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

Ivan Larionov xeron.oskom at gmail.com
Tue Jan 16 21:40:24 UTC 2018

So it's definitely not related to Vary, there's no such header in requests
I tried. Also this issue affects squid even with 1 worker if shared memory
is forced to on.

Interesting thing I noticed is that according to log file a lot of images
are actually cached in memory but sound files are not (mp3/wav). It's not
like it makes any sense but how about random example url:


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?

On Tue, Jan 16, 2018 at 6:25 AM, Alex Rousskov <
rousskov at measurement-factory.com> 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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