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" <dorma...@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
> > > >
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