A few different servers (5 / 205) experienced a segfault all within an hour or so. Unfortunately at this point I'm a bit out of my depth. I have the dmesg output, which is identical for all 5 boxes:
[46545.316351] memcached[2789]: segfault at 0 ip 000000000040e007 sp 00007f362ceedeb0 error 4 in memcached[400000+1d000] I can possibly supply the binary file if needed, though we didn't do anything besides the standard setup and compile. On Tuesday, September 29, 2015 at 10:27:59 PM UTC-7, Dormando wrote: > > If you look at the new branch there's a commit explaining the new stats. > > You can watch slab_reassing_evictions vs slab_reassign_saves. you can also > test automove=1 vs automove=2 (please also turn on the lru_maintainer and > lru_crawler). > > The initial branch you were running didn't add any new stats. It just > restored an old feature. > > On Tue, 29 Sep 2015, Scott Mansfield wrote: > > > An unrelated prod problem meant I had to stop after about an hour. I'm > turning it on again tomorrow morning. > > Are there any new metrics I should be looking at? Anything new in the > stats output? I'm about to take a look at the diffs as well. > > > > On Tuesday, September 29, 2015 at 12:37:45 PM UTC-7, Dormando wrote: > > excellent. if automove=2 is too aggressive you'll see that come in > in a > > hit ratio reduction. > > > > the new branch works with automove=2 as well, but it will attempt > to > > rescue valid items in the old slab if possible. I'll still be > working on > > it for another few hours today though. I'll mail again when I'm > done. > > > > On Tue, 29 Sep 2015, Scott Mansfield wrote: > > > > > I have the first commit (slab_automove=2) running in prod right > now. Later today will be a full load production test of the latest code. > I'll just let it run for a few days unless I spot any problems. We have > good metrics for latency et. al. from the client side, though network > normally dwarfs memcached time. > > > > > > On Tuesday, September 29, 2015 at 3:10:03 AM UTC-7, Dormando > wrote: > > > That's unfortunate. > > > > > > I've done some more work on the branch: > > > https://github.com/memcached/memcached/pull/112 > > > > > > It's not completely likely you would see enough of an > improvement from the > > > new default mode. However if your item sizes change > gradually, items are > > > reclaimed during expiration, or get overwritten (and thus > freed in the old > > > class), it should work just fine. I have another patch > coming which should > > > help though. > > > > > > Open to feedback from any interested party. > > > > > > On Fri, 25 Sep 2015, Scott Mansfield wrote: > > > > > > > I have it running internally, and it runs fine under > normal load. It's difficult to put it into the line of fire for a > production workload because of social reasons... As well it's a degenerate > case that we normally don't run in to (and actively try to avoid). I'm > going to run some heavier load tests on it today. > > > > > > > > On Wednesday, September 9, 2015 at 10:23:32 AM UTC-7, > Scott Mansfield wrote: > > > > I'm working on getting a test going internally. > I'll let you know how it goes. > > > > > > > > > > > > Scott Mansfield > > > > On Mon, Sep 7, 2015 at 2:33 PM, dormando wrote: > > > > Yo, > > > > > > > > > https://github.com/dormando/memcached/commits/slab_rebal_next - would you > > > > mind playing around with the branch here? You can > see the start options in > > > > the test. > > > > > > > > This is a dead simple modification (a restoration > of a feature that was > > > > arleady there...). The test very aggressively > writes and is able to shunt > > > > memory around appropriately. > > > > > > > > The work I'm exploring right now will allow > savings of items being > > > > rebalanced from, and increasing the aggression of > page moving without > > > > being so brain damaged about it. > > > > > > > > But while I'm poking around with that, I'd be > interested in knowing if > > > > this simple branch is an improvement, and if so > how much. > > > > > > > > I'll push more code to the branch, but the changes > should be gated behind > > > > a feature flag. > > > > > > > > On Tue, 18 Aug 2015, 'Scott Mansfield' via > memcached wrote: > > > > > > > > > > > > > > No worries man, you're doing us a favor. Let me > know if there's anything you need from us, and I promise I'll be quicker > this time :) > > > > > > > > > > On Aug 18, 2015 12:01 AM, "dormando" < > dorm...