Re: [Boost-users] Thread local storage

Subject: Re: [Boost-users] Thread local storage
From: Oliver Abert (abert_at_[hidden])
Date: 2009-04-20 10:15:15

Hi,

two or three weeks ago I reported a problem concerning very slow  
access to thread specific storage pointers on Mac OS in multithreaded  
environments. When using eight threads performance was reduced by  
about 80%. I have to say that I use more than a million calls to the  
thread specific storage per second, so you won't notice that problem  
with only a few calls.

In the meantime I have not found a clean solution, but I found the  
error, so the maintainer(s) or anybody more smart than me can think  
about a clean solution. The problem is caused in pthread/once.cpp in  
the method get_once_per_thread_epoch()

         boost::uintmax_t& get_once_per_thread_epoch()
         {
             BOOST_VERIFY(! 
pthread_once(&epoch_tss_key_flag,create_epoch_tss_key));
             void* data=pthread_getspecific(epoch_tss_key);
             if(!data)
             {
                 data=malloc(sizeof(boost::uintmax_t));
                 BOOST_VERIFY(!pthread_setspecific(epoch_tss_key,data));
                 *static_cast<boost::uintmax_t*>(data)=UINTMAX_C(~0);
             }
             return *static_cast<boost::uintmax_t*>(data);
         }

On Mac OS X the first BOOST_VERIFY causes a fully executed call to  
pthread_once each time, which in turn uses mutexes to lock something.  
This is however not the case on Windows and Linux, where the  
performance is as expected. My "solution" to this problem was to  
simply comment the line out. As far as I understand the usage of  
BOOST_VERIFY it is only an assertion and not required to run the code  
properly. This then gaves me identical performance on all three  
platforms. I also tried to used different compilers, as I was told  
Apple gcc 4.0.1 had a problem with statics... but results were the  
same with the Intel compiler.

As far as I understand the Boost license, I am allowed to patch boost  
and distribute the compiled dynamic link library with my own software.  
I further understand, that I do not need to also distribute the  
patched source code. Is that correct?

Best regards,

        Oliver