FreeBSD src
ddab052725
Clang emits SSE instructions on amd64 in the common path of pthread_mutex_unlock. If the thread does not otherwise use SSE, this usage incurs a context-switch of the FPU/SSE state, which reduces the performance of multiple real-world applications by a non-trivial amount (3-5% in one application). Instead of this change, I experimented with eagerly switching the FPU state at context-switch time. This did not help. Most of the cost seems to be in the read/write of memory--as kib@ stated--and not in the #NM handling. I tested on machines with and without XSAVEOPT. One counter-argument to this change is that most applications already use SIMD, and the number of applications and amount of SIMD usage are only increasing. This is absolutely true. I agree that--in general and in principle--this change is in the wrong direction. However, there are applications that do not use enough SSE to offset the extra context-switch cost. SSE does not provide a clear benefit in the current libthr code with the current compiler, but it does provide a clear loss in some cases. Therefore, disabling SSE in libthr is a non-loss for most, and a gain for some. I refrained from disabling SSE in libc--as was suggested--because I can't make the above argument for libc. It provides a wide variety of code; each case should be analyzed separately. https://lists.freebsd.org/pipermail/freebsd-current/2015-March/055193.html Suggestions from: dim, jmg, rpaulo Approved by: kib (mentor) MFC after: 2 weeks Sponsored by: Dell Inc. |
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| crypto | ||
| etc | ||
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| gnu | ||
| include | ||
| kerberos5 | ||
| lib | ||
| libexec | ||
| release | ||
| rescue | ||
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| sys | ||
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| tools | ||
| usr.bin | ||
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| .arcconfig | ||
| .arclint | ||
| COPYRIGHT | ||
| LOCKS | ||
| MAINTAINERS | ||
| Makefile | ||
| Makefile.inc1 | ||
| ObsoleteFiles.inc | ||
| README | ||
| UPDATING | ||
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