Date: Mon, 21 Oct 1996 00:25:18 +1000 (EST) From: Douglas Thomas Crosher <dtc@scrooge.ee.swin.oz.au> To: FreeBSD-gnats-submit@freebsd.org Subject: docs/1851: madvise MADV_FREE manual page clarification Message-ID: <199610201425.AAA16935@scrooge.ee.swin.oz.au> Resent-Message-ID: <199610201430.HAA22102@freefall.freebsd.org>
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>Number: 1851
>Category: docs
>Synopsis: madvise MADV_FREE manual page clarification
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: freebsd-bugs
>State: open
>Class: doc-bug
>Submitter-Id: current-users
>Arrival-Date: Sun Oct 20 07:30:01 PDT 1996
>Last-Modified:
>Originator: Douglas Crosher
>Organization:
Swinburne University
>Release: FreeBSD 2.2-CURRENT i386
>Environment:
FreeBSD-current, Oct 18.
>Description:
>From the madvise manual page:
"MADV_FREE gives the VM system the freedom to free pages, and tells the
system that information in the specified page range is no longer impor-
tant. This is an efficient way of allowing malloc(3) to free pages any-
where in the address space, while keeping the address space valid. The
next time that the page is referenced, the page might be demand zeroed,
or might contain the data that was there before the MADV_FREE call. ..."
I have a slight query on the behaviour of MADV_FREE. My initial
interpretation was that after a MADV_FREE a page would either be zero
filled or remain unchanged. However I found that pages could have
different data in them after the MADV_FREE call.
>How-To-Repeat:
I've seen this behaviour while experimenting with page freeing
strategies for a garbage collector for CMUCL. I tried to zero fill
the pages then call madvise MADV_FREE in the hope they would be zero
the next time there were accessed, however some pages were not zero
filled.
>Fix:
Perhaps the documentation could be made a little clearer, I suggest:
"... The next time that the page is referenced, the page might be
demand zeroed, or might contain random data."
>Audit-Trail:
>Unformatted:
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