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From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.unix.wizards
Subject: Re: Performance Tuning Ultrix 4.1
Keywords: paging swapping fast large load BSD
Message-ID: <12792@dog.ee.lbl.gov>
Date: 3 May 91 19:06:39 GMT
References: <12714@dog.ee.lbl.gov> <1991May2.052140.27048@milton.u.washington.edu> <12759@dog.ee.lbl.gov> <1991May2.231911.23612@milton.u.washington.edu>
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Organization: Lawrence Berkeley Laboratory, Berkeley
Lines: 87
X-Local-Date: Fri, 3 May 91 12:06:39 PDT

In article <1991May2.231911.23612@milton.u.washington.edu>
corey@milton.u.washington.edu (Corey Satten) writes:
>however I believe that code never executed on Ultrix or BSD because of
>the code a few lines before (BSD code fragment):
>
>		if ((rp->p_flag & (SSEQL|SUANOM)) == 0 &&
>		    rp->p_rssize <= rp->p_maxrss)
>			return (0);
>
>which the front hand does for valid pages.  I think this means that unless
>the process has executed a vadvise() to warn of sequential or anomalous
>paging behavior, the front hand never invalidates data pages.

Not in the old BSD kernel.  The code path is, for the front hand (on
the VAX):

	/*
	 * `page cluster' info is generally treated as `bits in the
	 * first pte mapping a page the cluster', hence the `mark first'
	 * code below.
	 */
	if (page cluster is valid) {
		mark page cluster invalid;
		mark process SPTECHG;
		if (any page in the cluster is modified)
			mark the first page modified;
		make all pages in the cluster look like the first one;
		if (it is a text page cluster)
			let other users know about changes;
		if (process is normal)
			we are done, return 0;
	}

The SEQL and SUANOM cases, and the rssize > maxrss case, are where
the process should not be paged in LRU fashion but rather in `almost
MRU'.  In this case, if the page was valid and we made it not-valid
we also try to page it out.  This is not quite right---we should
not be paging out text pages as the vadvise call is for data, not
text---but is probably `good enough'.

On the Tahoe, which has a reference bit, the valid bit and `fast
reclaim' stuff is unnecessary, and the code path looks like:

	if (this is a text page cluster)
		if (any process using it has referenced it)
			mark the first page as referenced;
	if (the page cluster has been referenced) {
		mark page cluster not-referenced;
		if (any page in the cluster is modified)
			mark the first page modified;
		make all pages in the cluster look like the first one;
		if (it is a text page cluster)
			let other users know about changes;
		if (process is normal)
			we are done, return 0;
	}

The back hand, of course, never looks at valid/referenced pages at
all.  Thus, the code in `>' above is meant only to cause the front hand
to do pageouts.  For most processes, the front hand merely paves the
way for the back hand to do pageouts.  Imagine a wall clock with both
hands moving at the same speed: the time it takes for the back hand to
pass the same spot as the front hand is the time a process is given to
`reclaim' a page.  We take it away, but leave it in memory, and if you
ask for it before the back hand gets around to it, you get to keep it.
If not, we dust off the page (write it to swap) if it is dirty, and
then put it in the `clean' pile (free pages).

For processes that have done a vadvise(SSEQL) or (SUANOM), we presume
instead that `recently used' means `unlikely to be reused', so in this
case we have the front hand dust it off instead---if you just used it,
we take it away.

The planned 4.2BSD `new VM' (which is only now being implemented by
beating the Mach VM into a different shape) had an `madvise' call which
was intended to mark anomalous or sequential behaviour on a per-region
or per-page basis, rather than per-process.  In the meantime the old
vadvise call was deprecated, but it lives on. . . .

Presumably someone broke this in the DEC MIPS port.  The MIPS chip does
not have PTEs, so PTEs must be done in software, so you can define your
own used/modified/ref'd/etc bits.  This is much easier in the new
Mach-based VM, where the responsibility for hardware management is in
a separate file.
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 415 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov