Date: Fri, 8 Mar 2002 04:01:28 -0800 From: Kris Kennaway <kris@obsecurity.org> To: Mauritz Sundell <mauritz.sundell@telia.com> Cc: questions@freebsd.org, dillon@freebsd.org Subject: Re: swap-usage Message-ID: <20020308040128.A27224@xor.obsecurity.org> In-Reply-To: <20020308115843.O29414-100000@morgan.upsys.se>; from mauritz.sundell@telia.com on Fri, Mar 08, 2002 at 12:54:14PM %2B0100 References: <20020308115843.O29414-100000@morgan.upsys.se>
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--vkogqOf2sHV7VnPd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Fri, Mar 08, 2002 at 12:54:14PM +0100, Mauritz Sundell wrote: > For me the swap-area is used only then the system have used > all available physical memory and need more and as soon as > the memory need decreases the swap is unused again. That's more or less correct, except that swap isn't freed up until the memory which was swapped out is actually called upon again, and the kernel needs to load those pages back into RAM. It's more efficient to leave it swapped out until needed and leave RAM for future use by other processes. > Further I do not think that where are many applications that > allocates more memory if there are more memory available. You'd be surprised. > So why should I have swap partions on each physical disk? Efficiency. With swap partitions on multiple disks the system can distribute pages across the different partitions, giving better performance because the disk I/O can occur simultaneously (under optimal conditions). > Why should I have 2x the swap-space as main memory? It's a good rule of thumb, because typical system workloads need up to that amount of swap. i.e. if you have a workstation with a typical amount 128MB of RAM, which is used for typical workstation tasks, then you'll typically need more than 128MB of RAM to actually run those tasks. If you start to run huge jobs on this machine, they won't fit into RAM at once, and the system performance will drop. The 2x rule accounts for running jobs plus all of the other background stuff which only needs to run occasionally, and so doesn't need to stay in RAM all the time. > A person that have a system with 64MB RAM and 128MB swap > wants to speed up and buy another 64MB RAM, installing the > RAM the swapping should decrease and the swap-area could > even by decreased. Ok, now the person feel that the system > goes smoother and tend to have more applications running > at the same time when before. But if he felt the system > was slow before update he probably dont want more swapping > to be done than before so why should the swap be increased > by an other 128MB? Why should the usage of memory suddenly > increase from 192MB to 384MB because of an upgrade with > 64MB? It's a rule of thumb, not a law of nature. A person setting up a system will choose the amount of memory based on the kind of workload the machine will be undertaking (you want to be able to fit all of the frequently-executing processes in RAM so it doesn't have to swap). But infrequently-running processes like login shells, sleeping daemons, and transient workloads like processing a huge logfile can demand a lot more memory, which needs to be available. You never ever want to run out of swap, because your machine will not be able to continue running without killing off processes, and that's almost always bad for your system operation. > So if I deside not to have any swap-areas what do I miss > besides a good place for crash-dumps? The ability to run more processes than can fit in RAM, even if not all of them are actually active at one time. There's really no reason you'd want to do this, unless you're building an embedded system where the hardware physically is not available. Kris --vkogqOf2sHV7VnPd Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (FreeBSD) Comment: For info see http://www.gnupg.org iD8DBQE8iKgXWry0BWjoQKURAi4UAKDzWJ0LmG8C/gh/q3abj6zuA2h+YwCg9WTt tgQEIqrXvICV3oj3LIqslh0= =wrfI -----END PGP SIGNATURE----- --vkogqOf2sHV7VnPd-- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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