mm: document highmem_is_dirtyable sysctl (d09b6468) · Commits · 戴 / test

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>

Documentation/sysctl/vm.txt

+20 −0

Original line number	Diff line number	Diff line
		@@ -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

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