Date: Tue, 15 Dec 2009 13:13:28 -0500 From: John Baldwin <jhb@freebsd.org> To: Harti Brandt <harti@freebsd.org> Cc: freebsd-arch@freebsd.org Subject: Re: network statistics in SMP Message-ID: <200912151313.28326.jhb@freebsd.org> In-Reply-To: <20091215183859.S53283@beagle.kn.op.dlr.de> References: <20091215103759.P97203@beagle.kn.op.dlr.de> <200912150812.35521.jhb@freebsd.org> <20091215183859.S53283@beagle.kn.op.dlr.de>
next in thread | previous in thread | raw e-mail | index | archive | help
On Tuesday 15 December 2009 12:45:13 pm Harti Brandt wrote: > On Tue, 15 Dec 2009, John Baldwin wrote: > > JB>On Tuesday 15 December 2009 4:38:04 am Harti Brandt wrote: > JB>> Hi all, > JB>> > JB>> I'm working on our network statistics (in the context of SNMP) and wonder, > JB>> to what extend we want them to be correct. I've re-read part of the past > JB>> discussions about 64-bit counters on 32-bit archs and got the impression, > JB>> that there are users that would like to have almost correct statistics > JB>> (for accounting, for example). If this is the case I wonder whether the > JB>> way we do the statistics today is correct. > JB>> > JB>> Basically all statistics are incremented or added to simply by a += b oder > JB>> a++. As I understand, this worked fine in the old days, where you had > JB>> spl*() calls at the right places. Nowadays when everything is SMP > JB>> shouldn't we use at least atomic operations for this? Also I read that on > JB>> architectures where cache coherency is not implemented in hardware even > JB>> this does not help (I found a mail from jhb why for the mutex > JB>> implementation this is not a problem, but I don't understand what to do > JB>> for the += and ++ operations). I failed to find a way, though, to > JB>> influence the caching policy (is there a function one can call to > JB>> change the policy?). > JB> > JB>Atomic ops will always work for reliable statistics. However, I believe > JB>Robert is working on using per-CPU statistics for TCP, UDP, etc. similar to > JB>what we do now for many of the 'cnt' stats (context switches, etc.). For > JB>'cnt' each CPU has its own count of stats that are updated using non-atomic > JB>ops (since they are CPU local). sysctl handlers then sum up the various per- > JB>CPU counts to report global counts to userland. > > I see. I was also thinking along these lines, but was not sure whether it > is worth the trouble. I suppose this does not help to implement 64-bit > counters on 32-bit architectures, though, because you cannot read them > reliably without locking to sum them up, right? Either that or you just accept that you have a small race since it is only stats. :) -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200912151313.28326.jhb>