[libdispatch-dev] Does GCD has a "global view" of all running processes?

Jordan K. Hubbard jkh at apple.com
Tue Sep 15 07:59:19 PDT 2009

On Sep 15, 2009, at 6:42 AM, Stéphane Letz wrote:

> After reading several things on GCD on the net, it is not yet clear  
> if GCD is mainly a "per process"  library, or is supposed to have a  
> "global view" of all processes, so that to allocate "not too much"  
> threads in each process, and avoid having a number of threads that  
> finally exceed the number of available cores on the machine.

"The net" is a notoriously unreliable source of information about,  
well, anything, but you're more than welcome to read the source code.   
It's available. :)

To answer your question in brief, it's both.  It's a per-process  
thread pool with cooperation from the kernel to determine,  
dynamically, how many threads should be in that pool given the overall  
load on the system.   The kernel provides the "global view" here  
through the pthread_workqueue interface and a compage variable which  
tells libdispatch when to start retiring threads (and what the new  
target should be) in order to deal with a resource depletion scenario.

- Jordan

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