Re: vmstat's entries type

On Saturday 29 July 2006 19:38, Michal Mertl wrote:
> Paul Allen wrote:
> > Surely all you need to do is a cheap crit_enter,crit_exit 
> > while updating the 64-bit per cpu counters.  and on
> > a 64-bit arch you skip the crit_enter,crit_exit.
> 
> Critical_enter/exit seem to be quite lightweight (single
> read/modify/write of a variable).
> 
> > Seriously this is a bike shed.  We can summarize it thus:
> > statistics should be maintained in 64-bit counters, these
> > counters should be per-cpu and consistent in that context,
> > nothing else should appear on the critical path.
> 
> Why do you call it a bikesched? I think that your proposal could work
> but as nobody proposed doing the stuff with critical_* before, the
> thread may be fruitful. 
> 
> Is critical_* good enough protection though? What if two threads were
> updating the same per-CPU counter on the same CPU at the same time? With
> 64bits counter on a 32bit architecture? I expect the cache coherency
> issues are completely eliminated with per-CPU data, aren't they?

critical_* would prevent an interrupt from preempting the thread, so you 
wouldn't have this case.

That said, I think just using a simple algo (like inc; jnc 1b; inc; 1:) would 
be fine.  With the counter being per-cpu you don't even need the 'lock' 
prefix for i386.  You would have to find similar solutions for other 32-bit 
archs (arm, ppc, mips).

-- 
John Baldwin