what're your start arguments? the settings look a little odd. ie; the full
commandline (censoring anything important) that you used to start
memcached
On Tue, 7 Jul 2020, Shweta Agrawal wrote:
> Sorry. Here it is.
>
> On Wednesday, July 8, 2020 at 12:38:38 AM UTC+5:30, Dormando wrote:
> 'stats settings' file is empty
>
> On Tue, 7 Jul 2020, Shweta Agrawal wrote:
>
> > Hi Dormando,
> > Got the stats for production. Please find attached files for stats
> settings. stats items, stats, stats slabs. Summary for all slabs.
> >
> > Other details that might help:
> > * TTL is two days or more.
> > * Key length is in the range of 40-80 bytes.
> > Below are the parameters that we plan to change from the current
> settings:
> > 1. slab_automove : from 0 to 1
> > 2. hash_algorithm: from jenkins to murmur
> > 3. chunk_size: from 48 to 297 (as we don't have data of size less
> than that)
> > 4. growth_factor: 1.25 to 1.20 ( Can reducing this more help? Do
> more slab classes affect performance?)
> > 5. max_item_size : from 4MB to 1MB (as our data will never be more
> than 1MB large)
> > Please let me know if different values for above paramters can be
> more beneficial.
> > Are there any other parameters which we should consider to change or
> set?
> >
> > Also below are the calculations used for columns in the summary
> shared. Can you please confirm if calculations are fine.
> > 1) Total_Mem = total_pages*page_size --> total memory
> > 2) Strg_ovrHd = (mem_requested/(used_chunks*chunk_size)) * 100 -->
> storage overhead
> > 3) Free Memory = free_chunks * chunk_size ---> free memory
> > 4) To Store = mem_requested --> actual memory requested for
> storing data
> >
> > Thank you for your time and efforts in explaining concepts.
> > Shweta
> >
> > > > 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
> > > > > > > >
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> > > > > > >
> > > > > > >
> > > > > >
> > > > > > prod_stats.png
> > > > > >
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