Date: Sat, 1 Jan 2000 14:35:02 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Kirk McKusick <mckusick@flamingo.McKusick.COM>, Greg Lehey <grog@lemis.com>, Mike Smith <msmith@FreeBSD.ORG>, Tom <tom@uniserve.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: Slight mistake in my DaemonNews column Message-ID: <200001012235.OAA93859@apollo.backplane.com>
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Ach, I also forgot the all-shadowed optimization, which is actually
fairly important. Well, if anyone brings it up here's the problem
and solution:
"But doesn't the object model cause an ever deeper stack of layers
as a server forks children out?".
Even though we do not optimize the dead-page case we *do* optimize the
all-shadowed case. Lets say that the parent, C1, touches all the pages
in B causing a copy-on-write to occur to C1. C1 now entirely shadows B
and therefore can BYPASS B.
+----+----+ +----+
| C1 | C2 | | C2 |
+----+----+ +---------+
| B | -> | C1 | B |
+---------+ +---------+
| A | | A |
+---------+ +---------+
Once the bypass occurs, B has only one reference and C2 and B can be
collapsed together.
Ok, so you might ask the question "But wait, what if B is large object
(represents the entire data area of the program)? Doesn't that mean
that neither C1 or C2 are likely to ever completely shadow B?".
Answer: This is often true at the second level (B), but usually not
true at the third level (C1 or C2). If C1 (the parent) or C2 (the child)
forks again, then it is likely that something in the new layer D? will
be able to completely overshadow C1 or C2 and thus D? will be able to
bypass C1 or C2.
The end result is that in typical useage, even with a lot of working going
on, the object stack does not run more then 4 deep.
-Matt
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