'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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