code review 6448132: runtime: use sched_getaffinity for runtime.NumCPU()

runtime: use sched_getaffinity for runtime.NumCPU() on Linux
        Fixes issue 3921.

[minux1, 2012-08-09 15:31:31]

Hello iant@golang.org (cc: golang-dev@googlegroups.com),

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

[iant, 2012-08-09 18:36:42]

LGTM

Thanks!

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux.c
File src/pkg/runtime/thread_linux.c (right):

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
src/pkg/runtime/thread_linux.c:83: extern runtime·sched_getaffinity(int32 pid,
int32 len, uintptr *buf);
Seems like both pid and len should be uintptr.

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
src/pkg/runtime/thread_linux.c:95: t = t - ((t >> 1) & 0x5555555555555555ULL);
It's a nice algorithm but of course in SSE4.2 there is a single machine
instruction for it.  Let's stick with this for now, though.

[minux1, 2012-08-10 02:05:38]

*** Submitted as http://code.google.com/p/go/source/detail?r=3532ab6bf5be ***

runtime: use sched_getaffinity for runtime.NumCPU() on Linux
        Fixes issue 3921.

R=iant
CC=golang-dev
http://codereview.appspot.com/6448132

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux.c
File src/pkg/runtime/thread_linux.c (right):

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
src/pkg/runtime/thread_linux.c:83: extern runtime·sched_getaffinity(int32 pid,
int32 len, uintptr *buf);
On 2012/08/09 18:36:42, iant wrote:
> Seems like both pid and len should be uintptr.
fixed.

http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
src/pkg/runtime/thread_linux.c:95: t = t - ((t >> 1) & 0x5555555555555555ULL);
On 2012/08/09 18:36:42, iant wrote:
> It's a nice algorithm but of course in SSE4.2 there is a single machine
> instruction for it.  Let's stick with this for now, though.
yes, the popcnt instruction. but i think this is not that performance
critical (besides, i will have to write 3 implementations if i want
to take advantage of popcnt)

[dave_cheney.net, 2012-08-10 02:11:04]

Very nice.

On Fri, Aug 10, 2012 at 12:05 PM,  <minux.ma@gmail.com> wrote:
> *** Submitted as
> http://code.google.com/p/go/source/detail?r=3532ab6bf5be ***
>
> runtime: use sched_getaffinity for runtime.NumCPU() on Linux
>         Fixes issue 3921.
>
> R=iant
> CC=golang-dev
> http://codereview.appspot.com/6448132
>
>
>
>
>
http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux.c
> File src/pkg/runtime/thread_linux.c (right):
>
>
http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
> src/pkg/runtime/thread_linux.c:83: extern
> runtime·sched_getaffinity(int32 pid, int32 len, uintptr *buf);
> On 2012/08/09 18:36:42, iant wrote:
>>
>> Seems like both pid and len should be uintptr.
>
> fixed.
>
>
>
http://codereview.appspot.com/6448132/diff/6001/src/pkg/runtime/thread_linux....
> src/pkg/runtime/thread_linux.c:95: t = t - ((t >> 1) &
> 0x5555555555555555ULL);
> On 2012/08/09 18:36:42, iant wrote:
>>
>> It's a nice algorithm but of course in SSE4.2 there is a single
>
> machine
>>
>> instruction for it.  Let's stick with this for now, though.
>
> yes, the popcnt instruction. but i think this is not that performance
> critical (besides, i will have to write 3 implementations if i want
> to take advantage of popcnt)
>
> http://codereview.appspot.com/6448132/