Date: Thu, 7 Mar 2002 16:30:13 +0100 From: Erik Trulsson <ertr1013@student.uu.se> To: cjp <cjp@sandstorm.net> Cc: mitko@rila.bg, freebsd-hackers@freebsd.org Subject: Re: Swapping performance Message-ID: <20020307153012.GA1942@student.uu.se> In-Reply-To: <05fe01c1c5e6$6a02e890$2400010a@eight> References: <20020307104518.0f73740b.mitko@rila.bg> <05fe01c1c5e6$6a02e890$2400010a@eight>
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On Thu, Mar 07, 2002 at 09:42:44AM -0500, cjp wrote: > This is a comparison of how fast Linux can do something > STUPID versus how fast a real OS can do something intelligently. Your > test is giving you misleading, and dangerous numbers. Do not go waving them > around until you have actually looked at mallocs behavior on the different > systems. > > Here's why: > > Linux implements a brain dead memory allocation > scheme called memory overcommit. It will let you malloc > as much memory as you want whether it is available as RAM or not > and only bitch when you try to use the memory. Therefore, > Linux malloc is much faster than any reasonable system, since all it is doing is > handing out address space out of unallocated address space, > not keeping track of how much memory there actually is. > > In order to handle the kruft that occurs, there is the out of memory killer, > oom_killer. > Which merrily goes through the list of processes, killing off the low priority > processes > until enough memory is free to satisfy what was most recently used. It's the > loan shark > repayment program, with OOMKiller performing the function of the deliquency > reminder. > > On any of the BSD system, you actually get memory you can use, and all the > overhead > of assuring its existence at the time of allocation. Much more robust, less > prone to abuse. > > Try it, you'll like it. If you want the nuts and bolts of it, read the source. Hate to disappoint you, but FreeBSD also overcommits memory. I believe most (or at least many) other Unix variants (including the other *BSD systems) also do that. Whether overcommit is brain-dead or a clever trick is a question which has been debated several times without any conclusive result. -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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