Got it. Thank you for the explanation :) On Sunday, July 5, 2020 at 9:10:23 AM UTC+5:30, Dormando wrote: > > On Sat, 4 Jul 2020, Shweta Agrawal wrote: > > > > the rest is free memory, which should be measured separately. > > free memory for a class will be : (free_chunks * chunk_size) > > And total memory reserved by a class will be : (total_pages*page_size) > > > > > If you're getting evictions in class A but there's too much free > memory in classes C, D, etc > > > then you have a balance issue. for example. An efficiency stat which > just > > > adds up the total pages doesn't tell you what to do with it. > > I see. Got your point.Storage overhead can help in deciding the > chunk_size and growth_factor. Let me add storage-overhead and free memory > as well for > > calculation. > > Most people don't have to worry about growth_factor very much. Especially > since the large item code was added, but it has its own caveats. Growth > factor is only typically useful if you have _very_ statically sized > objects. > > > One curious question: If we have an item of 500Bytes and there is free > memory only in class A(chunk_size: 100Bytes). Do cache evict items from > class with > > largeer chunk_size or use multiple chunks from class A? > > No, it will evict an item matching the 500 byte chunk size, and not touch > A. This is where the memory balancer comes in; it will move pages of > memory between slab classes to keep the tail age roughly the same between > classes. It does this slowly. > > > Example: > > In below scenario, when we try to store item with 3MB, even when there > was memory in class with smaller chunk_size, it evicts items from 512K > class and > > other memory is blocked by smaller slabs. > > Large (> 512KB) items are an exception. It will try to evict from the > "large item" bucket, which is 512kb. It will try to do this up to a few > times, trying to free up enough memory to make space for the large item. > > So to make space for a 3MB item, if the tail item is 5MB in size or 1MB in > size, they will still be evicted. If the tail age is low compared to all > other classes, the memory balancer will eventually move more pages into > the 512K slab class. > > If you tend to store a lot of very large items, it works better if the > instances are larger. > > Memcached is more optimized for performance with small items. if you try > to store a small item, it will evict exactly one item to make space. > However, for very large items (1MB+), the time it takes to read the data > from the network is so large that we can afford to do extra processing. > > > 3Mb_items_eviction.png > > > > > > Thank you, > > Shweta > > > > > > On Sunday, July 5, 2020 at 1:13:19 AM UTC+5:30, Dormando wrote: > > (memory_requested / (chunk_size * chunk_used)) * 100 > > > > is roughly the storage overhead of memory used in the system. the > rest is > > free memory, which should be measured separately. If you're > getting > > evictions in class A but there's too much free memory in classes > C, D, etc > > then you have a balance issue. for example. An efficiency stat > which just > > adds up the total pages doesn't tell you what to do with it. > > > > On Sat, 4 Jul 2020, Shweta Agrawal wrote: > > > > > > I'll need the raw output from "stats items" and "stats slabs". > I don't > > > > think that efficiency column is very helpful. ohkay no > worries. I can get by Tuesday and will share. > > > > > > Efficiency for each slab is calcuated as > > > (("stats slabs" -> memory_requested) / (("stats slabs" -> > total_pages) * page_size)) * 100 > > > > > > > > > Attaching script which has calculations for the same. The script > is from memcahe repo with additional calculation for efficiency. > > > Will it be possible for you to verify if the efficiency > calculation is correct? > > > > > > Thank you, > > > Shweta > > > > > > On Saturday, July 4, 2020 at 1:08:23 PM UTC+5:30, Dormando > wrote: > > > ah okay. > > > > > > I'll need the raw output from "stats items" and "stats > slabs". I don't > > > think that efficiency column is very helpful. > > > > > > On Fri, 3 Jul 2020, Shweta Agrawal wrote: > > > > > > > > > > > > > > > On Saturday, July 4, 2020 at 9:41:49 AM UTC+5:30, > Dormando wrote: > > > > No attachment > > > > > > > > On Fri, 3 Jul 2020, Shweta Agrawal wrote: > > > > > > > > > > > > > > Wooo...so quick. :):) > > > > > > Correct, close. It actually uses more like 3 > 512k chunks and then one > > > > > > smaller chunk from a different class to fit > exactly 1.6MB. > > > > > I see.Got it. > > > > > > > > > > >Can you share snapshots from "stats items" and > "stats slabs" for one of > > > > > these instances? > > > > > > > > > > Currently I have summary of it, sharing the same > below. I can get snapshot by Tuesday as