Re: Followup: [PATCH -mm] make swapin readahead skip over holes

From: Rik van Riel
Date: Mon Apr 16 2012 - 16:13:20 EST

On 04/16/2012 02:34 PM, Dan Magenheimer wrote:
Hi Rik --

I saw this patch in 3.4-rc1 (because it caused a minor merge
conflict with frontswap) and wondered about its impact.
Since I had a server still set up from running benchmarks
before LSFMM, I ran my kernel compile -jN workload (with
N varying from 4 to 40) on 1GB of RAM, on 3.4-rc2 both with
and without this patch.

For values of N=24 and N=28, your patch made the workload
run 4-9% percent faster. For N=16 and N=20, it was 5-10%
slower. And for N=36 and N=40, it was 30%-40% slower!

Is this expected? Since the swap "disk" is a partition
on the one active drive, maybe the advantage is lost due
to contention?

There are several things going on here:

1) you are running a workload that thrashes

2) the speed at which data is swapped in is increased
with this patch

3) with only 1GB memory, the inactive anon list is
the same size as the active anon list

4) the above points combined mean that less of the
working set could be in memory at once

One solution may be to decrease the swap cluster for
small systems, when they are thrashing.

On the other hand, for most systems swap is very much
a special circumstance, and you want to focus on quickly
moving excess stuff into swap, and moving it back into
memory when needed.

Workloads that thrash are very much an exception, and
probably not what we should optimize for.

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