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From:   Joel Fernandes <joelaf@google.com>
Date:   Wed, 30 Sep 2020 18:46:54 -0400
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Subject: Re: [PATCH 0/5] Speed up mremap on large regions
To:     Lokesh Gidra <lokeshgidra@google.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Kalesh Singh <kaleshsingh@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Minchan Kim <minchan@google.com>,
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        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
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On Wed, Sep 30, 2020 at 6:42 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> On Wed, Sep 30, 2020 at 3:32 PM Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Wed, Sep 30, 2020 at 10:21:17PM +0000, Kalesh Singh wrote:
> > > mremap time can be optimized by moving entries at the PMD/PUD level if
> > > the source and destination addresses are PMD/PUD-aligned and
> > > PMD/PUD-sized. Enable moving at the PMD and PUD levels on arm64 and
> > > x86. Other architectures where this type of move is supported and known to
> > > be safe can also opt-in to these optimizations by enabling HAVE_MOVE_PMD
> > > and HAVE_MOVE_PUD.
> > >
> > > Observed Performance Improvements for remapping a PUD-aligned 1GB-sized
> > > region on x86 and arm64:
> > >
> > >     - HAVE_MOVE_PMD is already enabled on x86 : N/A
> > >     - Enabling HAVE_MOVE_PUD on x86   : ~13x speed up
> > >
> > >     - Enabling HAVE_MOVE_PMD on arm64 : ~ 8x speed up
> > >     - Enabling HAVE_MOVE_PUD on arm64 : ~19x speed up
> > >
> > >           Altogether, HAVE_MOVE_PMD and HAVE_MOVE_PUD
> > >           give a total of ~150x speed up on arm64.
> >
> > Is there a *real* workload that benefit from HAVE_MOVE_PUD?
> >
> We have a Java garbage collector under development which requires
> moving physical pages of multi-gigabyte heap using mremap. During this
> move, the application threads have to be paused for correctness. It is
> critical to keep this pause as short as possible to avoid jitters
> during user interaction. This is where HAVE_MOVE_PUD will greatly
> help.

And that detail should totally have gone into the commit message :-/

Thanks,

 - Joel