On Sat, Oct 11, 2003 at 10:58:04PM +0200, Wolfgang Thaller wrote:You should also note that no Haskell implementation currently supports SMP; even when multiple kernel threads are used, there is a mutual exclusion lock on the Haskell heap, so a multithreaded Haskell program will use only one CPU on an SMP system. I hope my answer was useful...
That's a painful-sounding state of affairs, though not entirely unexpected. It would be interesting to hear of "BKL breakup" efforts for Haskell runtime systems, though anymore I'm totally ignorant of what the devil is going on in userspace except database-only syscalls.
I'm not sure I understand you. I found out that "BKL" refers to the "Big Kernel Lock" (in the Linux kernel), but I have no idea what "database-only syscalls" are.
The reason why we currently do not take advantage of SMP is that the Haskell Heap is a shared data structure which is modified whenever a thunk (an unevaluated expression) is evaluated. Using synchronisation primitives like pthread_mutex_lock for every evaluation of a thunk would be deadly for performance.
There is some old (1999) code in the GHC RTS that attempts this (using intel cmpxchg instructions for synchronisation), but it's currently "bitrotted" and I don't know how successful it was.
Cheers,
Wolfgang
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