Re: Efficient CPU usage with recursively parallelizable problem
Tomas Mikula wrote:
On Oct 18, 6:59 am, Dave Searles <sear...@hoombah.nurt.bt.uk> wrote:
Tomas Mikula wrote:
On Sat, 17 Oct 2009 08:10:05 -0700, Patricia Shanahan wrote:
Tomas Mikula wrote:
Hello!
I have a purely computational (no I/O) problem that can be
recursively split into concurrent tasks. Each task needs to wait
for its child tasks to complete. How would I implement this
efficiently, meaning to keep all the CPU cores busy, while
keeping the context-switching overhead low (i.e. maintain the
number of running threads roughly equal to the number of cores)?
...
Are you sure each task must wait, tying up a thread, for its
children? Have you considered dividing tasks up into active
computation phases, and treating each phase as a separate task?
You could then have a single manager thread that tracks
completion
of child tasks, and schedules completion tasks when their
preconditions are met.
Patricia
Patricia, thank you for your suggestion. I think my problem could
be
written this way with some effort. Though the suggested manager
thread would have to keep track of dependencies between tasks and
wait _actively_ for completion of subtasks.
Can't it sleep using wait() and be notify()d whenever any task is
completed?
Ah, yes, good point! Or it could wait on a BlockingQueue to become
non- empty and each completed task would insert itself into this
queue. (It would need some queue of completed tasks anyway.) But
this
may become a bottleneck on systems capable of _many_ simultaneous
threads - all would have to synchronize over this one queue.
I woundn't expect that to be a problem if the individual tasks were
large enough that the CPU spent running them is large compared to the
CPU spent doing scheduling overhead.
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