path: root/Documentation/nommu-mmap.txt
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authorPaul Mundt <lethal@linux-sh.org>2009-01-08 12:04:47 +0000
committerDavid Howells <dhowells@redhat.com>2009-01-08 12:04:47 +0000
commitdd8632a12e500a684478fea0951f380478d56fed (patch)
tree1a12f441f9de14fd233faa92cf13a5fbb0319f41 /Documentation/nommu-mmap.txt
parent8feae13110d60cc6287afabc2887366b0eb226c2 (diff)
NOMMU: Make mmap allocation page trimming behaviour configurable.
NOMMU mmap allocates a piece of memory for an mmap that's rounded up in size to the nearest power-of-2 number of pages. Currently it then discards the excess pages back to the page allocator, making that memory available for use by other things. This can, however, cause greater amount of fragmentation. To counter this, a sysctl is added in order to fine-tune the trimming behaviour. The default behaviour remains to trim pages aggressively, while this can either be disabled completely or set to a higher page-granular watermark in order to have finer-grained control. vm region vm_top bits taken from an earlier patch by David Howells. Signed-off-by: Paul Mundt <lethal@linux-sh.org> Signed-off-by: David Howells <dhowells@redhat.com> Tested-by: Mike Frysinger <vapier.adi@gmail.com>
Diffstat (limited to 'Documentation/nommu-mmap.txt')
1 files changed, 15 insertions, 0 deletions
diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index 02b89dcf38ac..b565e8279d13 100644
--- a/Documentation/nommu-mmap.txt
+++ b/Documentation/nommu-mmap.txt
Provision of shared mappings on block device files is exactly the same as for
character devices. If there isn't a real device underneath, then the driver
should allocate sufficient contiguous memory to honour any supported mapping.
+NOMMU mmap automatically rounds up to the nearest power-of-2 number of pages
+when performing an allocation. This can have adverse effects on memory
+fragmentation, and as such, is left configurable. The default behaviour is to
+aggressively trim allocations and discard any excess pages back in to the page
+allocator. In order to retain finer-grained control over fragmentation, this
+behaviour can either be disabled completely, or bumped up to a higher page
+watermark where trimming begins.
+Page trimming behaviour is configurable via the sysctl `vm.nr_trim_pages'.

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