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

[Wu Fengguang <fengguang.wu@intel.com>]

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

Thanks,
Fengguang