Re: [Qemu-devel] qemu-img convert cache mode for source

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Mar 05, 2014 at 07:09:13PM +0100, Peter Lieven wrote:
> Am 05.03.2014 18:38, schrieb Marcus:
> > On Wed, Mar 5, 2014 at 8:53 AM, Peter Lieven <pl@kamp.de> wrote:
> >> So you can confirm my oberservations and would be happy if
> >> this behaviour could be toggled with a cmdline switch?
> > Yes, I've seen the same behavior you mention just with 'cp'. It was
> > with a version of the CentOS 6.2 kernel, at least, before we added
> > FADV_DONTNEED into the backup scripts.
> Ok, Stefan would you be happy with it?

I'm happy with it.

But I'm a little frustrated that I've asked multiple times for a
concrete benchmark and it hasn't been provided.

In order to review patches that improve performance, we need a benchmark
that can be reproduced.  And the commit description must include results
that quantify the improvement.

Otherwise it's too easy to merge patches that sound reasonable but don't
have the effect that everyone thought they had.

I see two possible benchmarks:

1. Create memory pressure so that a benchmark performs worse unless we
   slim down our page cache usage.

2. Compare mm statistics before and after qemu-img to prove no longer
   bloats the page cache.

#2 doesn't prove that there is a practical performance issue, it just
optimizes the mm statistics.  But at least it quantifies that and serves
as a test case.

Either would be okay.

Stefan