[Haskell-cafe] Re: Bound threads

Simon Marlow simonmar at microsoft.com
Wed Mar 2 08:48:48 EST 2005

On 01 March 2005 11:21, Marcin 'Qrczak' Kowalczyk wrote:

> Marcin 'Qrczak' Kowalczyk <qrczak at knm.org.pl> writes:
>> Why is the main thread bound?
> I can answer myself: if the main thread is unbound, the end of the
> program can be reached in a different OS thread, which may be a
> problem if we want to return cleanly to the calling code.
> I've now implemented a threaded runtime in my language Kogut, based
> on the design of Haskell. The main thread is bound. The thread which
> holds the capability performs I/O multiplexing itself, without a
> separate service thread.

We found that doing this was excessively complex (well, I thought so
anyway).  The problem is, when there is no other work to do but there
are outstanding I/O requests to wait for, the thread holding the
capability has to wait in select().  But then, another OS thread making
an external call into Haskell has to somehow indicate that it needs the
capability, and wake up the thread blocked in select().  It can do this
with a pipe, but then you still have the problem that each call-in
requires a context switch, which is sub-optimal if you're implementing a
library in Haskell to be called from another language.

> Producer/consumer ping-pong is 15 times slower between threads running
> on different OS threads than on two unbound threads.

You might also want to measure throughput of call-ins.


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