Re: [PATCH]vmscan: handle underflow for get_scan_ratio

Shaohua Li
2010-04-12T01:57:59Z

On Sat, Apr 10, 2010 at 05:20:57AM +0800, Andrew Morton wrote:
> On Fri, 9 Apr 2010 14:51:04 +0800
> Shaohua Li <shaohua.li@intel.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.
Not needed.

> is the user-visible impact here?
Should be very small I think.

Thanks,
Shaohua