Re: [squid-users] cache_swap_low and high waternmark

Jigar Rasalawala wrote:

> I already used diskd and aufs and results are same. Why it does not hit
> high watermark ? According to me, if squid maintain space between
> high and low mark, and it should not have that much IO access.

How much IO access you have mainly depends on the amount of traffic you
have, not so much the high/low water marks.

> accessing disk ( clearing cache and writting data back should be as
> seperate process..).

It is when you use diskd or aufs.

> Which part of the code affactes the 5 sec delay in store.log file.

Squid never intentionally delays for 5 seconds, not even while deleting
objects as fast as it can.

If Squid delays for 5 seconds then most likely your OS have suspended
Squid for some reason, most likely memory pressure.

> Let me tell you one more thing about my test.
> 5. When I start squid, it just works file with 100 clients. Once it hit watermark and then it gives
> a gap of 5 secs. after every 4-5 minuts ( not exactly after same interval).

No idea unless there is a message in cache.log giving additional hints.

Maybe the OS is frenetically trying to write data out to disk and for
some reason suspended Squid while doing so or something.

Maybe you can gain a better understanding of what is going on if you run
"squid -k debug" while you see this 5 seconds gap. If the same kind of
gap is seen in cache.log then your Squid have been suspended. If the gap
occurs in different stages in cahce.log then you also know that this was
not under the control of Squid but decided freely by your OS for unknown
reasons. If the gap always occurs at the same stage in cache.log then
there might be a Squid bug in that part of the code.

I have seen pauses like this when you try to push the drive too hard,
causing writes to queue up in memory until the memory is full, forcing
the OS to suspend applications to catch up on writes to disk and
sometimes also swapping.

Regards
Henrik

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