Re: [PATCH]vmscan: handle underflow for get_scan_ratio

From: Rik van Riel
Date: Mon Apr 05 2010 - 23:41:13 EST

On 04/05/2010 11:31 PM, Wu Fengguang wrote:
On Tue, Apr 06, 2010 at 10:58:43AM +0800, KOSAKI Motohiro wrote:
Again, I didn't said his patch is no worth. I only said we don't have to
ignore the downside.

Right, we should document both the upside and downside.

The downside is obvious: streaming IO (used once data
that does not fit in the cache) can push out data that
is used more often - requiring that it be swapped in
at a later point in time.

I understand what Shaohua's patch does, but I do not
understand the upside. What good does it do to increase
the size of the cache for streaming IO data, which is
generally touched only once?

What kind of performance benefits can we get by doing

The main difference happens when file:anon scan ratio> 100:1.

For the current percent[] based computing, percent[0]=0 hence nr[0]=0
which disables anon list scan unconditionally, for good or for bad.

For the direct nr[] computing,
- nr[0] will be 0 for typical file servers, because with priority=12
and anon lru size< 1.6GB, nr[0] = (anon_size/4096)/100< 0
- nr[0] will be non-zero when priority=1 and anon_size> 100 pages,
this stops OOM for Shaohua's test case, however may not be enough to
guarantee safety (your previous reverting patch can provide this

I liked Shaohua's patch a lot -- it adapts well to both the
file-server case and the mostly-anon-pages case :)

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at