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SubjectRe: [PATCH 02/12] mm: supporting variables and functions for balanced zone aging
On Thu, Dec 01, 2005 at 02:37:14AM -0800, Andrew Morton wrote:
> Wu Fengguang <wfg@mail.ustc.edu.cn> wrote:
> >
> > The zone aging rates are currently imbalanced,
>
> ZONE_DMA is out of whack. It shouldn't be, and I'm not aware of anyone
> getting in and working out why. I certainly wouldn't want to go and add
> all this stuff without having a good understanding of _why_ it's out of
> whack. Perhaps it's just some silly bug, like the thing I pointed at in
> the previous email.

Yep, my rule is that if ever the DMA zone is reclaimed for watermark, it will
be running wild ;) So I leave it out by setting classzone_idx=0, and let the
age balancing code to catch it up. This scheme works fine: tested to be OK from
64M to 2G memory.

> > the gap can be as large as 3 times,
>
> What's the testcase?
>
> > which can severely damage read-ahead requests and shorten their
> > effective life time.
>
> Have you any performance numbers for this?

That's months ago, if I remember it right, the number of concurrent readers the
adaptive read-ahead code can handle without much thrashing was raised from ~100
to 800 with the balancing work.

This is my original announce back then:

The page aging problem showed up when I was testing many slow reads with
limited memory. Pages in the DMA zone were found out to be aged about 3
times faster than that of Normal zone in systems with 64-512M memory.
That is a BIG threat to the read-ahead pages. So I added some code to
make the aging rates synchronized. You can see the effect by running:
$ tar c / | cat > /dev/null &
$ watch -n1 'grep "age " /proc/zoneinfo'
There are still some extra DMA scans in the direct page reclaim path.
It tend to happen in large memory system and is therefore not a big
problem.

And here is some numbers collected by Magnus Damm:
http://lkml.org/lkml/2005/10/25/50

Good night!
Wu
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