Messages in this thread | | | Subject | Re: [PATCH 12/12] mm, page_alloc: Only enforce watermarks for order-0 allocations | From | Vlastimil Babka <> | Date | Wed, 30 Sep 2015 15:52:52 +0200 |
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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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