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SubjectRe: [RFC v2 PATCH 0/4] speed up page allocation for __GFP_ZERO
On Tue 22-12-20 22:42:13, Liang Li wrote:
> > > =====================================================
> > > QEMU use 4K pages, THP is off
> > > round1 round2 round3
> > > w/o this patch: 23.5s 24.7s 24.6s
> > > w/ this patch: 10.2s 10.3s 11.2s
> > >
> > > QEMU use 4K pages, THP is on
> > > round1 round2 round3
> > > w/o this patch: 17.9s 14.8s 14.9s
> > > w/ this patch: 1.9s 1.8s 1.9s
> > > =====================================================
> >
> > The cost of zeroing pages has to be paid somewhere. You've successfully
> > moved it out of this path that you can measure. So now you've put it
> > somewhere that you're not measuring. Why is this a win?
>
> Win or not depends on its effect. For our case, it solves the issue
> that we faced, so it can be thought as a win for us. If others don't
> have the issue we faced, the result will be different, maybe they will
> be affected by the side effect of this feature. I think this is your
> concern behind the question. right? I will try to do more tests and
> provide more benchmark performance data.

Yes, zeroying memory does have a noticeable overhead but we cannot
simply allow tasks to spil over this overhead to all other users by
default. So if anything this would need to be an opt-in feature
configurable by administrator.
--
Michal Hocko
SUSE Labs

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