct epitem *epi;
@@ -1861,6 +1871,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
if (f.file == tf.file || !is_file_epoll(f.file))
goto error_tgt_fput;
+ wait_flags = epds.events (EPOLLEXCLUSIVE | EPOLLROUNDROBIN);
+ if (wait_flags ((op == EPOLL_CTL_MOD) || ((op == EPOLL_CTL_ADD)
+ ((wait_flags == EPOLLROUNDROBIN) || (is_file_epoll(tf.file))
+ goto error_tgt_fput;
+
/*
* At this point it is safe to assume that the private_data contains
* our own data structure.
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index bc81fb2..10260a1 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -26,6 +26,12 @@
#define EPOLL_CTL_DEL 2
#define EPOLL_CTL_MOD 3
+/* Balance wakeups for a shared event source */
+#define EPOLLROUNDROBIN (1 27)
+
+/* Add exclusively */
+#define EPOLLEXCLUSIVE (1 28)
+
/*
* Request the handling of system wakeup events so as to prevent system
suspends
* from happening while those events are being processed.
--
1.8.2.rc2
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 03:18 PM, Andy Lutomirski wrote: On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, right? EPOLLET is still going to cause this thundering herd. If an event is a normal level-triggered non-one-shot event, then I don't understand how a round-robin wakeup makes any sense. It's level-triggered, after all. Yeah, so the current behavior is to wake up all of the threads. I'm trying to add a new mode where it load balances among the threads interested in the event. Perhaps, the test program I attached to 0/2 will show the issue better? Also, this originally came up in the context of a single listening socket which was attached to multiple epoll fds each in a separate thread. With the attached patch, I can measure a large decrease in cpu usage and better balancing behavior among the accepting threads. Thanks, -Jason -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron jba...@akamai.com wrote: On 02/09/2015 03:18 PM, Andy Lutomirski wrote: On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. Now I understand. Why are you using multiple epollfds? --Andy If an event has EPOLLET, then the considerations are similar to EPOLLONESHOT, right? EPOLLET is still going to cause this thundering herd. If an event is a normal level-triggered non-one-shot event, then I don't understand how a round-robin wakeup makes any sense. It's level-triggered, after all. Yeah, so the current behavior is to wake up all of the threads. I'm trying to add a new mode where it load balances among the threads interested in the event. Perhaps, the test program I attached to 0/2 will show the issue better? Also, this originally came up in the context of a single listening socket which was attached to multiple epoll fds each in a separate thread. With the attached patch, I can measure a large decrease in cpu usage and better balancing behavior among the accepting threads. Thanks, -Jason -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
On 02/09/2015 05:45 PM, Andy Lutomirski wrote: On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron jba...@akamai.com wrote: On 02/09/2015 03:18 PM, Andy Lutomirski wrote: On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. Now I understand. Why are you using multiple epollfds? --Andy So the multiple epollfds is really a way to partition the set of events. Otherwise, I have all the threads contending on all the events that are being generated. So I'm not sure if that is scalable. In the use-case I'm trying to describe, I've partitioned a large set of the events, but there may still be some event sources that we wish to share among all of the threads (or even subsets of them), so as not to overload any one in particular. More specifically, in the case of a single listen socket, its natural to call accept() on the thread that has been woken up, but without doing round robin, you quickly get into a very unbalanced load, and in addition you waste a lot of cpu doing unnecessary wakeups. There are other approaches to solve this, specifically using SO_REUSEPORT, which creates a separate socket per-thread and gets one back to the separately partitioned events case previously described. However, SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition does not have knowledge of the threads that are actively waiting as the epoll code does. Thanks, -Jason -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 2/2] epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN
Jason Baron jba...@akamai.com wrote: On 02/09/2015 05:45 PM, Andy Lutomirski wrote: On Mon, Feb 9, 2015 at 1:32 PM, Jason Baron jba...@akamai.com wrote: On 02/09/2015 03:18 PM, Andy Lutomirski wrote: On 02/09/2015 12:06 PM, Jason Baron wrote: Epoll file descriptors that are added to a shared wakeup source are always added in a non-exclusive manner. That means that when we have multiple epoll fds attached to a shared wakeup source they are all woken up. This can lead to excessive cpu usage and uneven load distribution. This patch introduces two new 'events' flags that are intended to be used with EPOLL_CTL_ADD operations. EPOLLEXCLUSIVE, adds the epoll fd to the event source in an exclusive manner such that the minimum number of threads are woken. EPOLLROUNDROBIN, which depends on EPOLLEXCLUSIVE also being set, can also be added to the 'events' flag, such that we round robin around the set of waiting threads. An implementation note is that in the epoll wakeup routine, 'ep_poll_callback()', if EPOLLROUNDROBIN is set, we return 1, for a successful wakeup, only when there are current waiters. The idea is to use this additional heuristic in order minimize wakeup latencies. I don't understand what this is intended to do. If an event has EPOLLONESHOT, then this only one thread should be woken regardless, right? If not, isn't that just a bug that should be fixed? hmm...so with EPOLLONESHOT you basically get notified once about an event. If i have multiple epoll fds (say 1 per-thread) attached to a single source in EPOLLONESHOT, then all threads will potentially get woken up once per event. Then, I would have to re-arm all of them. So I don't think this addresses this particular usecase...what I am trying to avoid is this mass wakeup or thundering herd for a shared event source. Now I understand. Why are you using multiple epollfds? --Andy So the multiple epollfds is really a way to partition the set of events. Otherwise, I have all the threads contending on all the events that are being generated. So I'm not sure if that is scalable. I wonder if EPOLLONESHOT + epoll_wait with a sufficiently large maxevents value is sufficient for you. All events would be shared, so they can migrate between threads(*). Each thread takes a largish set of events on every epoll_wait call and doesn't call epoll_wait again until it's done with the whole set it got. You'll hit more contention on EPOLL_CTL_MOD with shared events and a single epoll, but I think it's a better goal to make that lock-free. (*) Too large a maxevents will lead to head-of-line blocking, but from what I'm inferring, you already risk that with multiple epollfds and separate threads working on them. Do you have a userland use case to share? In the use-case I'm trying to describe, I've partitioned a large set of the events, but there may still be some event sources that we wish to share among all of the threads (or even subsets of them), so as not to overload any one in particular. More specifically, in the case of a single listen socket, its natural to call accept() on the thread that has been woken up, but without doing round robin, you quickly get into a very unbalanced load, and in addition you waste a lot of cpu doing unnecessary wakeups. There are other approaches to solve this, specifically using SO_REUSEPORT, which creates a separate socket per-thread and gets one back to the separately partitioned events case previously described. However, SO_REUSEPORT, I believe is very specific to tcp/udp, and in addition does not have knowledge of the threads that are actively waiting as the epoll code does. Did you try my suggestion of using a dedicated thread (or thread pool) which does nothing but loop on accept() + EPOLL_CTL_ADD? Those dedicated threads could do its own round-robin in userland to pick a different epollfd to call EPOLL_CTL_ADD on. -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
