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SubjectRe: [PATCH v2] fix get_scan_count for working well with small targets
On Wed, 27 Apr 2011 14:08:18 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> Hi Kame,
>
> On Wed, Apr 27, 2011 at 10:50 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Tue, 26 Apr 2011 13:59:34 -0700
> > Andrew Morton <akpm@linux-foundation.org> wrote:
> >
> >> What about simply removing the nr_saved_scan logic and permitting small
> >> scans?  That simplifies the code and I bet it makes no measurable
> >> performance difference.
> >>
> >
> > ok, v2 here. How this looks ?
> > For memcg, I think I should add select_victim_node() for direct reclaim,
> > then, we'll be tune big memcg using small memory on a zone case.
> >
> > ==
> > At memory reclaim, we determine the number of pages to be scanned
> > per zone as
> >        (anon + file) >> priority.
> > Assume
> >        scan = (anon + file) >> priority.
> >
> > If scan < SWAP_CLUSTER_MAX, the scan will be skipped for this time
> > and priority gets higher. This has some problems.
> >
> >  1. This increases priority as 1 without any scan.
> >     To do scan in this priority, amount of pages should be larger than 512M.
> >     If pages>>priority < SWAP_CLUSTER_MAX, it's recorded and scan will be
> >     batched, later. (But we lose 1 priority.)
>
> Nice catch! It looks to be much enhance.
>
> >     But if the amount of pages is smaller than 16M, no scan at priority==0
> >     forever.
>


> Before reviewing the code, I have a question about this.
> Now, in case of (priority = 0), we don't do shift operation with priority.>
So nr_saved_scan would be the number of lru list pages. ie, 16M.
> Why no-scan happens in case of (priority == 0 and 16M lru pages)?
> What am I missing now?
>
An, sorry. My comment is wrong. no scan at priority == DEF_PRIORITY.
I'll fix description.

But....
Now, in direct reclaim path
==
static void shrink_zones(int priority, struct zonelist *zonelist,
struct scan_control *sc)
{
....
if (scanning_global_lru(sc)) {
if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
continue;
if (zone->all_unreclaimable && priority != DEF_PRIORITY)
continue; /* Let kswapd poll it */
}
==
And in kswapd path
==
/*
* Scan in the highmem->dma direction for the highest
* zone which needs scanning
*/
for (i = pgdat->nr_zones - 1; i >= 0; i--) {
struct zone *zone = pgdat->node_zones + i;
if (!populated_zone(zone))
continue;
if (zone->all_unreclaimable && priority != DEF_PRIORITY)
continue;
....
for (i = 0; i <= end_zone; i++) {
if (zone->all_unreclaimable && priority != DEF_PRIORITY)
continue;
==

So, all_unreclaimable zones are only scanned when priority==DEF_PRIORITY.
But, in DEF_PRIORITY, scan count is always zero because of priority shift.
So, yes, no scan even if priority==0 even after setting all_unreclaimable == true.

Thanks,
-Kame

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