need to request for it. > > > > > > > > > > pages have value from total_pages from stats > slab for each slab > > > > > item_size have value from chunk_size from stats > slab for each slab > > > > > Used memory is calculated as pages*page size > ---> This has to corrected now. > > > > > > > > > > > > > > > prod_stats.png > > > > > > > > > > > > > > > > 90%+ are perfectly doable. You probably need > to look a bit more closely > > > > > > into why you're not getting the efficiency you > expect. The detailed stats > > > > > > output should point to why. I can help with > that if it's confusing. > > > > > > > > > > Great. Will surely ask for your input whenever I > have question. It is really kind of you to offer help. > > > > > > > > > > > Either the slab rebalancer isn't keeping up or > you actually do have 39GB > > > > > > of data and your expecations are a bit off. > This will also depending on > > > > > > the TTL's you're setting and how often/quickly > your items change size. > > > > > > Also things like your serialization method / > compression / key length vs > > > > > > data length / etc. > > > > > > > > > > We have much less data than 39 GB. As after > facing evictions, it has been always kept higher than expected data-size. > > > > > TTL is two days or more. > > > > > From my observation items size(data-length) is > in the range of 300Bytes to 500K after compression. > > > > > Key length is in the range of 40-80 bytes. > > > > > > > > > > Thank you, > > > > > Shweta > > > > > > > > > > On Saturday, July 4, 2020 at 8:38:31 AM > UTC+5:30, Dormando wrote: > > > > > Hey, > > > > > > > > > > > Putting my understanding to re-confirm: > > > > > > 1) Page size will always be 1MB and we > cannot change it.Moreover, it's not required to be changed. > > > > > > > > > > Correct. > > > > > > > > > > > 2) We can store items larger than 1MB > and it is done by combining chunks together. (example: let's say item size: > > ~1.6MB --> > > > 4 slab > > > > > chunks(512k slab) from > > > > > > 2 pages will be used) > > > > > > > > > > Correct, close. It actually uses more like > 3 512k chunks and then one > > > > > smaller chunk from a different class to > fit exactly 1.6MB. > > > > > > > > > > > We use memcache in production and in > past we saw evictions even when free memory was present. Also currently we > use > > cluster > > > with > > > > 39GB RAM in > > > > > total to > > > > > > cache data even when data size we expect > is ~15GB to avoid eviction of active items. > > > > > > > > > > Can you share snapshots from "stats items" > and "stats slabs" for one of > > > > > these instances? > > > > > > > > > > > But as our data varies in size, it is > possible to avoid evictions by tuning parameters: chunk_size, > growth_factor, > > > slab_automove. > > > > Also I > > > > > believe memcache > > > > > > is efficient and we can reduce cost by > reducing memory size for cluster. > > > > > > So I am trying to find the best possible > memory size and parameters we can have.So want to be clear with my > > understanding > > > and > > > > calculations. > > > > > > > > > > > > So while trying different parameters and > putting all calculations, I observed that total_pages * item_size_max > > > physical > > > memory for > > > > a > > > > > machine. And from > > > > > > all blogs,and docs it didnot match my > understanding. But it's clear now. Thanks to you. > > > > > > > > > > > > One last question: From my trials I find > that we can achieve ~90% storage efficiency with memcache. (i.e we need > > 10MB of > > > physical > > > > memory to > > > > > store 9MB of > > > > > > data. Do you recommend any idle > memory-size interms of percentage of expected data-size? > > > > > > > > > > 90%+ are perfectly doable. You probably > need to look a bit more closely > > > > > into why you're not getting the efficiency > you expect. The detailed stats > > > > > output should point to why. I can help > with that if it's confusing. > > > > > > > > > > Either the slab rebalancer isn't keeping > up or you actually do have 39GB > > > > > of data and your expecations are a bit > off. This will also depending on > > > > > the TTL's you're setting and how > often/quickly your items change size. > > > > > Also things like your serialization method > / compression / key length vs > > > > > data length / etc. > > > > > > > > > > -Dormando > > > > > > > > > > > On Saturday, July 4, 2020 at 12:23:09 AM > UTC+5:30, Dormando wrote: > > > > > > Hey, > > > > > > > > > > > > Looks like I never updated the > manpage. In the past the item size max was > > > > > > achieved by changing the slab page > size, but that hasn't been true for a > > > > > > long time. > > > > > > > > > > > > From ./memcached -h: > > > > > > -m, --memory-limit=<num> item > memory in megabytes (default: 64) > > > > > > > > > > > > ... -m just means the memory limit > in megabytes, abstract from the page > > > > > > size. I think that was always > true. > > > > > > > > > > > > In any recentish version, any item > larger than half a page size (512k) is > > > > > > created by stitching page chunks > together. This prevents waste when an > > > > > > item would be more than half a > page size. > > > > > > > > > > > > Is there a problem you're trying > to track down? > > > > > > > > > > > > I'll update the manpage. > > > > > > > > > > > > On Fri, 3 Jul 2020, Shweta Agrawal > wrote: > > > > > > > > > > > > > Hi, > > > > > > > Sorry if I am repeating the > question, I searched the list but could not find definite answer. So > posting it. > > > > > > > > > > > > > > Memcache version: 1.5.10 > > > > > > > I have started memcahce with > option: -I 4m (setting maximum item size to 4MB).Verified it is set by > > command stats > > > settings , > > > > I can > > > > > see STAT > > > > > > item_size_max > > > > > > > 4194304. > > > > > > > > > > > > > > Documentation from git > repository here stats that: > > > > > > > > > > > > > > -I, --max-item-size=<size> > > > > > > > Override the default size of > each slab page. The default size is 1mb. Default > > > > > > > value for this parameter is 1m, > minimum is 1k, max is 1G (1024 * 1024 * 1024). > > > > > > > Adjusting this value changes the > item size limit. > > > > > > > My understanding from > documentation is this option will allow to save items with size till 4MB > and the page > > size for > > > each > > > > slab will > > > > > be 4MB > > > > > > (as I set it as > > > > > > > -I 4m). > > > > > > > > > > > > > > I am able to save items till 4MB > but the page-size is still 1MB. > > > > > > > > > > > > > > -m memory size is default 64MB. > > > > > > > > > > > > > > Calculation: > > > > > > > -> Calculated total pages used > from stats slabs output parameter total_pages = 64 (If page size is 4MB > then > > total > > > pages > > > > should not > > > > > be more > > > > > > than 16. Also > > > > > > > when I store 8 items of ~3MB it > uses 25 pages but if page size is 4MB, it should use 8 pages right.) > > > > > > > > > > > > > > Can you please help me in > understanding the behaviour? > > > > > > > > > > > > > > Attached files with details for > output of command stats settings and stats slabs. > > > > > > > Below is the summarized view of > the distribution. > > > > > > > First added items with variable > sizes, then then added items with 3MB and above. > > > > > > > > > > > > > > data_distribution.png > > > > > > > > > > > > > > > > > > > > > > > > > > > > Please let me know in case more > details are required or question is not clear. > > > > > > > > > > > > > > Thank You, > > > > > > > Shweta > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > --- > > > > > > > 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 > memc...@googlegroups.com. > > > > > > > To view this discussion on the > web visit > > > > > > > https://groups.google.com/d/msgid/memcached/2b640e1f-9f59-4432-a930-d830cbe8566do%40googlegroups.com. > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > --- > > > > > > 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 memc...@googlegroups.com. > > > > > > To view this discussion on the web visit > > > > > > https://groups.google.com/d/msgid/memcached/586aad58-c6fb-4ed8-89ce-6b005d59ba12o%40googlegroups.com. > > > > > > > > > > > > > > > > > > > > > > > > prod_stats.png > > > > > > > > > > -- > > > > > > > > > > --- > > > > > 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 memc...@googlegroups.com. > > > > > To view this discussion on the web visit > > > > > https://groups.google.com/d/msgid/memcached/8d011c1a-deec-463f-a17e-4e9908d97bdfo%40googlegroups.com. > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > --- > > > > 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 memc...@googlegroups.com. > > > > To view this discussion on the web visit > > > > https://groups.google.com/d/msgid/memcached/f0c2bfe1-d65d-4b62-9a87-68fc42446c3do%40googlegroups.com. > > > > > > > > > > > > > > > > -- > > > > > > --- > > > 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 memc...@googlegroups.com. > > > To view this discussion on the web visit > > > https://groups.google.com/d/msgid/memcached/bcd4da5a-ae8e-470f-beb9-2705c0f0202ao%40googlegroups.com. > > > > > > > > > > > > -- > > > > --- > > 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 memc...@googlegroups.com <javascript:>. > > To view this discussion on the web visit > https://groups.google.com/d/msgid/memcached/5e76fa4f-7e06-468a-8b10-d99ab89d7ec2o%40googlegroups.com. > > > > > >
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