linux-kernel - Re: [PATCH]vmscan: handle underflow for get_scan

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Date:	Fri, 9 Apr 2010 14:20:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	"Wu, Fengguang" <fengguang.wu@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	riel@...hat.com
Subject: Re: [PATCH]vmscan: handle underflow for get_scan_ratio

On Fri, 9 Apr 2010 14:51:04 +0800
Shaohua Li <shaohua.li@...el.com> wrote:

> get_scan_ratio() calculates percentage and if the percentage is < 1%, it will
> round percentage down to 0% and cause we completely ignore scanning anon/file
> pages to reclaim memory even the total anon/file pages are very big.
> 
> To avoid underflow, we don't use percentage, instead we directly calculate
> how many pages should be scaned. In this way, we should get several scanned pages
> for < 1% percent.
> 
> This has some benefits:
> 1. increase our calculation precision
> 2. making our scan more smoothly. Without this, if percent[x] is underflow,
> shrink_zone() doesn't scan any pages and suddenly it scans all pages when priority
> is zero. With this, even priority isn't zero, shrink_zone() gets chance to scan
> some pages.
> 
> Note, this patch doesn't really change logics, but just increase precision. For
> system with a lot of memory, this might slightly changes behavior. For example,
> in a sequential file read workload, without the patch, we don't swap any anon
> pages. With it, if anon memory size is bigger than 16G, we will see one anon page
> swapped. The 16G is calculated as PAGE_SIZE * priority(4096) * (fp/ap). fp/ap
> is assumed to be 1024 which is common in this workload. So the impact sounds not
> a big deal.

I grabbed this.

Did we decide that this needed to be backported into 2.6.33.x?  If so,
some words explaining the reasoning would be needed.

Come to that, it's not obvious that we need this in 2.6.34 either.  What
is the user-visible impact here?

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