Fast sigblock (AKA rtld speedup)

[David Chisnall]

On 14 Jan 2013, at 17:47, Jilles Tjoelker wrote:

> The code which does that check is actually under contrib/gcc. Problem
> is, they designed __gthread_active_p() to distinguish threaded and
> unthreaded programming environments -- it must be known in advance and
> cannot be changed later. The code for the unthreaded environment then
> takes advantage of this by not even allocating memory for mutexes in
> some cases.

It's worth taking a step back and asking why this code exists at all, and the main reason is that acquiring a mutex used to be really expensive.  It still is on some fruit-flavoured operating systems, but elsewhere it's a single atomic operation in the uncontended case, and in that case the cache line will already be exclusively owned by the calling core in single-threaded code.  

I would much rather that we followed the example of Solaris and made the multithreaded case fast and the default than keep piling on hacks that allow code to shave off a few clock cycles in the single-threaded case.  In particular, the popularity of multicore systems means that it is increasingly rare for code to be both single threaded and performance critical, so this seems like misplaced optimisation.

I strongly suspect that making it possible to inline the uncontended lock case for a pthread mutex and eliminating all of the branches on __isthreaded would give us a net speedup in both single and multithreaded cases.

> This __gthread_active_p() thing is another barrier to bringing in a
> threaded plugin in an unthreaded application. Ports people spend a fair
> amount of time adding -pthread flags to things (such as perl) to work
> around this.


This and the similar checks in libc cause a lot of pain, and it seems that the correct fix is ensuring that the performance penalty for linking libthr is so small that there is no point in avoiding it.

David

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 841 bytes
Desc: Message signed with OpenPGP using GPGMail
URL: <http://lists.freebsd.org/pipermail/freebsd-arch/attachments/20130114/8e4a857b/attachment.sig>