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From: "Robert N. M. Watson" <rwatson@freebsd.org>
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To: Gabor PALI <pgj@FreeBSD.org>
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Subject: Re: svn commit: r209119 - head/sys/sys
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On 11 Jul 2010, at 04:18, Gabor PALI wrote:

> On Sat, Jul 10, 2010 at 5:24 PM, Robert N. M. Watson
> <rwatson@freebsd.org> wrote:
>> If we can do it in one atomic in the common case, and two atomics in =
an edge case, that sounds fine. I think any use of locking(9) would be =
sufficiently costly as to not be worth the improvements in consistency, =
given the frequency of statistics operations.
>=20
> I have tried to use atomic operations for counting (without
> locking(9)), but they turned out to be significantly slower than the
> naive case indeed.  If consistency is not so important for statistics,
> whether would it be safe to simply use 64-bit variables for counters
> everywhere on all architectures?

I think the worry comes down to: an occasional missed packet is OK, but =
a duplicated carry(for example)  from the lower 32 bits to the upper 32 =
bits would put the counter off by 4 billion, which is not really =
acceptable.

What sort of measurement did you do to show the speed loss, BTW?

Robert