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
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