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Re: Dummy pthread functions in libc considered harmful

On Tue, 2015-08-25 at 13:41 +0200, Samuel Thibault wrote:
> Andreas Schwab, le Tue 25 Aug 2015 09:09:58 +0200, a Ãcrit :
> > Samuel Thibault <> writes:
> > > It's usually not programs which call pthread_mutex, but libraries which
> > > want to be thread-safe without actually bringing the libpthread
> > > dependencye.
> > 
> > Does the reason for avoiding the dependency still exist?  Surely the
> > overhead of libpthread has been greatly reduced since the days of
> > linuxthreads.
> The overhead of pthread_mutex_lock in the uncontended case has not
> really changed. E.g. in glibc 2.2 in 2002 it was already a mere compare
> and swap instruction after just checking the time of lock.

It has on archs such as x86 simply because the overhead of atomic HW
instructions has been reduced significantly.

> A quick dumb measurement of for() { mutex_lock(); mutex_unlock(); } on
> my laptop gives a 4x-5x factor between the empty hook without -lpthread
> and a non-contended actual mutex_lock call with -lpthread.

You should do at least something in the critical section to create a
somewhat realistic benchmark.

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