[Interest] QSqlDatabase and Multithreading

[Thiago Macieira]

On quarta-feira, 22 de janeiro de 2014 22:01:39, Philipp Kursawe wrote:
> sorry got that mixed up. Qt uses Events under Windows (was that changed in
> Qt5?), which is still not as efficient as CriticalSections.

Yes, QMutex got a large rewrite in Qt 5.

I'd like to see your benchmarks. This is what the one in 
tests/benchmarks/corelib/thread/qmutex say:

Non-recursive mutex:
- QMutex construction time and CriticalSection construction time are very 
efficient. QMutex is 15.8% faster.

- QMutex performance with uncontended locks is very good too. QMutex is 29.8% 
faster

- for contended mutexes, QMutex also wins: 36.5% faster on the 1-mutex 
testcase, 49.2% faster on the 2-mutex case

Recursive mutex:
- QMutex construction is much worse, since it needs to allocate memory; 5.4x 
worse

- uncontended locks: QMutex is 48% slower

- contended locks: QMutex is between 100% to 500% slower

The recursive QMutex and the critical section objects are very much alike. The 
only reason I can think of for why QMutex is that much slower is due to the 
indirection and extra memory allocation.

For Qt 6, it might be a good idea to make QMutex bigger so it can store the 
fields for a recursive mutex without indirection and memory allocation.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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