Concurrency has been implemented in a development version of Yhc, but is not yet available in the standard tree.
Two of the biggest issues are detailed here, along with their solutions.
The first thing to note is that concurrency is implemented in Yhc by the interpretter running some instructions for one process, then some more instructions for the next process and so on.
This is rather than using one OS thread per Haskell process because all processes can access the global heap, so if using OS threads it would be necessary to either lock every heap access (incredibly slow) or use some of the truely nasty tricks that concurrent GHC uses.
1 Supporting Multiple Execution Stacks
The first problem, is that originally we only had one stack but now every process needs its own stack. After considering possible options I decided the easiest way was:
- process stacks become normal nodes in the heap, and are garbage collected like other heap nodes. This preserves the property that we can't run out of heap but not stack (and vice versa).
- a process is initially allocated a small stack space in the heap and if that runs out we simply allocate a new stack space (twice as large as before) and let the old stack be garbage collected.
- I considered using stacks spread out over multiple 'pages', but in practice that I found that
- it's really hard to implement correctly, especially when you consider garbage collection can shift all your addresses around (don't forget frames on the stack reference the previous frame, possibly in a different stack page!)
- most stacks don't grow very large so there's little penalty to just throwing an old stack away if it gets too small.
- it would also be easy to recycle old stacks i.e. perhaps one process's discarded stack would be the right size for another process. This would be especially true if stacks came in standard sizes (64,128,256 etc). I haven't implemented this yet.
2 Blocking I/O and FFI calls
Now going back, the approach of running some instructions from one thread then some more from another is fine providing every instruction runs quickly. They all do, with one exception, PRIMITIVE. This is because PRIMITIVE is used for IO actions/FFI calls.
The solution? Well the basic idea is "just spawn an OS thread to do the IO action while the main thread continues running Haskell".
In detail, when PRIMITIVE is called for an IO action/FFI call:
- an OS thread is taken from a pool of threads
- the main thread tells the OS thread to run the IO action/FFI call
- the main thread then reschedules so another haskell process can run
- eventually the OS thread complete it's action and is put back in the pool. It adds the process to the list of processes that have completed their IO actions and are waiting to be rescheduled.
- when the main thread next reschedules it checks the list of completed IO actions and makes the corresponding blocked processes ready to run again.
There is a simple optimisation used which is if there is only one ready process in the program then it doesn't bother using a seperate thread, it just does the IO action directly.