RE: [RFC][PATCH] mm: cut down __GFP_NORETRY page allocation failures

[Satoru Moriya]

Hi Wu,
 
> On Mon, May 02, 2011 at 09:29:58PM +0800, Wu Fengguang wrote:
> > > > +                     if (preferred_zone &&
> > > > +                         zone_watermark_ok_safe(preferred_zone, sc->order,
> > > > +                                     high_wmark_pages(preferred_zone),
> > > > +                                     zone_idx(preferred_zone), 0))
> > > > +                             goto out;
> > > > +             }
> > >
> > > As I said, I think direct reclaim path sould be fast if possbile and
> > > it should not a function of min_free_kbytes.
> >
> > It can be made not a function of min_free_kbytes by simply changing
> > high_wmark_pages() to low_wmark_pages() in the above chunk, since
> > direct reclaim is triggered when ALLOC_WMARK_LOW cannot be satisfied,
> > ie. it just dropped below low_wmark_pages().
> >
> > But still, it costs 62ms reclaim latency (base kernel is 29ms).
> 
> I got new findings: the CPU schedule delays are much larger than
> reclaim delays. It does make the "direct reclaim until low watermark
> OK" latency less a problem :)
> 
> 1000 dd test case:
>                 RECLAIM delay   CPU delay       nr_alloc_fail   CAL (last CPU)
> base kernel     29ms            244ms           14586           218440
> patched         62ms            215ms           5004            325

Hmm, in your system, the latency of direct reclaim may be a less problem.

But, generally speaking, in a latency sensitive system in enterprise area
there are two kind of processes. One is latency sensitive -(A) the other
is not-latency sensitive -(B). And usually we set cpu affinity for both processes
to avoid scheduling issue in (A). In this situation, CPU delay tends to be lower
than the above and a less problem but reclaim delay is more critical. 

Regards,
Satoru

> 
> Thanks,
> Fengguang
> 

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