(using a thread per connection) to event driven servers (which are supposedly
oh so much faster.) He shows how you can CPS the thread
per connection server and end up with the event driven server.
That way you don't need to manually mangle your code in an ad hoc way.
Of course, as Peter points out, you can build cooperative multitasking in
Haskell using monads rather that face the pain of using Java like Demsky.
If I recall correctly, the main performance advantages came from avoiding
- thread creation overhead
- excessive context switching
- synchronization due to potential interruptions at inconvenient moments
I suspect Haskell threads are much lighter than Java threads, so
there may not be as large of a payoff.
I'm curious about the difficulties with the monadic cooperative threads.
(I assume Peter meant cooperative threads where a scheduler chooses the next
bit-o-code to run rather than coroutines where the yielder explicitly calls
some other coroutine.) Were the asynchronous IO primitives too ugly to
deal with cleanly, or was the performance gain too small to be worth while?
Regards,
Paul
On Tuesday, November 4, 2003, at 02:58 PM, Peter Simons wrote:
S Alexander Jacobson writes:
Is there a reasonably efficient Haskell httpd implementation around that uses poll/select?
There is a web server written in Haskell: HWS-WP -- or "Haskell Web Server with Plug-ins". You'll find it at:
http://sourceforge.net/forum/forum.php?forum_id=253134 http://cvs.sourceforge.net/viewcvs.py/haskell-libs/libs/hws-wp/
The following paper from Simon Marlow provides a detailed description of the server's architecture:
Writing High-Performance Server Applications in Haskell, Case Study: A Haskell Web Server
http://www.haskell.org/~simonmar/bib.html
Note that HWS-WP does not use poll(2) directly. Instead, it relies an Haskell's forkIO function to spawn concurrent threads of execution. These are _not_ necessarily system threads, though. GHC, for instance, does implement IO threads with poll() internally. Other compilers or run-time systems may use other techniques.
I have experimented with direct poll()-based scheduling in Haskell (using a CSP monad to implement co-routines) and honestly didn't find it to be worth the extra effort. Eventually, I threw all the code away and just used forkIO, like everybody else does.
Peter
P. S.: If you're interested in a poll()-based web server written in C++, though, let me know. I have some code I'm more than willing to share. The server speaks HTTP/1.1 and can deliver static pages. No dynamic content, though. But it _is_ fast. :-)
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