Re: [PATCH]vmscan: handle underflow for get_scan_ratio

[Wu Fengguang]

On Tue, Apr 06, 2010 at 09:25:36AM +0800, Li, Shaohua wrote:
> On Sun, Apr 04, 2010 at 10:19:06PM +0800, KOSAKI Motohiro wrote:
> > > On Fri, Apr 02, 2010 at 05:14:38PM +0800, KOSAKI Motohiro wrote:
> > > > > > > This patch makes a lot of sense than previous. however I think <1% anon ratio
> > > > > > > shouldn't happen anyway because file lru doesn't have reclaimable pages.
> > > > > > > <1% seems no good reclaim rate.
> > > > > > 
> > > > > > Oops, the above mention is wrong. sorry. only 1 page is still too big.
> > > > > > because under streaming io workload, the number of scanning anon pages should
> > > > > > be zero. this is very strong requirement. if not, backup operation will makes
> > > > > > a lot of swapping out.
> > > > > Sounds there is no big impact for the workload which you mentioned with the patch.
> > > > > please see below descriptions.
> > > > > I updated the description of the patch as fengguang suggested.
> > > > 
> > > > Umm.. sorry, no.
> > > > 
> > > > "one fix but introduce another one bug" is not good deal. instead, 
> > > > I'll revert the guilty commit at first as akpm mentioned.
> > > Even we revert the commit, the patch still has its benefit, as it increases
> > > calculation precision, right?
> > 
> > no, you shouldn't ignore the regression case.

> I don't think this is serious. In my calculation, there is only 1 page swapped out
> for 6G anonmous memory. 1 page should haven't any performance impact.

1 anon page scanned for every N file pages scanned?

Is N a _huge_ enough ratio so that the anon list will be very light scanned?

Rik: here is a little background.

Under streaming IO, the current get_scan_ratio() will get a percent[0]
that is (much) less than 1, so underflows to 0.

It has the bad effect of completely disabling the scan of anon list,
which leads to OOM in Shaohua's test case. OTOH, it also has the good
side effect of keeping anon pages in memory and totally prevent swap
IO.

Shaohua's patch improves the computation precision by computing nr[]
directly in get_scan_ratio(). This is good in general, however will
enable light scan of the anon list on streaming IO.

Thanks,
Fengguang
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