On Mon, 25 Apr 2011, Alex Rousskov wrote:
> On 04/25/2011 05:31 PM, david_at_lang.hm wrote:
>> On Mon, 25 Apr 2011, david_at_lang.hm wrote:
>>> On Mon, 25 Apr 2011, Alex Rousskov wrote:
>>>> On 04/14/2011 09:06 PM, david_at_lang.hm 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
Received on Tue Apr 26 2011 - 00:15:02 MDT
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