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SubjectRe: [PATCH] Reduce vm_stat cacheline contention in __vm_enough_memory
On Fri, Oct 14, 2011 at 10:18:24AM -0500, Christoph Lameter wrote:
> Also the whole thing could be optimized by concentrating updates to the
> vm_stat array at one point in time. If any local per cpu differential
> overflows then update all the counters in the same cacheline for which we have per cpu
> differentials.
>
> That will defer another acquisition of the cacheline for the next delta
> overflowing. After an update all the per cpu differentials would be zero.
>
> This could be added to zone_page_state_add....
>
>
> Something like this patch? (Restriction of the updates to the same
> cacheline missing. Just does everything and the zone_page_state may need
> uninlining now)

This patch doesn't have much, if any, effect, at least in the 46 writer thread
case (NR_VM_EVENT_ITEMS-->NR_VM_ZONE_STAT_ITEMS allowed it to boot :) ).
I applied this with the change to align vm_stat.

So far cache alignment of vm_data and increasing ZVC delta has the greatest
effect.

>
> ---
> include/linux/vmstat.h | 19 ++++++++++++++++---
> mm/vmstat.c | 10 ++++------
> 2 files changed, 20 insertions(+), 9 deletions(-)
>
> Index: linux-2.6/include/linux/vmstat.h
> ===================================================================
> --- linux-2.6.orig/include/linux/vmstat.h 2011-10-14 09:58:03.000000000 -0500
> +++ linux-2.6/include/linux/vmstat.h 2011-10-14 10:08:00.000000000 -0500
> @@ -90,10 +90,23 @@ static inline void vm_events_fold_cpu(in
> extern atomic_long_t vm_stat[NR_VM_ZONE_STAT_ITEMS];
>
> static inline void zone_page_state_add(long x, struct zone *zone,
> - enum zone_stat_item item)
> + enum zone_stat_item item, s8 new_value)
> {
> - atomic_long_add(x, &zone->vm_stat[item]);
> - atomic_long_add(x, &vm_stat[item]);
> + enum zone_stat_item i;
> +
> + for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
> + long y;
> +
> + if (i == item)
> + y = this_cpu_xchg(zone->pageset->vm_stat_diff[i], new_value) + x;
> + else
> + y = this_cpu_xchg(zone->pageset->vm_stat_diff[i], 0);
> +
> + if (y) {
> + atomic_long_add(y, &zone->vm_stat[item]);
> + atomic_long_add(y, &vm_stat[item]);
> + }
> + }
> }
>
> static inline unsigned long global_page_state(enum zone_stat_item item)
> Index: linux-2.6/mm/vmstat.c
> ===================================================================
> --- linux-2.6.orig/mm/vmstat.c 2011-10-14 10:04:20.000000000 -0500
> +++ linux-2.6/mm/vmstat.c 2011-10-14 10:08:39.000000000 -0500
> @@ -221,7 +221,7 @@ void __mod_zone_page_state(struct zone *
> t = __this_cpu_read(pcp->stat_threshold);
>
> if (unlikely(x > t || x < -t)) {
> - zone_page_state_add(x, zone, item);
> + zone_page_state_add(x, zone, item, 0);
> x = 0;
> }
> __this_cpu_write(*p, x);
> @@ -262,8 +262,7 @@ void __inc_zone_state(struct zone *zone,
> if (unlikely(v > t)) {
> s8 overstep = t >> 1;
>
> - zone_page_state_add(v + overstep, zone, item);
> - __this_cpu_write(*p, -overstep);
> + zone_page_state_add(v + overstep, zone, item, -overstep);
> }
> }
>
> @@ -284,8 +283,7 @@ void __dec_zone_state(struct zone *zone,
> if (unlikely(v < - t)) {
> s8 overstep = t >> 1;
>
> - zone_page_state_add(v - overstep, zone, item);
> - __this_cpu_write(*p, overstep);
> + zone_page_state_add(v - overstep, zone, item, overstep);
> }
> }
>
> @@ -343,7 +341,7 @@ static inline void mod_state(struct zone
> } while (this_cpu_cmpxchg(*p, o, n) != o);
>
> if (z)
> - zone_page_state_add(z, zone, item);
> + zone_page_state_add(z, zone, item, 0);
> }
>
> void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,


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