Seems like this is a borderline use case, but it might still work for you.
How did you verify you found the cause? Can you share snapshots from
"stats items" and "stats slabs" output after your test was run?
Memory isn't evenly distributed; it's assigned where objects actually
exist. so either you've filled the server with other objects or there's
been a miscalculation. It's possible you're hitting client timeouts or
something.
On Tue, 26 Mar 2019, Jerome Kieffer wrote:
> Hi,
>
> I am completely new to memcached but it looks like the right tool for what I
> need to do:
> I would like to do temporary storage of some data coming from a detector at a
> pretty high data-rate and share them with other computer
> via memcached. The image size varies from 512x512 to 4096x4096 which makes
> the raw data size from 0.5 to 32 MB (hence compatible with
> memcached). I wish to use some fast (nvme) SSD as extra memory as well.
>
> I setup a quick benchmark, initially to measure speeds (memcached 1.5.8 from
> debian10):
> * Memcached is setup to accept object up to 128m and use up to 64G of RAM
> (optional SSD storage coming later ...)
> * Store 1024 images of 2048x2048 as uint16 which represents ~8G of RAM.
>
> Actually only a limited number of frames were still available in the cache
> after the end of the write (between 7 and 120 frames).
>
> I found the cause: all frames have the same size hence fall into the same
> slab (the one for the largest objects), which is of limited
> size because chunk slabs are evenly distributed over object sizes.
>
> This raises 2 questions:
> * How can I assign "most" of the available memory to a given size avoid
> dropping frames ? (I know in advance what will be the size of
> the objects)
> * Maybe memcached is not the right tool and you could indicate another tool
> better adapted ?
>
> Thanks in advance for your help.
>
> Cheers,
>
> Jérôme
>
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