RFR(m): 8214271: Fast primitive to wake many threads

[Robbin Ehn]

Hi David,

On 12/20/18 7:08 AM, David Holmes wrote:
> Hi Robbin,
> 
> Looks good, small doc follow up below ...

Thanks!

> Sound reasonable?

Yes, I also added the comment from my other mail, let me know what you think.

Inc:
http://cr.openjdk.java.net/~rehn/8214271/4/inc/webrev/

Full:
http://cr.openjdk.java.net/~rehn/8214271/4/full/webrev/

/Robbin

> 
> Otherwise this all looks good!
> 
> Thanks,
> David
> -----
> 
> 
>> Full:
>> http://cr.openjdk.java.net/~rehn/8214271/3/full/webrev/
>>
>> Thanks, Robbin
>>
>> On 11/23/18 5:55 PM, Robbin Ehn wrote:
>>> Forgot RFR in subject.
>>>
>>> /Robbin
>>>
>>> On 2018-11-23 17:51, Robbin Ehn wrote:
>>>> Hi all, please review.
>>>>
>>>> When a safepoint is ended we need a way to get back to 100% utilization as fast
>>>> as possible. 100% utilization means no idle cpu in the system if there is a
>>>> JavaThread that could be executed. The traditional ways to wake many, e.g.
>>>> semaphore, pthread_cond, is not implemented with a single syscall instead they
>>>> typical do one syscall per thread to wake.
>>>>
>>>> This change-set contains that primitive, the WaitBarrier, and a gtest for it.
>>>> No actual users, which is in coming patches.
>>>>
>>>> The WaitBarrier solves by doing a cooperative semaphore posting, threads woken
>>>> will also post. On Linux we can instead directly use a futex and with one
>>>> syscall wake all. Depending on how many threads and cpus the performance vary,
>>>> but a good utilization of the machine, just on the edge of saturated, the 
>>>> time to reach 100% utilization is around 3 times faster with the WaitBarrier 
>>>> (where futex is faster than semaphore).
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~rehn/8214271/webrev/
>>>>
>>>> CR:
>>>> https://bugs.openjdk.java.net/browse/JDK-8214271
>>>>
>>>> Passes 100 iterations of gtest on our platforms, both fastdebug and release.
>>>> And have been stable when used in safepoints (t1-8) (coming patches).
>>>>
>>>> Thanks, Robbin