ping,
anything new on this issue? (including any patches for me to test?)
David Lang
On Mon, 25 Apr 2011, david@xxxxxxx wrote:
Date: Mon, 25 Apr 2011 17:14:52 -0700 (PDT)
From: david@xxxxxxx
To: Alex Rousskov <rousskov@xxxxxxxxxxxxxxxxxxxxxxx>
Cc: Marcos <mczueira@xxxxxxxxxxxx>, squid-users@xxxxxxxxxxxxxxx,
squid-dev@xxxxxxxxxxxxxxx
Subject: Re: Res: squid 3.2.0.5 smp scaling issues
On Mon, 25 Apr 2011, Alex Rousskov wrote:
On 04/25/2011 05:31 PM, david@xxxxxxx wrote:
On Mon, 25 Apr 2011, david@xxxxxxx wrote:
On Mon, 25 Apr 2011, Alex Rousskov wrote:
On 04/14/2011 09:06 PM, david@xxxxxxx wrote:
In addition, there seems to be some sort of locking betwen the multiple
worker processes in 3.2 when checking the ACLs
There are pretty much no locks in the current official SMP code. This
will change as we start adding shared caches in a week or so, but even
then the ACLs will remain lock-free. There could be some internal
locking in the 3rd-party libraries used by ACLs (regex and such), but I
do not know much about them.
what are the 3rd party libraries that I would be using?
See "ldd squid". Here is a sample based on a randomly picked Squid:
libnsl, libresolv, libstdc++, libgcc_s, libm, libc, libz, libepol
Please note that I am not saying that any of these have problems in SMP
environment. I am only saying that Squid itself does not lock anything
runtime so if our suspect is SMP-related locks, they would have to
reside elsewhere. The other possibility is that we should suspect
something else, of course. IMHO, it is more likely to be something else:
after all, Squid does not use threads, where such problems are expected.
BTW, do you see more-or-less even load across CPU cores? If not, you may
need a patch that we find useful on older Linux kernels. It is discussed
in the "Will similar workers receive similar amount of work?" section of
http://wiki.squid-cache.org/Features/SmpScale
the load is pretty even across all workers.
with the problems descripted on that page, I would expect uneven utilization
at low loads, but at high loads (with the workers busy serviceing requests
rather than waiting for new connections), I would expect the work to even out
(and the types of hacks described in that section to end up costing
performance, but not in a way that would scale with the ACL processing load)
one thought I had is that this could be locking on name lookups. how
hard would it be to create a quick patch that would bypass the name
lookups entirely and only do the lookups by IP.
I did not realize your ACLs use DNS lookups. Squid internal DNS code
does not have any runtime SMP locks. However, the presence of DNS
lookups increases the number of suspects.
they don't, everything in my test environment is by IP. But I've seen other
software that still runs everything through name lookups, even if what's
presented to the software (both in what's requested and in the ACLs) is all
done by IPs. It's a easy way to bullet-proof the input (if it's a name it
gets resolved, if it's an IP, the IP comes back as-is, and it works for IPv4
and IPv6, no need to have logic that looks at the value and tries to figure
out if the user intended to type a name or an IP). I don't know how squid is
working internally (it's a pretty large codebase, and I haven't tried to
really dive into it) so I don't know if squid does this or not.
A patch you propose does not sound difficult to me, but since I cannot
contribute such a patch soon, it is probably better to test with ACLs
that do not require any DNS lookups instead.
if that regains the speed and/or scalability it would point fingers
fairly conclusively at the DNS components.
this is the only think that I can think of that should be shared between
multiple workers processing ACLs
but it is _not_ currently shared from Squid point of view.
Ok, I was assuming from the description of things that there would be one DNS
process that all the workers would be accessing. from the way it's described
in the documentation it sounds as if it's already a separate process, so I
was thinking that it was possible that if each ACL IP address is being put
through a single DNS process, I could be running into contention on that
process (and having to do name lookups for both IPv6 and then falling back to
IPv4 would explain the severe performance hit far more than the difference
between IPs being 128 bit values instead of 32 bit values)
David Lang