@rydia.net> wrote: > > > > > Hey, > > > > > > > > > > I'm still really interested in working on > this. I'll be taking a careful > > > > > look soon I hope. > > > > > > > > > > On Mon, 3 Aug 2015, Scott Mansfield wrote: > > > > > > > > > > > I've tweaked the program slightly, so > I'm adding a new version. It prints more stats as it goes and runs a bit > faster. > > > > > > > > > > > > On Monday, August 3, 2015 at 1:20:37 AM > UTC-7, Scott Mansfield wrote: > > > > > > Total brain fart on my part. > Apparently I had memcached 1.4.13 on my path (who knows how...) Using the > actual one that I've built works. Sorry for the confusion... can't believe > I didn't realize that before. I'm testing against the compiled one now to > see how it behaves. > > > > > > On Monday, August 3, 2015 at > 1:15:06 AM UTC-7, Dormando wrote: > > > > > > You sure that's 1.4.24? None > of those fail for me :( > > > > > > > > > > > > On Mon, 3 Aug 2015, Scott > Mansfield wrote: > > > > > > > > > > > > > The command line I've used > that will start is: > > > > > > > > > > > > > > memcached -m 64 -o > slab_reassign,slab_automove > > > > > > > > > > > > > > > > > > > > > the ones that fail are: > > > > > > > > > > > > > > > > > > > > > memcached -m 64 -o > slab_reassign,slab_automove,lru_crawler,lru_maintainer > > > > > > > > > > > > > > memcached -o lru_crawler > > > > > > > > > > > > > > > > > > > > > I'm sure I've missed > something during compile, though I just used ./configure and make. > > > > > > > > > > > > > > > > > > > > > On Monday, August 3, 2015 > at 12:22:33 AM UTC-7, Scott Mansfield wrote: > > > > > > > I've attached a > pretty simple program to connect, fill a slab with data, and then fill > another slab slowly with data of a different size. I've been trying to get > memcached to run with the lru_crawler and lru_maintainer flags, but I get ' > > > > > > > > > > > > > > Illegal suboption > "(null)"' every time I try to start with either in any configuration. > > > > > > > > > > > > > > > > > > > > > I haven't seen it > start to move slabs automatically with a freshly installed 1.2.24. > > > > > > > > > > > > > > > > > > > > > On Tuesday, July 21, > 2015 at 4:55:17 PM UTC-7, Scott Mansfield wrote: > > > > > > > I realize I've > not given you the tests to reproduce the behavior. I should be able to > soon. Sorry about the delay here. > > > > > > > In the mean time, I wanted > to bring up a possible secondary use of the same logic to move items on > slab rebalancing. I think the system might benefit from using the same > logic to crawl the pages in a slab and compact the data in the background. > In the case where we have memory that is assigned to the slab > > but not > > > > being used > > > > > because > > > > > > of replaced > > > > > > > or TTL'd out data, > returning the memory to a pool of free memory will allow a slab to grow > with that memory first instead of waiting for an event where memory is > needed at that instant. > > > > > > > > > > > > > > It's a change in approach, > from reactive to proactive. What do you think? > > > > > > > > > > > > > > On Monday, July 13, 2015 > at 5:54:11 PM UTC-7, Dormando wrote: > > > > > > > > First, more detail > for you: > > > > > > > > > > > > > > > > We are running > 1.4.24 in production and haven't noticed any bugs as of yet. The new LRUs > seem to be working well, though we nearly always run memcached scaled to > hold all data without evictions. Those with evictions are behaving well. > Those without evictions haven't seen crashing or any other > > noticeable > > > bad > > > > behavior. > > > > > > > > > > > > > > Neat. > > > > > > > > > > > > > > > > > > > > > > > OK, I think I see > an area where I was speculating on functionality. If you have a key in slab > 21 and then the same key is written again at a larger size in slab 23 I > assumed that the space in 21 was not freed on the second write. With that > assumption, the LRU crawler would not free up that space. > > Also just > > > > by observation > > > > > in > > > > > > the > > > > > > > macro, the space is > not freed > > > > > > > > fast enough to be > effective, in our use case, to accept the writes that are happening. Think > in the hundreds of millions of "overwrites" in a 6 - 10 hour period across > a cluster. > > > > > > > > > > > > > > Internally, "items" > (a key/value pair) are generally immutable. The only > > > > > > > time when it's not > is for INCR/DECR, and it still becomes immutable if two > > > > > > > INCR/DECR's collide. > > > > > > > > > > > > > > What