Date: Thu, 22 Feb 1996 13:46:19 +0100 From: root <root@spase.nl> To: hackers@freebsd.org Message-ID: <199602221246.NAA11132@mercurius.spase.nl>
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From: tinguely@plains.nodak.edu (Mark Tinguely)
Newsgroups: rtd.freebsd.hackers
Subject: Re: Paging questions
Date: Tue, 20 Feb 1996 18:19:03 -0600 (CST)
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> 1)
> Assume a page is swapped to disk, then copied back in core,
> and not modified afterwards. Do we keep a reference to the disk
> block for the page, so as to save subsequent pageouts ? Do we
> have an idea on how frequently this occurs ? I have no data on this.
Before the VM code grabbed pages around the faulted page (back in 386bsd,
early FreeBSD 1.0.x days) I did count the number of pages that were stored in
swap, read and restored. It was VERY RARE that the page was not modified again
before being placed back into storage. I checked this because I changed the
code so that the VM free the page from swap when read back into memory and this
went a long way to fix the swap depletion problem we had then.
But I have not counted with the new VM code. if I remember correctly, it has
been a long time since I was looking in that code, a larger number of adjacent
pages are brought back in when a page is faulted and some of those pages may
or may not be kept in memory after the transfer. I argued last time this talk
came up (within a year) that in swap depletion, *one* (not the only) strategy
would be to again remove the pages from swap when that page is faulted back in.
but there is performance concerns and hardware is cheap counter arguement was
raised and I never went back down that exploritory road.
--mark.
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