linux-kernel - [BUG] fadvise POSIX_FADV

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Message-Id: <1185695837.6665.22.camel@perkele>
Date:	Sun, 29 Jul 2007 03:57:17 -0400
From:	Eric St-Laurent <ericstl34@...patico.ca>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [BUG] fadvise POSIX_FADV_NOREUSE does nothing

Related to my other bug report today, calling posix_fadvise (which uses
fadvise64) with the POSIX_FADV_NOREUSE flag does nothing.  The pages are
not dropped behind.

I also tried call fadvise with POSIX_FADV_SEQUENTIAL first.

This is expected as the POSIX_FADV_NOREUSE is a no-op in the recent
kernels.

Also, POSIX_FADV_SEQUENTIAL only does the readahead window. It doesn't
hint the VM in any way to possibly drop-behind the pages.

(See the previous bug report for more details of the test case)

Relevant numbers:

Copying (using fadvise_cp) a large file test:

1st run: 0m9.018s
2nd run: 0m3.444s
Copying large file...
3rd run: 0m14.024s    <<< page cache trashed
4th run: 0m3.449s

Test programs and batch files are attached.

- Eric

View attachment "fadvise_cp.c" of type "text/x-csrc" (875 bytes)

View attachment "Makefile" of type "text/x-makefile" (84 bytes)

Download attachment "use-once-test.sh" of type "application/x-shellscript" (2864 bytes)

View attachment "working_set_simul.c" of type "text/x-csrc" (637 bytes)