this means, is > that the new item is staged in a piece of free memory > > > > > > > while the "upload" > stage of the SET happens. When memcached has all of the > > > > > > > data in memory to > replace the item, it does an internal swap under a lock. > > > > > > > The old item is > removed from the hash table and LRU, and the new item gets > > > > > > > put in its place (at > the head of the LRU). > > > > > > > > > > > > > > Since items are > refcounted, this means that if other users are downloading > > > > > > > an item which just > got replaced, their memory doesn't get corrupted by the > > > > > > > item changing out > from underneath them. They can continue to read the old > > > > > > > item until they're > done. When the refcount reaches zero the old memory is > > > > > > > reclaimed. > > > > > > > > > > > > > > Most of the time, > the item replacement happens then the old memory is > > > > > > > immediately removed. > > > > > > > > > > > > > > However, this does > mean that you need *one* piece of free memory to > > > > > > > replace the old one. > Then the old memory gets freed after that set. > > > > > > > > > > > > > > So if you take a > memcached instance with 0 free chunks, and do a rolling > > > > > > > replacement of all > items (within the same slab class as before), the first > > > > > > > one would cause an > eviction from the tail of the LRU to get a free chunk. > > > > > > > Every SET after that > would use the chunk freed from the replacement of the > > > > > > > previous memory. > > > > > > > > > > > > > > > After that last > sentence I realized I also may not have explained well enough the access > pattern. The keys are all overwritten every day, but it takes some time to > write them all (obviously). We see a huge increase in the bytes metric as > if the new data for the old keys was being written for the > > first > > > time. > > > > Since the > > > > > "old" > > > > > > slab for > > > > > > > the same key doesn't > > > > > > > > proactively > release memory, it starts to fill up the cache and then start evicting data > in the new slab. Once that happens, we see evictions in the old slab > because of the algorithm you mentioned (random picking / freeing of > memory). Typically we don't see any use for "upgrading" an item as the new > > data > > > > would be entirely > > > > > > new and > > > > > > > should wholesale > replace the > > > > > > > > old data for that > key. More specifically, the operation is always set, with different data > each day. > > > > > > > > > > > > > > Right. Most of your > problems will come from two areas. One being that > > > > > > > writing data > aggressively into the new slab class (unless you set the > > > > > > > rebalancer to > always-replace mode), the mover will make memory available > > > > > > > more slowly than you > can insert. So you'll cause extra evictions in the > > > > > > > new slab class. > > > > > > > > > > > > > > The secondary > problem is from the random evictions in the previous slab > > > > > > > class as stuff is > chucked on the floor to make memory moveable. > > > > > > > > > > > > > > > As for testing, > we'll be able to put it under real production workload. I don't know what > kind of data you mean you need for testing. The data stored in the caches > are highly confidential. I can give you all kinds of metrics, since we > collect most of the ones that are in the stats and some from the > > stats > > > > slabs output. If > > > > > > you have > > > > > > > some specific ones > that > > > > > > > > need collecting, > I'll double check and make sure we can get those. Alternatively, it might > be most beneficial to see the metrics in person :) > > > > > > > > > > > > > > I just need stats > snapshots here and there, and actually putting the thing > > > > > > > under load. When I > did the LRU work I had to beg for several months > > > > > > > before anyone tested > it with a production load. This slows things down and > > > > > > > demotivates me from > working on the project. > > > > > > > > > > > > > > Unfortunately my > dayjob keeps me pretty busy so ~internet~ would probably > > > > > > > be best. > > > > > > > > > > > > > > > I can create a > driver program to reproduce the behavior on a smaller scale. It would write > e.g. 10k keys of 10k size, then rewrite the same keys with different size > data. I'll work on that and post it to this thread when I can reproduce the > behavior locally. > > > > > > > > > > > > > > Ok. There're slab > rebalance unit tests in the t/ directory which do things > > > > > > > like this, and I've > used mc-crusher to slam the rebalancer. It's pretty > > > > > > > easy to run one > config to load up 10k objects, then flip to the other > > > > > > > using the same key > namespace. > > > > > > > > > > > > > > > Thanks, > > > > > > > > Scott > > > > > > > > > > > > > > > > On Saturday, July > 11, 2015 at 12:05:54 PM UTC-7, Dormando wrote: > > > > > > > > Hey, > > > > > > > > > > > > > > > > On Fri, 10 > Jul 2015, Scott Mansfield wrote: > > > > > > > > > > > > > > > > > We've seen > issues recently where we run a cluster that typically has the majority of > items overwritten in the same slab every day and a sudden change in data > size evicts a ton of data, affecting downstream systems. To be clear that > is our problem, but I think there's a tweak in memcached > > that might > > > > be useful and > > > > > > another > > > > > > > possible feature > that > > > > > > > > would be > even > > > > > > > > > better. > > > > > > > > > The data > that is written to this cache is overwritten every day, though the TTL is 7 > days. One slab takes up the majority of the space in the cache. The > application wrote e.g. 10KB (slab 21) every day for each key consistently. > One day, a change occurred where it started writing 15KB (slab > > 23), > > > > causing a migration > > > > > > of data > > > > > > > from one slab to > > > > > > > > another. We > had -o > > > > > > > > > > slab_reassign,slab_automove=1 set on the server, causing large numbers of > evictions on the initial slab. Let's say the cache could hold the data at > 15KB per key, but the old data was not technically TTL'd out in it's old > slab. This means that memory was not being freed by the lru crawler > > thread (I > > > > think) because > > > > > its > > > > > > expiry > > > > > > > had not come > > > > > > > > around. > > > > > > > > > > > > > > > > > > lines 1199 > and 1200 in items.c: > > > > > > > > > if > ((search->exptime != 0 && search->exptime < current_time) || > is_flushed(search)) { > > > > > > > > > > > > > > > > > > If there > was a check to see if this data was "orphaned," i.e. that the key, if > accessed, would map to a different slab than the current one, then these > orphans could be reclaimed as free memory. I am working on a patch to do > this, though I have reservations about performing a hash on the > > key on the > > > > lru crawler > > > > > > thread (if > > > > > > > the hash is not > > > > > > > > already > available). > > > > > > > > > I have > very little experience in the memcached codebase so I don't know the most > efficient way to do this. Any help would be appreciated. > > > > > > > > > > > > > > > > There seems > to be a misconception about how the slab classes work. A key, > > > > > > > > if already > existing in a slab, will always map to the slab class it > > > > > > > > currently > fits into. The slab classes always exist, but the amount of > > > > > > > > memory > reserved for each of them will shift with the slab_reassign. ie: 10 > > > > > > > > pages in > slab class 21, then memory pressure on 23 causes it to move over. > > > > > > > > > > > > > > > > So if you > examine a key that still exists in slab class 21, it has no > > > > > > > > reason to > move up or down the slab classes. > > > > > > > > > > > > > > > > > > Alternatively, and possibly more beneficial is compaction of data in a slab > using the same set of criteria as lru crawling. Understandably, compaction > is a very difficult problem to solve since moving the data would be a pain > in the ass. I saw a couple of discussions about this in the > > mailing list, > > > > though I didn't > > > > > > see any > > > > > > > firm thoughts about > > > > > > > > it. I think > it > > > > > > > > > can > probably be done in O(1) like the lru crawler by limiting the number of > items it touches each time. Writing and reading are doable in O(1) so > moving should be as well. Has anyone given more thought on compaction? > > > > > > > > > > > > > > > > I'd be > interested in hacking this up for you folks if you can provide me > > > > > > > > testing and > some data to work with. With all of the LRU work I did in > > > > > > > > 1.4.24, the > next things I wanted to do is a big improvement on the slab > > > > > > > > reassignment > code. > > > > > > > > > > > > > > > > Currently it > picks essentially a random slab page, empties it, and moves > > > > > > > > the slab > page into the class