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

[Vlastimil Babka <vbabka@suse.cz>]

On 09/30/2015 10:51 AM, Vitaly Wool wrote:
> On Wed, Sep 9, 2015 at 2:39 PM, Mel Gorman <mgorman@techsingularity.net> 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@techsingularity.net>:
>>>> 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

Does a better job regarding what exactly? It does fix the CMA-specific 
issue, but so does this patch - without affecting allocation fastpaths 
by making them update another counter. But the issues discussed here are 
not related to that CMA problem.

> 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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