code review 7275049: runtime: limit number of OS threads

runtime: limit number of OS threads

Add a semaphore around entersyscall/exitsyscall to limit the maximum
number of OS threads to 256. The new runtime.MaxOSThreads func allows
the limit to be changed.

Fixes issue 4056.

[rick, 2013-02-04 13:30:20]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go

[rsc, 2013-02-05 04:01:07]

The system call entry and exit are performance sensitive. I'd like to think
about this a bit more to see if we can avoid the semacquire and semrelease.

Russ

[rick, 2013-02-09 05:44:29]

PTAL

I got rid of the semaphore. Does this version address your performance concerns?

Thanks.

[rsc, 2013-02-13 21:43:40]

Now the code has a race.

I think it would probably be better to leave this one alone for now. We'll
get to it with other scheduler changes later.

Russ

[rick, 2013-02-14 22:53:06]

On 2013/02/13 21:43:40, rsc wrote:
> Now the code has a race.
> 
> I think it would probably be better to leave this one alone for now. We'll
> get to it with other scheduler changes later.
> 
> Russ

No problem. I learned a bit about the runtime, which was my main goal.

Thanks.

[rick, 2013-02-14 22:53:29]

*** Abandoned ***