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Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations |
Date: |
Mon, 25 Mar 2013 09:02:02 -0400 (EDT) |
> Maybe I should have explained the output more detailed. The percentages
> are added. 35.8% in the second last column means that
> 35.8% have a return value that is less than TARGET_PAGE_SIZE.
> This was meant to illustrate at how many 64-bit chunks you have
> to look to grab a certain percentage of non-zero pages.
Ok, I wrongly understood that many pages had 4088 zero bytes but
the last 8 were not zero. Now it's clearer, and more logical too. :)
> Looking e.g. at the third value it means that looking at the first
> three 64-bit chunks it will catch 34.0% of all pages.
> It turns out that the non-zeroness of a page can be detected looking
> at the first 256 or so bits and only a low
> percentage turns out to be non-zero at a later position. So after
> having checked the first chunks one by one
> there is no big penalty looking at the remaining chunks with the
> vectorized loop.
I think it makes most sense to unroll the first four non-vectorized
iterations, i.e. not use SSE and use three or four ifs. Either:
if (foo[0]) return 0;
if (foo[1]) return 8;
if (foo[2]) return 16;
if (foo[3]) return 24;
or
if (foo[0]) return 0;
if (foo[1] | foo[2] | foo[3]) return 8;
and then proceed on the remaining 4096-4*sizeof(long) bytes with
the vectorized loop. foo+4 is aligned for SIMD operations on both
32- and 64-bit machines, which makes this a nice choice.
Paolo
> Here is the distribution of return values for the Windows XP example:
>
> 25.62% 0.49% 7.86% 0.12% 0.15% 0.05% 0.05% 0.04% 0.05% 0.02% 0.03%
> 0.02% 0.03% 0.02% 0.02% 0.01% 0.03% 0.02% 0.01% 0.02% 0.02% 0.01%
> 0.02% 0.01% 0.01% 0.01% 0.01% 0.01% 0.02% 0.00% 0.01% 0.02% 0.03%
> 0.01% 0.01% 0.01% 0.01% 0.01% 0.01% 0.00% 0.00% 0.01% 0.07% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.01% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.01% 0.02% 0.00% 0.00% 0.00%
> 0.00% 0.01% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.02% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.03% 0.00% 0.00% 0.00%
> 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.02% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.01%
> 0.00% 0.00% 0.00% 0.02% 0.00% 0.00% 0.02% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.01% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01%
> 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
> 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 64.23%
>
> The last value is the percentage of return value of TARGET_PAGE_SIZE
> meaning the page is all zero.
>
> Peter
>
>
- [Qemu-devel] [PATCHv4 7/9] migration: do not sent zero pages in bulk stage, (continued)
- [Qemu-devel] [PATCHv4 7/9] migration: do not sent zero pages in bulk stage, Peter Lieven, 2013/03/22
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Paolo Bonzini, 2013/03/22
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/22
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Paolo Bonzini, 2013/03/22
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/23
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Paolo Bonzini, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Paolo Bonzini, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/25
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Peter Lieven, 2013/03/26
- Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations, Paolo Bonzini, 2013/03/26