Date: Mon, 2 Oct 2006 14:23:11 -0400 From: Charles Swiger <cswiger@mac.com> To: Bob <bob@tania.servebbs.org> Cc: freebsd-questions@freebsd.org Subject: Re: SWAP priority Message-ID: <BA05A137-3CE6-421B-8EE3-2ACFB396F0B4@mac.com> In-Reply-To: <200610021406.39143.bob@tania.servebbs.org> References: <200610020048.47955.bob@tania.servebbs.org> <452110B6.2010800@mac.com> <200610021406.39143.bob@tania.servebbs.org>
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On Oct 2, 2006, at 2:06 PM, Bob wrote: > On Monday 02 October 2006 09:14, Chuck Swiger wrote: >> The swap system knows how to interleave data between the >> additional swap >> areas relatively efficiently, > > Yes I discovered that. The additional swap space was instantly used > as soon as > I activated it; and the added swap improved things measurably. Does > the swap > system take into account current disk activity when it decides to > use a > particular swap? Sort of. The syncer process runs at idle priority, so normal I/O initiated by your processes will take priority over paging/swapping idle pages of RAM out. There may be additional logic involved to help balance I/O in terms of which swapfile is being used if one drive remains busier than another, but I am not completely familiar with FreeBSD's implementation. >> that you need to use more than 2GB of swapspace on a machine with >> 1GB of >> RAM, you should add more RAM, not more swapspace.... > > It is on order. > > The basis for my question about swap priority was based on an > observation that > the slowdown was due to swapping AND heavy disk usage. I noticed > that when > snapshots were being made on the main drive (the one I am using all > the > time), all other processes went to slow-mode. You see, the lack of > enough > memory caused the system to swap, and it swapped to the heaviest > used raid > array. I thought if I could force the system to swap to the other > raid array > (much less used) with the new swapfile, things would improve even > more. Well, you might try benchmarking the system with both arrays used for swapping and with only the less-busy RAID array being used for swapping, and see which one does better. -- -Chuck
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