linux-kernel - Re: [RFC v2 PATCH 0/4] speed up page allocation for __GFP

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Date:   Tue, 22 Dec 2020 22:42:13 +0800
From:   Liang Li <liliang324@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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>,
        David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...e.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 0/4] speed up page allocation for __GFP_ZERO

> > =====================================================
> > 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.

> > Speed up kernel routine
> > =======================
> > This can’t be guaranteed because we don’t pre zero out all the free pages,
> > but is true for most case. It can help to speed up some important system
> > call just like fork, which will allocate zero pages for building page
> > table. And speed up the process of page fault, especially for huge page
> > fault. The POC of Hugetlb free page pre zero out has been done.
>
> Try kernbench with and without your patch.

OK. Thanks for your suggestion!

Liang