Getting the current thread ID without a syscall?

[Konstantin Belousov]

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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