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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Message-ID: <71953119-06ff-0bb8-1879-09e24bf80446@redhat.com>
Date:   Tue, 5 Jan 2021 10:29:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>, Dave Hansen <dave.hansen@...el.com>
Cc:     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@...ck.org,
        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 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.

-- 
Thanks,

David / dhildenb