[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 04:25:27 UTC 2018

My total hit ratio decreased in ~2 times from 40% to 20% (it could be cold
cache but it lasted at this level for a day without sign of improvement).

I'll retry tests with making sure there're no Vary header and will also try
1 worker with shared cache tomorrow. But even if it is this bug looks like
caching works as expected in squid 4 so it'll be better solution than
messing up with headers probably.

On Mon, Jan 15, 2018 at 10:26 AM, Alex Rousskov <
rousskov at measurement-factory.com> wrote:

> 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%.
> This could be a side effect of not supporting Vary caching in shared
> memory: https://bugs.squid-cache.org/show_bug.cgi?id=3806#c9
> > And disk cache hit ratio went up from 15-50% to stable 60-65%.
> I hope your total/combined hit ratio improved overall.
> > it looks like in SMP/rock mode squid avoids using memory for small
> > files like 1-3KB but uses it for 10KB+ files.
> No, there is no such size-discrimination code in Squid.
> > I started tracking down the issue with disabling disk cache completely
> > and it didn't change anything, I just started to get MISS every time for
> > the URL which was getting MEM_HIT with an old configuration. Then I
> > changed "workers 2" to "workers 1" and started getting memory hits as
> > before.
> For a clean apples-to-apples test, make sure you use
> "memory_cache_shared on" when using a single worker without rock
> cache_dirs.
> > Am I doing anything wrong? Which debug options should I enable to
> > provide more information if it seems like a bug?
> Vary caching should be fixed as well, of course, but perhaps there is
> another problem we do not know about. I would start by eliminating Vary
> as the known problem. When using a test transaction, make sure the
> response does not have a Vary header. Or configure Squid to log the Vary
> header and remove the corresponding transactions when computing
> adjusted-for-Vary memory cache hit ratio.
> HTH,
> Alex.

With best regards, Ivan Larionov.
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