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" <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 > > > > > > > > > > -- > > > > > > > > > > --- > > > > > 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+...@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. > > > > > > > > > > -- > > > > --- > > 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. > > > > > > > -- > > --- > 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. > >
