On Sat, 4 Sep 2010 11:23:11 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > Still, given the improvements in performance from this patchset,
> > I'd say inclusion is a no-braniner....
>
> In your case it's not really high memory pressure, but maybe too many
> concurrent direct reclaimers, so that when one reclaimed some free
> pages, others kick in and "steal" the free pages. So we need to kill
> the second cond_resched() call (which effectively gives other tasks a
> good chance to steal this task's vmscan fruits), and only do
> drain_all_pages() when nothing was reclaimed (instead of allocated).
Well... cond_resched() will only resched when this task has been
marked for preemption. If that's happening at such a high frequency
then Something Is Up with the scheduler, and the reported context
switch rate will be high.
> Dave, will you give a try of this patch? It's based on Mel's.
>
>
> --- linux-next.orig/mm/page_alloc.c 2010-09-04 11:08:03.000000000 +0800
> +++ linux-next/mm/page_alloc.c 2010-09-04 11:16:33.000000000 +0800
> @@ -1850,6 +1850,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
>
> cond_resched();
>
> +retry:
> /* We now go into synchronous reclaim */
> cpuset_memory_pressure_bump();
> p->flags |= PF_MEMALLOC;
> @@ -1863,26 +1864,23 @@ __alloc_pages_direct_reclaim(gfp_t gfp_m
> lockdep_clear_current_reclaim_state();
> p->flags &= ~PF_MEMALLOC;
>
> - cond_resched();
> -
> - if (unlikely(!(*did_some_progress)))
> + if (unlikely(!(*did_some_progress))) {
> + if (!drained) {
> + drain_all_pages();
> + drained = true;
> + goto retry;
> + }
> return NULL;
> + }
>
> -retry:
> page = get_page_from_freelist(gfp_mask, nodemask, order,
> zonelist, high_zoneidx,
> alloc_flags, preferred_zone,
> migratetype);
>
> - /*
> - * If an allocation failed after direct reclaim, it could be because
> - * pages are pinned on the per-cpu lists. Drain them and try again
> - */
> - if (!page && !drained) {
> - drain_all_pages();
> - drained = true;
> + /* someone steal our vmscan fruits? */
> + if (!page && *did_some_progress)
> goto retry;
> - }
Perhaps the fruit-stealing event is worth adding to the
userspace-exposed vm stats somewhere. But not in /proc - somewhere
more temporary, in debugfs.
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