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Re: [Qemu-devel] [PATCH v2 2/3] qed: add zero write detection support

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v2 2/3] qed: add zero write detection support
Date: Thu, 8 Dec 2011 15:54:43 +0000

On Thu, Dec 8, 2011 at 2:29 PM, Mark Wu <address@hidden> wrote:
> I tried to optimize the zero detecting code with SSE instruction.   The idea
> comes from Paolo's patch "migration: vectorize is_dup_page".  It's expected
> to give us an noticeable improvement. But I didn't find any improvement in
> the qemu-io test even though I increased the image size to 5GB.  The
> following is my test patch.  Could you please review it to see if I made any
> mistake and SSE can help for zero detecting?

Please put the zero detection function in a common location before
adding serious optimization so that qemu-img.c:is_not_zero() can also
use it.

Out of interest here is the code generated by gcc 4.6.2 from the non-SSE code:

    1d50:       89 c2                   mov    %eax,%edx
    1d52:       c1 fa 03                sar    $0x3,%edx
    1d55:       48 63 d2                movslq %edx,%rdx
    1d58:       48 83 3c d6 00          cmpq   $0x0,(%rsi,%rdx,8)
    1d5d:       0f 85 03 ff ff ff       jne    1c66 <qed_aio_write_data+0x146>
    1d63:       83 c0 08                add    $0x8,%eax
    1d66:       48 63 d0                movslq %eax,%rdx
    1d69:       48 39 d1                cmp    %rdx,%rcx
    1d6c:       77 e2                   ja     1d50 <qed_aio_write_data+0x230>

Once you have the zero detection code in a utility function it's easy
to write a small test program to run a performance benchmark.


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