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Re: [PATCH 2/2] meson: Set avx512f option to auto
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From: |
Richard Henderson |
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Subject: |
Re: [PATCH 2/2] meson: Set avx512f option to auto |
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Date: |
Fri, 16 Dec 2022 15:50:10 -0800 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.2 |
On 12/16/22 15:08, Paolo Bonzini wrote:
Because that's what configure used to do
(https://lists.nongnu.org/archive/html/qemu-devel/2022-02/msg00650.html).
<https://lists.nongnu.org/archive/html/qemu-devel/2022-02/msg00650.html).>..
Yeah, but I wondered if that was just a bug.
It can surely be changed but AVX512 is known to limit processor frequency. I am not sure
if the limitation is per core or extends to multiple cores, and it would be a pity if
guests were slowed down even further during migration.
Hmm. Should we simply remove it?
Especially after the bulk phase buffer_is_zero performance matters a lot less so you'd pay
the price of AVX512 for little gain. After the bulk phase it may even make sense to just
use SSE, since even AVX requires a voltage transition[1] from what I saw at
https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html
<https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html>.
Ouch, never heard of that.
I'm not going to worry about it, because glibc str* routines make the same choice to use
AVX2, as does TCG, so I can only imagine that for the most part we're continually in and
out of 256-bit avx mode.
Anyway, I'll drop this patch.
r~