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Re: Dummy pthread functions in libc considered harmful
- From: Samuel Thibault <samuel dot thibault at ens-lyon dot org>
- To: Andreas Schwab <schwab at suse dot de>
- Cc: Rich Felker <dalias at libc dot org>, libc-alpha at sourceware dot org
- Date: Tue, 25 Aug 2015 13:41:04 +0200
- Subject: Re: Dummy pthread functions in libc considered harmful
- Authentication-results: sourceware.org; auth=none
- References: <mvmr3ms4sbj dot fsf at hawking dot suse dot de> <20150824153816 dot GC3210 at type dot bordeaux dot inria dot fr> <20150824162250 dot GD32742 at brightrain dot aerifal dot cx> <20150824162641 dot GH3210 at type dot bordeaux dot inria dot fr> <mvmbndv4zeh dot fsf at hawking dot suse dot de>
Andreas Schwab, le Tue 25 Aug 2015 09:09:58 +0200, a écrit :
> Samuel Thibault <samuel.thibault@ens-lyon.org> 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.
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.
Samuel