under pressure. > > > > > > > > > > > > > > > > One thing we > can do is first examine for free memory in the existing slab, > > > > > > > > IE: > > > > > > > > > > > > > > > > - Take a > page from slab 21 > > > > > > > > - Scan the > page for valid items which need to be moved > > > > > > > > - Pull free > memory from slab 21, migrate the item (moderately complicated) > > > > > > > > - When the > page is empty, move it (or give up if you run out of free > > > > > > > > chunks). > > > > > > > > > > > > > > > > The next > step is to pull from the LRU on slab 21: > > > > > > > > > > > > > > > > - Take page > from slab 21 > > > > > > > > - Scan page > for valid items > > > > > > > > - Pull free > memory from slab 21, migrate the item > > > > > > > > - If no > memory free, evict tail of slab 21. use that chunk. > > > > > > > > - When the > page is empty, move it. > > > > > > > > > > > > > > > > Then, when > you hit this condition your least-recently-used data gets > > > > > > > > culled as > new data migrates your page class. This should match a natural > > > > > > > > occurrance > if you would already be evicting valid (but old) items to make > > > > > > > > room for new > items. > > > > > > > > > > > > > > > > A bonus to > using the free memory trick, is that I can use the amount of > > > > > > > > free space > in a slab class as a heuristic to more quickly move slab pages > > > > > > > > around. > > > > > > > > > > > > > > > > If it's > still necessary from there, we can explore "upgrading" items to a > > > > > > > > new slab > class, but that is much much more complicated since the item has > > > > > > > > to shift > LRU's. Do you put it at the head, the tail, the middle, etc? It > > > > > > > > might be > impossible to make a good generic decision there. > > > > > > > > > > > > > > > > What version > are you currently on? If 1.4.24, have you seen any > > > > > > > > instability? > I'm currently torn between fighting a few bugs and start on > > > > > > > > improving > the slab rebalancer. > > > > > > > > > > > > > > > > -Dormando > > > > > > > > > > > > > > > > > > > > > > > > On Saturday, July > 11, 2015 at 12:05:54 PM UTC-7, Dormando wrote: > > > > > > > > Hey, > > > > > > > > > > > > > > > > On Fri, 10 > Jul 2015, Scott Mansfield wrote: > > > > > > > > > > > > > > > > > We've seen > issues recently where we run a cluster that typically has the majority of > items overwritten in the same slab every day and a sudden change in data > size evicts a ton of data, affecting downstream systems. To be clear that > is our problem, but I think there's a tweak in memcached > > that might > > > > be useful and > > > > > > another > > > > > > > possible feature > that > > > > > > > > would be > even > > > > > > > > > better. > > > > > > > > > The data > that is written to this cache is overwritten every day, though the TTL is 7 > days. One slab takes up the majority of the space in the cache. The > application wrote e.g. 10KB (slab 21) every day for each key consistently. > One day, a change occurred where it started writing 15KB (slab > > 23), > > > > causing a migration > > > > > > of data > > > > > > > from one slab to > > > > > > > > another. We > had -o > > > > > > > > > > slab_reassign,slab_automove=1 set on the server, causing large numbers of > evictions on the initial slab. Let's say the cache could hold the data at > 15KB per key, but the old data was not technically TTL'd out in it's old > slab. This means that memory was not being freed by the lru crawler > > thread (I > > > > think) because > > > > > its > > > > > > expiry > > > > > > > had not come > > > > > > > > around. > > > > > > > > > > > > > > > > > > lines 1199 > and 1200 in items.c: > > > > > > > > > if > ((search->exptime != 0 && search->exptime < current_time) || > is_flushed(search)) { > > > > > > > > > > > > > > > > > > If there > was a check to see if this data was "orphaned," i.e. that the key, if > accessed, would map to a different slab than the current one, then these > orphans could be reclaimed as free memory. I am working on a patch to do > this, though I have reservations about performing a hash on the > > key on the > > > > lru crawler > > > > > > thread (if > > > > > > > the hash is not > > > > > > > > already > available). > > > > > > > > > I have > very little experience in the memcached codebase so I don't know the most > efficient way to do this. Any help would be appreciated. > > > > > > > > > > > > > > > > There seems > to be a misconception