On 28 November 2005 11:47, Joel Reymont wrote:

On Nov 28, 2005, at 11:32 AM, Simon Marlow wrote:

...

On Unix with -threaded, all blocking I/O using Handles or sockets goes via the I/O manager thread, which multiplexes all the blocked I/O requests into a single select() call. If you have 20k blocked socket reads, there will be 20k lightweight Haskell threads, and probably 2 or 3 real OS threads. Without -threaded the outcome is pretty much the same, except the runtime does the select().

I'm not well-versed on the differences between select in the I/O manager and the runtime. What is the difference? Should I just plug - threaded into my cabal file and not worry about it? How big of an impact is select() in the runtime?

When the runtime is doing select(), you'll get more calls to select() because the runtime calls it for every thread switch, whereas with -threaded select() is only called when something changes - we need to wait on a new descriptor, or I/O is available on one or more descriptors. So you should get better performance with -threaded for large numbers of descriptors. Also you might get a small amount of parallelism on a multi-processor because the select() will run on a different CPU. I'm aware that there are various better alternatives to select(), but we don't use any of them yet. Cheers, Simon