On Tue, Jan 15, 2013 at 06:03:30PM -0500, Trent Nelson wrote: > On Tue, Jan 15, 2013 at 02:33:41PM -0800, Ian Lepore wrote: > > On Tue, 2013-01-15 at 14:29 -0800, Alfred Perlstein wrote: > > > On 1/15/13 1:43 PM, Konstantin Belousov wrote: > > > > On Tue, Jan 15, 2013 at 04:35:14PM -0500, Trent Nelson wrote: > > > >> > > > >> Luckily it's for an open source project (Python), so recompilation > > > >> isn't a big deal. (I also check the intrinsic result versus the > > > >> syscall result during startup to verify the same ID is returned, > > > >> falling back to the syscall by default.) > > > > For you, may be. For your users, it definitely will be a problem. > > > > And worse, the problem will be blamed on the operating system and not > > > > to the broken application. > > > > > > > Anything we can do to avoid this would be best. > > > > > > The reason is that we are still dealing with an "optimization" that perl > > > did, it reached inside of the opaque struct FILE to "do nasty things". > > > Now it is very difficult for us to fix "struct FILE". > > > > > > We are still paying for this years later. > > > > > > Any way we can make this a supported interface? > > > > > > -Alfred > > > > Re-reading the original question, I've got to ask why pthread_self() > > isn't the right answer? The requirement wasn't "I need to know what the > > OS calls me" it was "I need a unique ID per thread within a process." > > The identity check is performed hundreds of times per second. The > overhead of (Py_MainThreadId == __readgsdword(0x48) ? A() : B()) is > negligible -- I can't say the same for a system/function call. > > (I'm experimenting with an idea I had to parallelize Python such > that it can exploit all cores without impeding the performance > of normal single-threaded execution (like previous-GIL-removal > attempts and STM). It's very promising so far -- presuming we > can get the current thread ID in a couple of instructions. If > not, single-threaded performance suffers too much.)
If the only thing you ever need is to get a unique handle for the current thread, without the requirement that it corresponds to any other identifier, everything becomes much easier. On amd64, use 'movq %fs:0,%register', on i386 'movl %gs:0,%register'. This instruction is guaranteed to return the thread-unique address of the tcb. See an article about ELF TLS for more details. Even better, this instruction is portable among all ELF Unixes which support TLS.
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