you said you were seeing evictions? Was this on a different instance?
I don't really have any control or influence over what amazon deploys for
elasticache. They've also changed the daemon. Some of your settings are
different from the defaults that 1.5.10 has (automove should default to 1
and hash_Algo should default to murmur).
Also your instance hasn't even malloc'ed half of its memory limit. You
have over 6 gigabytes unused. There aren't any evictions despite the
uptime being over two months.
So far as I can see you don't have to do anything? Unless a different
instance was giving you trouble.
Otherwise:
1. is the default in 1.5 anyway
2. is the default in 1.5.
3. don't bother changing this; it'll change the way the slabs scale.
4. 1.20 is probably fine. reducing it only helps if you have very little
memory.
5. also fine.
but mainly 1) I can't really guarantee anything I say has relevance since
I don't know what code is in elasticache and 2) your instance isn't even
remotely full so I don't have any recommendations.
If it were full and automove was off like it is now, you would see
problems over time.
On Tue, 7 Jul 2020, Shweta Agrawal wrote:
> yes
>
> On Wednesday, July 8, 2020 at 9:35:19 AM UTC+5:30, Dormando wrote:
> Oh, so this is amazon elasticache?
>
> On Tue, 7 Jul 2020, Shweta Agrawal wrote:
>
> > We use aws for deployment and don't have that information. What
> particularly looks odd in settings?
> >
> > On Wednesday, July 8, 2020 at 8:10:04 AM UTC+5:30, Dormando wrote:
> > 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
> > > > > > > > > >
> > > > > > > > > >
> --
> > > > > > > > > >
> > > > > > > > > >
> ---
> > > > > > > > > >
> 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.
> > > > > To view this discussion on the web visit
> > > >
> https://groups.google.com/d/msgid/memcached/5e76fa4f-7e06-468a-8b10-d99ab89d7ec2o%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/71fd5680-7bd2-473b-9944-6cda8271ad5fo%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/372169f1-2a2e-4163-bf48-ca8176e76443o%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/d89d1650-801b-4632-8a5d-3a29b98c161fo%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 memcached+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/memcached/19c7f2f3-df06-4f08-b279-28293e39a4bco%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 memcached+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/memcached/alpine.DEB.2.21.2007072125200.18887%40dskull.
