Date: Tue, 15 Dec 2009 09:08:09 -0800 From: Julian Elischer <julian@elischer.org> To: John Baldwin <jhb@freebsd.org> Cc: Harti Brandt <harti@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: network statistics in SMP Message-ID: <4B27C279.8030402@elischer.org> In-Reply-To: <200912150812.35521.jhb@freebsd.org> References: <20091215103759.P97203@beagle.kn.op.dlr.de> <200912150812.35521.jhb@freebsd.org>
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John Baldwin wrote: > On Tuesday 15 December 2009 4:38:04 am Harti Brandt wrote: >> Hi all, >> >> I'm working on our network statistics (in the context of SNMP) and wonder, >> to what extend we want them to be correct. I've re-read part of the past >> discussions about 64-bit counters on 32-bit archs and got the impression, >> that there are users that would like to have almost correct statistics >> (for accounting, for example). If this is the case I wonder whether the >> way we do the statistics today is correct. >> >> Basically all statistics are incremented or added to simply by a += b oder >> a++. As I understand, this worked fine in the old days, where you had >> spl*() calls at the right places. Nowadays when everything is SMP >> shouldn't we use at least atomic operations for this? Also I read that on >> architectures where cache coherency is not implemented in hardware even >> this does not help (I found a mail from jhb why for the mutex >> implementation this is not a problem, but I don't understand what to do >> for the += and ++ operations). I failed to find a way, though, to >> influence the caching policy (is there a function one can call to >> change the policy?). > > Atomic ops will always work for reliable statistics. However, I believe > Robert is working on using per-CPU statistics for TCP, UDP, etc. similar to > what we do now for many of the 'cnt' stats (context switches, etc.). For > 'cnt' each CPU has its own count of stats that are updated using non-atomic > ops (since they are CPU local). sysctl handlers then sum up the various per- > CPU counts to report global counts to userland. the trouble is that PCPU and VNET collide. you then need to have Per-CPU, per VNET counters. which would be yet a different pool of linker set symbols.. >
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