network statistics in SMP

Harti Brandt hartmut.brandt at
Tue Dec 15 09:45:17 PST 2009

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>> 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>> 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>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?


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