Date: Wed, 9 May 2001 12:41:39 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Peter Jeremy <peter.jeremy@alcatel.com.au> Cc: Kirk McKusick <mckusick@mckusick.com>, Rik van Riel <riel@conectiva.com.br>, arch@FreeBSD.ORG, linux-mm@kvack.org, sfkaplan@cs.amherst.edu Subject: Re: on load control / process swapping Message-ID: <200105091941.f49JfdD98861@earth.backplane.com> References: <200105082052.NAA08757@beastie.mckusick.com> <200105090018.f490IGR87881@earth.backplane.com> <20010509120743.Y59150@gsmx07.alcatel.com.au>
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:I don't think this follows. A program that does something like:
:{
: extern char memory[BIG_NUMBER];
: int i;
:
: for (i = 0; i < BIG_NUMBER; i += PAGE_SIZE)
: memory[i]++;
:}
:will thrash nicely (assuming BIG_NUMBER is large compared to the
:currently available physical memory). Occasionally, it will be
:runnable - at which stage it has a footprint of only two pages, but
Why only two pages? It looks to me like the footprint is BIG_NUMBER
bytes.
:after executing a couple of instructions, it'll have another page
:fault. Old pages will remain resident for some time before they age
:enough to be paged out. If the VM system is stressed, swapping this
:process out completely would seem to be a win.
Not exactly. Page aging works both ways. Just accessing a page
once does not give it priority over everything else in the page
queues.
:...
:you ignore spikes due to process initialisation etc, a process that
:faults very quickly after being given the CPU wants a working set size
:that is larger than the VM system currently allows. The fault rate
:would seem to be proportional to the ratio between the wanted WSS and
:allowed RSS. This would seem to be a useful parameter to help decide
:which process to swap out - in an ideal world the VM subsystem would
:swap processes to keep the WSS of all in-core processes at about the
:size of non-kernel RAM.
:
:Peter
Fault rate isn't useful -- maybe faults that require large disk seeks
would be useful, but just counting the faults themselves is not useful.
-Matt
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