about how the slab classes work. A key, > > > > > > > > if already > existing in a slab, will always map to the slab class it > > > > > > > > currently > fits into. The slab classes always exist, but the amount of > > > > > > > > memory > reserved for each of them will shift with the slab_reassign. ie: 10 > > > > > > > > pages in > slab class 21, then memory pressure on 23 causes it to move over. > > > > > > > > > > > > > > > > So if you > examine a key that still exists in slab class 21, it has no > > > > > > > > reason to > move up or down the slab classes. > > > > > > > > > > > > > > > > > > Alternatively, and possibly more beneficial is compaction of data in a slab > using the same set of criteria as lru crawling. Understandably, compaction > is a very difficult problem to solve since moving the data would be a pain > in the ass. I saw a couple of discussions about this in the > > mailing list, > > > > though I didn't > > > > > > see any > > > > > > > firm thoughts about > > > > > > > > it. I think > it > > > > > > > > > can > probably be done in O(1) like the lru crawler by limiting the number of > items it touches each time. Writing and reading are doable in O(1) so > moving should be as well. Has anyone given more thought on compaction? > > > > > > > > > > > > > > > > I'd be > interested in hacking this up for you folks if you can provide me > > > > > > > > testing and > some data to work with. With all of the LRU work I did in > > > > > > > > 1.4.24, the > next things I wanted to do is a big improvement on the slab > > > > > > > > reassignment > code. > > > > > > > > > > > > > > > > Currently it > picks essentially a random slab page, empties it, and moves > > > > > > > > the slab > page into the class under pressure. > > > > > > > > > > > > > > > > One thing we > can do is first examine for free memory in the existing slab, > > > > > > > > IE: > > > > > > > > > > > > > > > > - Take a > page from slab 21 > > > > > > > > - Scan the > page for valid items which need to be moved > > > > > > > > - Pull free > memory from slab 21, migrate the item (moderately complicated) > > > > > > > > - When the > page is empty, move it (or give up if you run out of free > > > > > > > > chunks). > > > > > > > > > > > > > > > > The next > step is to pull from the LRU on slab 21: > > > > > > > > > > > > > > > > - Take page > from slab 21 > > > > > > > > - Scan page > for valid items > > > > > > > > - Pull free > memory from slab 21, migrate the item > > > > > > > > - If no > memory free, evict tail of slab 21. use that chunk. > > > > > > > > - When the > page is empty, move it. > > > > > > > > > > > > > > > > Then, when > you hit this condition your least-recently-used data gets > > > > > > > > culled as > new data migrates your page class. This should match a natural > > > > > > > > occurrance > if you would already be evicting valid (but old) items to make > > > > > > > > room for new > items. > > > > > > > > > > > > > > > > A bonus to > using the free memory trick, is that I can use the amount of > > > > > > > > free space > in a slab class as a heuristic to more quickly move slab pages > > > > > > > > around. > > > > > > > > > > > > > > > > If it's > still necessary from there, we can explore "upgrading" items to a > > > > > > > > new slab > class, but that is much much more complicated since the item has > > > > > > > > to shift > LRU's. Do you put it at the head, the tail, the middle, etc? It > > > > > > > > might be > impossible to make a good generic decision there. > > > > > > > > > > > > > > > > What version > are you currently on? If 1.4.24, have you seen any > > > > > > > > instability? > I'm currently torn between fighting a few bugs and start on > > > > > > > > improving > the slab rebalancer. > > > > > > > > > > > > > > > > -Dormando > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > > > --- > > > > > > > > You received this > message because you are subscribed to the Google Groups "memcached" group. > > > > > > > > To unsubscribe > from this group and stop receiving emails from it, send an email to > memcached+...@googlegroups.com. > > > > > > > > For more options, > visit https://groups.google.com/d/optout. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > ... -- --- You received this message because you are subscribed to the Google Groups "memcached" group. To unsubscribe from this group and stop receiving emails from it, send an email to memcached+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
