[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 14:25:23 UTC 2018

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.



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