Re: [PATCH 2/2] meson: Set avx512f option to auto

[Paolo Bonzini]

Because that's what configure used to do (https://lists.nongnu.org/archive/html/qemu-devel/2022-02/msg00650.html)...

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.

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.

Paolo

[1] voltage transitions slow down the processor during the transition

Il dom 4 dic 2022, 02:51 Richard Henderson <richard.henderson@linaro.org> ha scritto:

I'm not sure why this option wasn't set the same as avx2.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 meson_options.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/meson_options.txt b/meson_options.txt
index 4b749ca549..f98ee101e2 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -102,7 +102,7 @@ option('membarrier', type: 'feature', value: 'disabled',

 option('avx2', type: 'feature', value: 'auto',
        description: 'AVX2 optimizations')
-option('avx512f', type: 'feature', value: 'disabled',
+option('avx512f', type: 'feature', value: 'auto',
        description: 'AVX512F optimizations')
 option('keyring', type: 'feature', value: 'auto',
        description: 'Linux keyring support')
--
2.34.1