Yeah I meant Very in response to requests I tried.
The fact that you can't reproduce it with this url and the fact that it affects mostly mp3/wav files gave me an idea.
Our env specific is squid's cache_peer parent which transcodes audio files to ulaw (format which our backend supports). This explains why audio files. Doesn't explain why it works with squid 3 non-shared memory and squid 4 shared-memory.
I verified direct request and memory cache works with squid 3 +shared memory.
The difference between direct and non-direct (transcoded) response for http://techslides.com/demos/samples/sample.mp3:
* "Content-Type: audio/mpeg" for direct, "Content-Type: audio/ulaw" for non-direct.
* No "Content-Length" header for non-direct.
What do you think? Could these 2 points lead to the issue I see? Why does it work in all situations except squid 3 + shared memory?
On Tue, Jan 16, 2018 at 3:17 PM, Alex Rousskov <rousskov@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
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.
Alex.
> 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.
With best regards, Ivan Larionov.
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