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Date:   Mon, 4 Jan 2021 13:51:22 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Liang Li <liliang324@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Liang Li <liliangleo@didiglobal.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [RFC v2 PATCH 0/4] speed up page allocation for __GFP_ZERO
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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