Date: Mon, 21 Jan 2002 01:12:30 +0100 (CET) From: Michal Mertl <mime@traveller.cz> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: arch@FreeBSD.ORG Subject: Re: 64 bit counters again Message-ID: <Pine.BSF.4.41.0201210105090.94442-100000@prg.traveller.cz> In-Reply-To: <200201202224.g0KMO8E56230@whizzo.transsys.com>
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On Sun, 20 Jan 2002, Louis A. Mamakos wrote: > If a simple spin-lock was used, then you could update an arbitrarily > sized counter for essentially the same cost. If you were clever > enough in writing your device drivers, you could arrange to > update more than one counter (e.g., packets received, octets received) > using the same lock, with the additional counters for free. > That's all nice but 1) would require quite a lot of bigger changes in kernel and 2) could sometimes create a contention for whole struct when only one member is needed (not too probable I guess). > This needs to be weighed against the per-CPU counters and the complexity > associated with them. > > My gut feeling is that the level of contention is low enough that spin > locks should be more than adequate to protect updating a couple of > counters. This assumes that spin locks have not become expensive or > bloated to invalidate their usefulness, I suppose. The 64 bit atomic ops on i386 work probably exactly the same. I guarantee there's no bloat :-). > > The larger thought is that there's this unstated assumption which > probably needs to be state: We have and maintain accurate statistics on > various operations that the FreeBSD kernel performs. For network > elements, it's assumed that there are the typical network-interface > stats which we also provide. We can agonize over the cost of having > to implement all these statistics, but I'd claim that the kernel > is defective if it doesn't do so. I won't be that harsh - some people may need them and they have their cost. We have lot of servers but none of them is longer time doing more than say 10Mbit. I would consider atomic ops free in this operation. -- Michal Mertl mime@traveller.cz To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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