This is the mail archive of the
mailing list for the libstdc++ project.
Re: libstdc++/7445: poor performance of std::locale::classic() inmulti-threaded applications
Benjamin Kosnik wrote:
I still think the two problems I mentioned are still there even with
this version of the patch, namely
a) Two threads can enter the method at once, the first will get the
lock, set up _S_classic, etc, and release the lock, then the 2nd
will do exactly the same things again.
This can be fixed by checking _S_classic again after the auto lock
b) If a second thread enters while another is in the process of
up _S_classic, ie it has assigned _S_classic, but not initialised
c_locale, the 2nd thread will return the unconstructed c_locale
This can be fixed by setting _S_classic as the last thing in the
I think the location of the static mutex doesn't matter.
So, again, I think the entire routine needs to be changed to something
like this... (comments included to indicate reasoning...)
I'm not excited by this patch. There has got to be a better way. Loren,
can you commet on this? I should have just waited for you before I
checked this in anyway.
Fair enough. It was just a quick patch to fix the problems introduced by
moving the location of the locking. I agree that there is probably a better
way to do it though.
// libstdc++/7445 - Optimisation to avoid the auto-lock once the
// Initial creation has been done
static _STL_mutex_lock __lock __STL_MUTEX_INITIALIZER;
// In case multiple threads call locale::classic() at the same
// time with _S_classic unset, check if another thread might
>> // have already done the work
locale::_Impl* __tmp_S_classic = 0;
// 26 Standard facets, 2 references.
// One reference for _M_classic, one for _M_global
facet** f = new(&facet_vec) facet*[_GLIBCPP_NUM_FACETS];
for (size_t __i = 0; __i < _GLIBCPP_NUM_FACETS; ++__i)
f[__i] = 0;
__tmp_S_classic = new (&c_locale_impl) _Impl(f, 2, true);
new (&c_locale) locale(__tmp_S_classic);
// Set _S_classic last when we know we have done all the
_S_classic = _S_global = __tmp_S_classic;
// Just call destructor, so that locale_impl_c's memory is
// not deallocated via a call to delete.
_S_classic = _S_global = 0;
Might be "weak", should do the job though :-)
Andrew Pollard - Senior Software Engineer (APF)
Brooks-PRI Automation - Planning and Logistics Solutions
Email: Andrew.Pollard@brooks-pri.com - Tel: +44 (0)118 9215603