Re: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Mon, Jul 20, 2015 at 09:00:19AM +0100, Mel Gorman wrote:
> From: Mel Gorman <mgorman@suse.de>
> 
> The primary purpose of watermarks is to ensure that reclaim can always
> make forward progress in PF_MEMALLOC context (kswapd and direct reclaim).
> These assume that order-0 allocations are all that is necessary for
> forward progress.
> 
> High-order watermarks serve a different purpose. Kswapd had no high-order
> awareness before they were introduced (https://lkml.org/lkml/2004/9/5/9).
> This was particularly important when there were high-order atomic requests.
> The watermarks both gave kswapd awareness and made a reserve for those
> atomic requests.
> 
> There are two important side-effects of this. The most important is that
> a non-atomic high-order request can fail even though free pages are available
> and the order-0 watermarks are ok. The second is that high-order watermark
> checks are expensive as the free list counts up to the requested order must
> be examined.
> 
> With the introduction of MIGRATE_HIGHATOMIC it is no longer necessary to
> have high-order watermarks. Kswapd and compaction still need high-order
> awareness which is handled by checking that at least one suitable high-order
> page is free.

I totally agree removing watermark checking for order from
PAGE_ALLOC_COSTLY_ORDER to MAX_ORDER. It doesn't make sense to
maintain such high-order freepage that MM don't guarantee allocation
success. For example, in my system, when there is 1 order-9 freepage,
allocation request for order-9 fails because watermark check requires
at least 2 order-9 freepages in order to succeed order-9 allocation.

But, I think watermark checking with order up to PAGE_ALLOC_COSTLY_ORDER is
different. If we maintain just 1 high-order freepages, successive
high-order allocation request that should be success always fall into
allocation slow-path and go into the direct reclaim/compaction. It enlarges
many workload's latency. We should prepare at least some number of freepage
to handle successive high-order allocation request gracefully.

So, how about following?

1) kswapd checks watermark as is up to PAGE_ALLOC_COSTLY_ORDER. It
guarantees kswapd prepares some number of high-order freepages so
successive high-order allocation request will be handlded gracefully.
2) In case of !kswapd, just check whether appropriate freepage is
in buddy or not.

Thanks.

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