Re: [PATCH]vmscan: handle underflow for get_scan_ratio

[Wu Fengguang]

On Tue, Apr 06, 2010 at 10:06:19AM +0800, KOSAKI Motohiro wrote:
> > 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.
> 
> The problem is, the VM are couteniously discarding no longer used file
> cache. if we are scan extra anon 1 page, we will observe tons swap usage
> after few days.
> 
> please don't only think benchmark.

OK the days-of-streaming-io typically happen in file servers.  Suppose
a file server with 16GB memory, 1GB of which is consumed by anonymous
pages, others are for page cache.

Assume that the exact file:anon ratio computed by the get_scan_ratio()
algorithm is 1000:1. In that case percent[0]=0.1 and is rounded down
to 0, which keeps the anon pages in memory for the few days.

Now with Shaohua's patch, nr[0] = (262144/4096)/1000 = 0.06 will also
be rounded down to 0. It only becomes >=1 when
- reclaim runs into trouble and priority goes low
- anon list goes huge

So I guess Shaohua's patch still has reasonable "underflow" threshold :)

Thanks,
Fengguang

> 
> > 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.
> 
> In such case, percent[0] should be big. I think underflowing is not point.
> His test case is merely streaming io copy, why can't we drop tmpfs cached
> page? his /proc/meminfo describe his machine didn't have droppable file cache.
> so, big percent[1] value seems makes no sense. no?
> 
> I'm not sure we need either below detection. I need more investigate.
>  1) detect no discardable file cache
>  2) detect streaming io on tmpfs (as regular file)
> 
> 
> 
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