[squid-users] squid doesn't cache objects in memory when using SMP and shared memory cache

Alex Rousskov rousskov at measurement-factory.com
Tue Jan 16 23:17:32 UTC 2018

On 01/16/2018 02:40 PM, Ivan Larionov wrote:
> So it's definitely not related to Vary, there's no such header in
> requests I tried.

Just to avoid misunderstanding, please note that Vary is a response header.

> Also this issue affects squid even with 1 worker if
> shared memory is forced to on.

This matches the current suspicion that there is something wrong with
the shared memory cache (in your environment).

> It's not like it makes any sense but how about random example url:
> http://techslides.com/demos/samples/sample.mp3
> squid 4, 2 workers, shared cache, no disk cache – MEM_HIT
> squid 3, same config – MISS every time
> squid 3, no shared cache – MEM_HIT
> Could you do a brief test with this URL may be and confirm that I'm not
> the only one who see this issue?

I cannot confirm that: In primitive wget tests with Squid v3.5 (bzr
r14182), I am getting shared memory hits with the above URL, with or
without cache_dirs, with or without SMP.


> On Tue, Jan 16, 2018 at 6:25 AM, Alex Rousskov wrote:
>     On 01/15/2018 09:25 PM, Ivan Larionov wrote:
>     > My total hit ratio decreased in ~2 times from 40% to 20%
>     >     On 01/14/2018 10:53 PM, Ivan Larionov wrote:
>     >
>     >     > After migrating squid from non-SMP/aufs to SMP/rock memory cache hit
>     >     > ratio dropped significantly. Like from 50-100% to 1-5%.
>     >
>     >     > And disk cache hit ratio went up from 15-50% to stable 60-65%.
>     The combination of the three statements above may be a sign of a problem
>     unrelated to Vary: Since the disk cache can cache everything the memory
>     cache can and is typically much larger than the memory cache, the
>     incapacitation of a memory cache (due to Vary) should not have a
>     significant effect on overall hit ratio. It should only affect hit
>     response time.
>     The only known culprit I can think of in this context are hits for
>     being-cached objects: Rock lacks code that allows Squid to read
>     being-written objects. The shared memory cache has that code already. If
>     your workload has a lot of cases where clients request a being-written
>     object, then the overall hit ratio should go down after the memory cache
>     incapacitation (due to Vary).
>     I suspect something else is in play here though, especially if you see a
>     different picture with Squid v4 -- the known problem discussed above is
>     present in all Squid versions. I second Amos's recommendation to focus
>     on v4 because it is unlikely that any complicated problems are going to
>     be fixed in v3, even if you triage them well.
>     HTH,
>     Alex.
> -- 
> With best regards, Ivan Larionov.

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