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mm: document highmem_is_dirtyable sysctl

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It seems that there are still people using 32b kernels which a lot of
memory and the IO tend to suck a lot for them by default.  Mostly
because writers are throttled too when the lowmem is used.  We have
highmem_is_dirtyable to work around that issue but it seems we never
bothered to document it.  Let's do it now, finally.

Link: http://lkml.kernel.org/r/20170626093200.18958-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alkis Georgopoulos <alkisg@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michal Hocko 2017-07-10 15:49:38 -07:00 committed by Linus Torvalds
parent e3d3910a57
commit d09b646882
1 changed files with 20 additions and 0 deletions
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Documentation/sysctl/vm.txt
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@ -240,6 +240,26 @@ fragmentation index is <= extfrag_threshold. The default value is 500.
==============================================================
highmem_is_dirtyable
Available only for systems with CONFIG_HIGHMEM enabled (32b systems).
This parameter controls whether the high memory is considered for dirty
writers throttling. This is not the case by default which means that
only the amount of memory directly visible/usable by the kernel can
be dirtied. As a result, on systems with a large amount of memory and
lowmem basically depleted writers might be throttled too early and
streaming writes can get very slow.
Changing the value to non zero would allow more memory to be dirtied
and thus allow writers to write more data which can be flushed to the
storage more effectively. Note this also comes with a risk of pre-mature
OOM killer because some writers (e.g. direct block device writes) can
only use the low memory and they can fill it up with dirty data without
any throttling.
==============================================================
hugepages_treat_as_movable
This parameter controls whether we can allocate hugepages from ZONE_MOVABLE