(2012/06/24 19:32), Wanpeng Li wrote:
On Sun, Jun 24, 2012 at 12:19:48PM +0200, Johannes Weiner wrote:
On Sun, Jun 24, 2012 at 06:08:26PM +0800, Wanpeng Li wrote:
On Sun, Jun 24, 2012 at 11:46:14AM +0200, Johannes Weiner wrote:
On Sun, Jun 24, 2012 at 10:16:09AM +0800, Wanpeng Li wrote:
From: Wanpeng Li <liwp@xxxxxxxxxxxxxxxxxx>
Since exceeded unused cached charges would add pressure to
mem_cgroup_do_charge, more overhead would burn cpu cycles when
mem_cgroup_do_charge cause page reclaim or even OOM be triggered
just for such exceeded unused cached charges. Add MAX_CHARGE_BATCH
to limit max cached charges.
Signed-off-by: Wanpeng Li <liwp.linux@xxxxxxxxx>
---
mm/memcontrol.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0e092eb..1ff317a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1954,6 +1954,14 @@ void mem_cgroup_update_page_stat(struct page *page,
* TODO: maybe necessary to use big numbers in big irons.
*/
#define CHARGE_BATCH 32U
+
+/*
+ * Max size of charge stock. Since exceeded unused cached charges would
+ * add pressure to mem_cgroup_do_charge which will cause page reclaim or
+ * even oom be triggered.
+ */
+#define MAX_CHARGE_BATCH 1024U
+
struct memcg_stock_pcp {
struct mem_cgroup *cached; /* this never be root cgroup */
unsigned int nr_pages;
@@ -2250,6 +2258,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
unsigned int batch = max(CHARGE_BATCH, nr_pages);
int nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
struct mem_cgroup *memcg = NULL;
+ struct memcg_stock_pcp *stock;
int ret;
/*
@@ -2320,6 +2329,13 @@ again:
rcu_read_unlock();
}
+ stock = &get_cpu_var(memcg_stock);
+ if (memcg == stock->cached && stock->nr_pages) {
+ if (stock->nr_pages > MAX_CHARGE_BATCH)
+ batch = nr_pages;
+ }
+ put_cpu_var(memcg_stock);
The only way excessive stock can build up is if the charging task gets
rescheduled, after trying to consume stock a few lines above, to a cpu
it was running on when it built up stock in the past.
consume_stock()
memcg != stock->cached:
return false
do_charge()
<reschedule>
refill_stock()
memcg == stock->cached:
stock->nr_pages += nr_pages
__mem_cgroup_try_charge() {
unsigned int batch = max(CHARGE_BATCH, nr_pages);
[...]
mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
[...]
if(batch > nr_pages)
refill_stock(memcg, batch - nr_pages);
}
Consider this scenario, If one task wants to charge nr_pages = 1,
then batch = max(32,1) = 32, this time 31 excess charges
will be charged in mem_cgroup_do_charge and then add to stock by
refill_stock. Generally there are many tasks in one memory cgroup and
maybe charges frequency. In this situation, limit will reach soon,
and cause mem_cgroup_reclaim to call try_to_free_mem_cgroup_pages.
But the stock is not a black hole that gets built up for giggles! The
next time the processes want to charge a page on this cpu, they will
consume it from the stock. Not add more pages to it. Look at where
consume_stock() is called.
if(nr_pages == 1 && consume_stock(memcg))
goto done;
Only when charge one page will call consume_stock. You can see the codes
in mem_cgroup_charge_common() which also call __mem_cgroup_try_charge,
when both transparent huge and hugetlbfs pages, nr_pages will larger than 1.
Because THP charges 2M bytes at once, the optimization by 'stock' will have no
effects. (It merges 512page faults into a page fault.)
I think you can't see any performance difference even if we handle THP
pages with 'stock'.
And I think MAX_CHARGE_BATCH=1024 is too big...If you have 256cpus, you'll
have 1GB of cached charges...it means 1GB of inaccuracy of usage.
If you want to enlarge it, please show performance benefit.
Thanks,
-Kame
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