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Date: Mon, 26 Jun 2017 11:32:00 +0200 From: Michal Hocko <mhocko@...nel.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>, <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>, Alkis Georgopoulos <alkisg@...il.com>, Michal Hocko <mhocko@...e.com> Subject: [PATCH] mm: document highmem_is_dirtyable sysctl From: Michal Hocko <mhocko@...e.com> 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. Cc: Alkis Georgopoulos <alkisg@...il.com> Signed-off-by: Michal Hocko <mhocko@...e.com> --- Hi, JFYI this came out from recent discussion [1]. [1] http://lkml.kernel.org/r/20170622123736.1d80f1318eac41cd661b7757@linux-foundation.org Documentation/sysctl/vm.txt | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt index b4ad97f10b8e..48244c42ff52 100644 --- a/Documentation/sysctl/vm.txt +++ b/Documentation/sysctl/vm.txt @@ -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 -- 2.11.0
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