On segunda-feira, 21 de maio de 2012 08.34.32, bradley.hug...@nokia.com wrote: > On May 18, 2012, at 8:34 PM, ext Thiago Macieira wrote: > > Recommendations (priority): > > > > (P0) de-inline QBasicMutex locking functions until we solve some or all of > > the below problems > > I agree with this, so that it gives a chance to fix the performance > regressions on Mac at a later date (since it probably won't be fixed before > 5.0 is released).
Some notes from the IRC discussion this morning between Olivier, Brad and
myself:
* QMutex contended performance has dropped considerably on Mac from 4.8 to
5.0 (it's 10x slower)
* QMutex contended performance on Mac is now actually similar to the
pthread_mutex_t performance (read: contended QMutex on 4.8 is 10x faster than
pthread_mutex_t)
* changing the QMutex implementation to use the generic Unix codepath on Mac
makes it 2x slower
* the non-Linux code in QBasicMutex::lockInternal is considered complex and
hard to read by both Brad and myself
Brad: could you please provide what is, to the best of your knowledge today,
the combination of tricks that made 4.8 fast?
* QMutex de-inlining and the Mac performance issues are orthogonal.
* QMutex "de-inlining" should be understood more correctly as: removing the
testAndSet calls from the inline functions. The inline functions should remain
inline.
* The de-inlining is important for Valgrind (helgrind / DRD) to work
properly, even in release mode
Note that there's another trick that QMutex can apply under valgrind but
QBasicMutex cannot: if the QMutex constructor initialises the d pointer to
anything non-null and different from 3, the inlined testAndSet will fail, so
valgrind can properly hijack the lock and unlock functions.
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
Intel Sweden AB - Registration Number: 556189-6027
Knarrarnäsgatan 15, 164 40 Kista, Stockholm, Sweden
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