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
> > > > > > > > > >
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> > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > > > > prod_stats.png
> > > > > > > >
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