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SubjectRe: Free memory never fully used, swapping
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On Fri, 2010-11-26 at 09:25 +0800, Mel Gorman wrote:
> On Fri, Nov 26, 2010 at 09:05:56AM +0800, Shaohua Li wrote:
> > > @@ -2168,6 +2180,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> > > static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
> > > {
> > > int all_zones_ok;
> > > + int any_zone_ok;
> > > int priority;
> > > int i;
> > > unsigned long total_scanned;
> > > @@ -2201,6 +2214,7 @@ loop_again:
> > > disable_swap_token();
> > >
> > > all_zones_ok = 1;
> > > + any_zone_ok = 0;
> > >
> > > /*
> > > * Scan in the highmem->dma direction for the highest
> > > @@ -2310,10 +2324,12 @@ loop_again:
> > > * spectulatively avoid congestion waits
> > > */
> > > zone_clear_flag(zone, ZONE_CONGESTED);
> > > + if (i <= pgdat->high_zoneidx)
> > > + any_zone_ok = 1;
> > > }
> > >
> > > }
> > > - if (all_zones_ok)
> > > + if (all_zones_ok || (order && any_zone_ok))
> > > break; /* kswapd: all done */
> > > /*
> > > * OK, kswapd is getting into trouble. Take a nap, then take
> > > @@ -2336,7 +2352,7 @@ loop_again:
> > > break;
> > > }
> > > out:
> > > - if (!all_zones_ok) {
> > > + if (!(all_zones_ok || (order && any_zone_ok))) {
> > > cond_resched();
> > >
> > > try_to_freeze();
> > > @@ -2361,7 +2377,13 @@ out:
> > > goto loop_again;
> > > }
> > >
> > > - return sc.nr_reclaimed;
> > > + /*
> > > + * Return the order we were reclaiming at so sleeping_prematurely()
> > > + * makes a decision on the order we were last reclaiming at. However,
> > > + * if another caller entered the allocator slow path while kswapd
> > > + * was awake, order will remain at the higher level
> > > + */
> > > + return order;
> > > }
> > This seems always fail. because you have the protect in the kswapd side,
> > but no in the page allocation side. so every time a high order
> > allocation occurs, the protect breaks and kswapd keeps running.
> >
>
> I don't understand your question. sc.nr_reclaimed was being unused. The
> point of returning order was to tell kswapd that "the order you were
> reclaiming at may or may not be still valid, make your decisions on the
> order I am currently reclaiming at". The key here is if that multiple
> allocation requests come in for higher orders, kswapd will get reworken
> multiple times. Unless it gets rewoken multiple times, kswapd is willing
> to go back to sleep to avoid reclaiming an excessive number of pages.
yes, I thought is rewoken multiple times, in the workload Simon reported
Node 0, zone DMA32 20741 29383 6022 134 272 123 4 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20476 29370 6024 117 48 116 4 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20343 29369 6020 110 23 10 2 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21592 30477 4856 22 10 4 2 0 0 0 0
order >=3 pages are reduced a lot in a second



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