> On Apr 9, 2015, at 2:23 PM, Clark Smith Cox III <clark....@apple.com>
> wrote:
>
>> On Apr 9, 2015, at 12:07, Carl Hoefs <newsli...@autonomy.caltech.edu>
>> wrote:
>>>
>>> I have a highly CPU-bound project that parallelizes well. A small case
>>> has
>>> about 5000 tasks that can execute concurrently (max is ~100,000). I
>>> would
>>> like to use GCD queues to take advantage of the 24 cores I have on my
>>> MacPro. Does GCD automatically limit the number of tasks that become
>>> current on its queues to the number of cores (hw.logicalcpu)?
>>
>> Yes.
>
> Well, it's not as simple as limiting to the number of cores. It will
> continue to spawn worker threads so long as there is spare overall system
> capacity (up to some internal limit, but that's not the number of cores).
>
> If there are other processes/threads doing work on your system, your
> process will use fewer threads than cores.
>
> If some of your tasks are blocked (due to I/O or locks or even sleeps),
> then you may have more tasks in flight than there are cores, because the
> blocked tasks don't consume CPU capacity. This can actually be a problem
> if you have tasks which are mixed I/O and computation, because GCD will
> over-subscribe the CPU. When the tasks unblock, there are more tasks
> wanting CPU than there are cores.
>
> As much as possible, use dispatch I/O or dispatch sources for I/O. Keep
> tasks segregated to doing either I/O or computation, not a mix. If you
> can't use dispatch I/O or sources, then try to self-limit the number of
> I/O tasks in flight at once (for example by submitting them to a serial
> queue or low-concurrency-limit NSOperationQueue per device). Once you do
> that, you should be safe to submit large numbers of CPU tasks and let GCD
> manage them.
Interesting point. That could present a problem, as I do some I/O within
the block. I'll have to try it out and see how it performs. Or perhaps
there is there a way to tell a GCD concurrent queue to limit itself to N
tasks?
On another note, I don't know why this is having trouble:
dispatch_apply(4950,
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH,
0),
^(size_t idx) { <--- error
});
Incompatible block pointer types passing 'int (^)(size_t)' to parameter of
type 'void (^)(size_t)'
-Carl
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