On Fri, Aug 02, 2013 at 06:06:32PM +0200, Andrea Arcangeli wrote:
> The min wmark should be satisfied with just 1 hugepage.
This depends on the size of the machine and if THP is enabled or not
(which adjusts min_free_kbytes). I expect that it is generally true but
wonder how often it is true on something like ARM which does high-order
allocators for stack.
> And the other
> wmarks should be adjusted accordingly. We need to succeed the low
> wmark check if there's some significant amount of 0 order pages, but
> we don't need plenty of high order pages because the PF_MEMALLOC paths
> don't require those. Creating a ton of high order pages that cannot be
> allocated by the high order allocation paths (no PF_MEMALLOC) is quite
> wasteful because they can be splitted in lower order pages before
> anybody has a chance to allocate them.
>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> ---
> mm/page_alloc.c | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 092b30d..4401983 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1643,6 +1643,23 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
>
> if (free_pages - free_cma <= min + lowmem_reserve)
> return false;
> + if (!order)
> + return true;
> +
> + /*
> + * Don't require any high order page under the min
> + * wmark. Invoking compaction to create lots of high order
> + * pages below the min wmark is wasteful because those
> + * hugepages cannot be allocated without PF_MEMALLOC and the
> + * PF_MEMALLOC paths must not depend on high order allocations
> + * to succeed.
> + */
> + min = mark - z->watermark[WMARK_MIN];
> + WARN_ON(min < 0);
It would be hard to hit but you may be able to trigger this warning if
process a process b
read min watermark
increase min_free_kbytes
__zone_watermark_ok
if (min < 0)
return false;
?
> + if (alloc_flags & ALLOC_HIGH)
> + min -= min / 2;
> + if (alloc_flags & ALLOC_HARDER)
> + min -= min / 4;
> for (o = 0; o < order; o++) {
> /* At the next order, this order's pages become unavailable */
> free_pages -= z->free_area[o].nr_free << o;
--
Mel Gorman
SUSE Labs
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