Re: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0 allocations

[Vitaly Wool]

On Wed, Sep 9, 2015 at 2:39 PM, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Sep 08, 2015 at 05:26:13PM +0900, Joonsoo Kim wrote:
>> 2015-08-24 21:30 GMT+09:00 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>:
>> > 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 still don't think that this one suitable high-order page is enough.
>> If fragmentation happens, there would be no order-2 freepage. If kswapd
>> prepares only 1 order-2 freepage, one of two successive process forks
>> (AFAIK, fork in x86 and ARM require order 2 page) must go to direct reclaim
>> to make order-2 freepage. Kswapd cannot make order-2 freepage in that
>> short time. It causes latency to many high-order freepage requestor
>> in fragmented situation.
>>
>
> So what do you suggest instead? A fixed number, some other heuristic?
> You have pushed several times now for the series to focus on the latency
> of standard high-order allocations but again I will say that it is outside
> the scope of this series. If you want to take steps to reduce the latency
> of ordinary high-order allocation requests that can sleep then it should
> be a separate series.

I do believe https://lkml.org/lkml/2015/9/9/313 does a better job
here. I have to admit the patch header is a bit misleading here since
we don't actually exclude CMA pages, we just _fix_ the calculation in
the loop which is utterly wrong otherwise.

~vitaly
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