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From: pcg@cs.aber.ac.uk (Piercarlo Grandi)
Newsgroups: comp.unix.wizards,comp.unix.i386,comp.sys.att
Subject: Re: Is VHANDFRAC --> VHANDL dynamic?
Message-ID: <PCG.90Jul17125739@odin.cs.aber.ac.uk>
Date: 17 Jul 90 11:57:39 GMT
References: <562@oglvee.UUCP> <PCG.90Jul7174558@odin.cs.aber.ac.uk>
	<565@oglvee.UUCP> <1289@tuewsd.win.tue.nl>
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In-reply-to: wsinpdb@lso.win.tue.nl's message of 13 Jul 90 10:56:40 GMT


In article <1289@tuewsd.win.tue.nl> wsinpdb@lso.win.tue.nl (Paul de Bra)
writes:

   The page-stealing demon will copy a page to swap space and mark
   it as 'free'.

Different versions do different things, but most will not save a page
to swap until it is required, just in case it is reused and modified
again, so avoiding some IO traffic.

Some pagers will even (and it is a big mistake) preferentially select as
victims pages that have not been modified, to avoid having to save them
prior to reuse.

   It does not zero the page or anything, so if the process wants the
   page back and the page has not been ackuired by another process in
   the meantime the original process can get its original page back. It
   need not be paged-in from the swap space.

   Is this correct?

Yes.

   The problem which remains is what happens when a process suddenly
   needs more pages that are currently marked free.

The page stealer (clock hand) would be invoked; and/or the swapper would
be invoked and some process (possibly the one that requested the extra
page) will be expansion swapped. I suspect that in the current System
V/386 the latter course is taken, with the outswap candidate being the
process requesting the extra page (which is often a poor choice for
obvious reasons). Again, Bach hints that the algorithms used to select
outswap (especially if outswap was because of expansion of memory
allocation) and inswap candidates should be rewritten.

In practice there is very poor interaction between the balance set
manager (swapper) and the working set manager (pager, the clock
algorithm), because their functions (block is a *global* policy) do
overlap somehow, and their logic has not been well integrated and
designed.

Again, the solution recommended by AT&T is to avoid exercising the
swapper and pager, by allocating 2-3 times more real memory than the
expected worst case usage (e.g. 512KB to 1MB per user, when the average
working set size of a user command is well below 100-300KB).
--
Piercarlo "Peter" Grandi           | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcsun!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk