Re: [Boost-users] Thread local storage

Subject: Re: [Boost-users] Thread local storage
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2009-04-20 11:34:57

AMDG

Oliver Abert wrote:
> 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.

The pthread_once call is necessary for the code to function correctly.
BOOST_VERIFY differs from BOOST_ASSERT in that it always
evaluates its argument.

> 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?

Yes.

In Christ,
Steven Watanabe