Date: Tue, 1 Jan 2002 15:49:20 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Bruce Evans <bde@zeta.org.au> Cc: Mike Smith <msmith@FreeBSD.ORG>, Bernd Walter <ticso@cicely8.cicely.de>, Michal Mertl <mime@traveller.cz>, <arch@FreeBSD.ORG> Subject: Re: When to use atomic_ functions? (was: 64 bit counters) Message-ID: <200201012349.g01NnKA40071@apollo.backplane.com> References: <20020102075650.L11121-100000@gamplex.bde.org>
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:On Tue, 1 Jan 2002, Mike Smith wrote:
:
:> > MBs are not needed for the variable itself, but they are making this
:> > family of functions very expensive.
:> > It's not very wise to handle counters with atomic_ functions unless
:> > the need to have MBs in them is not removed.
:>
:> It's imperative to use atomic operations for counters on SMP systems.
:
:Not true. Atomic operations for counters are not needed on SMP systems
:in at least the following cases:
:- if there is a lock that prevents other processes from accessing the
: counter
:- if the counters are per-CPU. See previous mail by someone named msmith.
:
:Bruce
Well, I'm not sure how I got on the Cc list but I agree with Bruce
on this one. An SMP-synchronized counter increment is a ridiculous
waste of time. They should be per-cpu and then we don't care *how*
wide the counters are. Having programs like netstat, or our sysctl
mechanism, aggregate the count values is easy.
-Matt
Matthew Dillon
<dillon@backplane.com>
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