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

[Vlastimil Babka]

On 09/30/2015 10:51 AM, Vitaly Wool wrote:
> 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

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/