Re: [RFC]mm: batch activate_page() to reduce lock contention

[Minchan Kim <minchan.kim@gmail.com>]

On Thu, Jul 22, 2010 at 9:27 AM, Shaohua Li <shaohua.li@intel.com> wrote:
>> > But we did see some strange regression. The regression is small (usually < 2%)
>> > and most are from multithread test and none heavily use activate_page(). For
>> > example, in the same system, we create 64 threads. Each thread creates a private
>> > mmap region and does read access. We measure the total time and saw about 2%
>> > regression. But in such workload, 99% time is on page fault and activate_page()
>> > takes no time. Very strange, we haven't a good explanation for this so far,
>> > hopefully somebody can share a hint.
>>
>> Mabye it might be due to lru_add_drain.
>> You are adding cost in lru_add_drain and it is called several place.
>> So if we can't get the gain in there, it could make a bit of regression.
>> I might be wrong and it's a just my guessing.
> The workload with regression doesn't invoke too many activate_page, so
> basically activate_page_drain_cpu() is a nop, it should not take too much.

I think it's culprit. little call activate_page, many call lru_drain_all.
It would make losing pagevec's benefit.
But as your scenario, I think it doesn't call lru_drain_all frequently.
That's because it is called when process call things related unmap
operation or swapping.
Do you have a such workload in test case?

>
>> > Signed-off-by: Shaohua Li <shaohua.li@intel.com>
>> >
>> > diff --git a/mm/swap.c b/mm/swap.c
>> > index 3ce7bc3..4a3fd7f 100644
>> > --- a/mm/swap.c
>> > +++ b/mm/swap.c
>> > @@ -39,6 +39,7 @@ int page_cluster;
>> >
>> >  static DEFINE_PER_CPU(struct pagevec[NR_LRU_LISTS], lru_add_pvecs);
>> >  static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
>> > +static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
>> >
>> >  /*
>> >   * This path almost never happens for VM activity - pages are normally
>> > @@ -175,11 +176,10 @@ static void update_page_reclaim_stat(struct zone *zone, struct page *page,
>> >  /*
>> >   * FIXME: speed this up?
>> >   */
>> Couldn't we remove above comment by this patch?
> ha, yes.
>
>> > -void activate_page(struct page *page)
>> > +static void __activate_page(struct page *page)
>> >  {
>> >     struct zone *zone = page_zone(page);
>> >
>> > -   spin_lock_irq(&zone->lru_lock);
>> >     if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
>> >             int file = page_is_file_cache(page);
>> >             int lru = page_lru_base_type(page);
>> > @@ -192,7 +192,46 @@ void activate_page(struct page *page)
>> >
>> >             update_page_reclaim_stat(zone, page, file, 1);
>> >     }
>> > -   spin_unlock_irq(&zone->lru_lock);
>> > +}
>> > +
>> > +static void activate_page_drain_cpu(int cpu)
>> > +{
>> > +   struct pagevec *pvec = &per_cpu(activate_page_pvecs, cpu);
>> > +   struct zone *last_zone = NULL, *zone;
>> > +   int i, j;
>> > +
>> > +   for (i = 0; i < pagevec_count(pvec); i++) {
>> > +           zone = page_zone(pvec->pages[i]);
>> > +           if (zone == last_zone)
>> > +                   continue;
>> > +
>> > +           if (last_zone)
>> > +                   spin_unlock_irq(&last_zone->lru_lock);
>> > +           last_zone = zone;
>> > +           spin_lock_irq(&last_zone->lru_lock);
>> > +
>> > +           for (j = i; j < pagevec_count(pvec); j++) {
>> > +                   struct page *page = pvec->pages[j];
>> > +
>> > +                   if (last_zone != page_zone(page))
>> > +                           continue;
>> > +                   __activate_page(page);
>> > +           }
>> > +   }
>> > +   if (last_zone)
>> > +           spin_unlock_irq(&last_zone->lru_lock);
>> > +   release_pages(pvec->pages, pagevec_count(pvec), pvec->cold);
>> > +   pagevec_reinit(pvec);
>>
>> In worst case(DMA->NORMAL->HIGHMEM->DMA->NORMA->HIGHMEM->......),
>> overhead would is big than old. how about following as?
>> static DEFINE_PER_CPU(struct pagevec[MAX_NR_ZONES], activate_page_pvecs);
>> Is it a overkill?
> activate_page_drain_cpu is a two level loop. In you case, the drain order
> will be DMA->DMA->NORMAL->NORMAL->HIGHMEM->HIGHMEM. Since pagevec size is
> 14, the loop should finish quickly.
Yes. so why do we separates lru pagevec with  pagevec[NR_LRU_LISTS]?
I think It can remove looping unnecessary looping overhead but of
course we have to use more memory.




-- 
Kind regards,
Minchan Kim

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