Re: [RFC v2 PATCH 0/4] speed up page allocation for __GFP_ZERO

[Michal Hocko]

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