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Date: Sun, 6 Jul 2003 17:49:51 -0700
From: David Schultz <das@FreeBSD.ORG>
To: Chuck Swiger <cswiger@mac.com>, freebsd-stable@FreeBSD.ORG
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References: <200307051728.24681.me@farid-hajji.de>
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Subject: Re: Weird vmstat -s stats
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On Mon, Jul 07, 2003, Peter Jeremy wrote:
> On 2003-Jul-05 18:55:43 -0400, Chuck Swiger <cswiger@mac.com> wrote:
> >Farid Hajji wrote:
> >[ ... ]
> >>Shouldn't such counters be at least 64 bit wide?
> >
> >You betcha.  :-)  The problem is that a 32-bit CPU, like the Intel x86 
> >family, can't increment a 64-bit counter atomicly.
> 
> This isn't absolutely true.  You _can_ perform atomic 64-bit
> operations on an x86 (for x>=5), they are just extremely expensive.
> 
> There are regular threads on this sort of problem and I don't believe
> anyone has come up with a solution that did not involve overheads that
> were considered unacceptable in the general case.

The overhead of a 64-bit atomic increment isn't so bad on x86.  It
takes just a few instructions to do the arithmetic (since namei
only needs to do increments), plus a cmpxchg8b.  But at the
moment, we actually use non-atomic increments, which appears to be
a bug.  We should probably just eat the overhead of 64-bit
atomically-incremented counters; the cache_lookup() path isn't so
critical that we need every cycle.