Re: ghci and ghc -threaded [slowdown]

Malcolm Wallace wrote:

Simon Marlow wrote:

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Malcolm Wallace wrote:

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For the only application I tried, using the threaded RTS imposes a 100% performance penalty - i.e. computation time doubles, compared to the non-threaded RTS. This was with ghc-6.8.2, and maybe the overhead has improved since then? This is a guess, but I wonder if this program is concurrent, and does a lot of communication between the main thread and other threads?

Exactly so - it hits the worst case behaviour. This was a naive attempt to parallelise an algorithm by shifting some work onto a spare processor. Unfortunately, there is a lot of communication to the main thread, because the work that was shifted elsewhere computes a large data structure in chunks, and passes those chunks back. The main thread then runs OpenGL calls using this data -- and I believe OpenGL calls must run in a bound thread.

This all suggests that one consequence of ghc's RTS implementation choices is that it will never be cheap to compute visualization data in parallel with rendering it in OpenGL. That would be a shame. This was exactly the parallelism I was hoping for.

I'm not sure how we could do any better here. To get parallelism you need to run the OpenGL thread and the worker thread on separate OS threads, which we do. So what aspect of the RTS design is preventing you from getting the parallelism you want? It seems that the problem you have is that moving to the multithreaded runtime imposes an overhead on the communication between your two threads, when run on a *single CPU*. But performance on a single CPU is not what you're interested in - you said you wanted parallelism, and for that you need multiple CPUs, and hence multiple OS threads. I suspect the underlying problem in your program is that the communication is synchronous. To get good parallelism you'll need to use asynchronous communication, otherwise even on multiple CPUs you'll see little parallelism. If you still do asynchronous communication and yet don't get good parallelism, then we should look into what's causing that. Cheers, Simon