linux-kernel - Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP

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Date:   Tue, 5 Jan 2021 17:39:27 +0800
From:   Liang Li <liliang324@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Liang Li <liliangleo@...iglobal.com>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page
 allocation for __GFP_ZERO

On Tue, Jan 5, 2021 at 5:30 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 05.01.21 10:20, Michal Hocko wrote:
> > On Mon 04-01-21 15:00:31, Dave Hansen wrote:
> >> On 1/4/21 12:11 PM, David Hildenbrand wrote:
> >>>> Yeah, it certainly can't be the default, but it *is* useful for
> >>>> thing where we know that there are no cache benefits to zeroing
> >>>> close to where the memory is allocated.
> >>>>
> >>>> The trick is opting into it somehow, either in a process or a VMA.
> >>>>
> >>> The patch set is mostly trying to optimize starting a new process. So
> >>> process/vma doesn‘t really work.
> >>
> >> Let's say you have a system-wide tunable that says: pre-zero pages and
> >> keep 10GB of them around.  Then, you opt-in a process to being allowed
> >> to dip into that pool with a process-wide flag or an madvise() call.
> >> You could even have the flag be inherited across execve() if you wanted
> >> to have helper apps be able to set the policy and access the pool like
> >> how numactl works.
> >
> > While possible, it sounds quite heavy weight to me. Page allocator would
> > have to somehow maintain those pre-zeroed pages. This pool will also
> > become a very scarce resource very soon because everybody just want to
> > run faster. So this would open many more interesting questions.
>
> Agreed.
>
> >
> > A global knob with all or nothing sounds like an easier to use and
> > maintain solution to me.
>
> I mean, that brings me back to my original suggestion: just use
> hugetlbfs and implement some sort of pre-zeroing there (worker thread,
> whatsoever). Most vfio users should already be better of using hugepages.
>
> It's a "pool of pages" already. Selected users use it. I really don't
> see a need to extend the buddy with something like that.
>

OK, since most people prefer hugetlbfs, I will send another revision for this.

Thanks
Liang