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From: Michal Mertl <mime@traveller.cz>
To: Paul Allen <nospam@ugcs.caltech.edu>
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Date: Sun, 30 Jul 2006 01:38:39 +0200
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Cc: Peter Jeremy <peterjeremy@optushome.com.au>, freebsd-current@freebsd.org,
	Brian Candler <B.Candler@pobox.com>
Subject: Re: vmstat's entries type
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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?

I did not think of preventing the migration of the thread to different
CPU before. I would have expected this was expensive operation...

Michal