From: Michal Hocko <mhocko@xxxxxxxx> Subject: mm: document highmem_is_dirtyable sysctl 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@xxxxxxxxxx Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Alkis Georgopoulos <alkisg@xxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/sysctl/vm.txt | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff -puN Documentation/sysctl/vm.txt~mm-document-highmem_is_dirtyable-sysctl Documentation/sysctl/vm.txt --- a/Documentation/sysctl/vm.txt~mm-document-highmem_is_dirtyable-sysctl +++ a/Documentation/sysctl/vm.txt @@ -240,6 +240,26 @@ fragmentation index is <= extfrag_thresh ============================================================== +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 _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
