RFR(S) 8205921: Optimizing best-of-2 work stealing queue selection

[Zhengyu Gu]

On 07/05/2018 03:33 PM, Kim Barrett wrote:
>> On Jul 5, 2018, at 3:16 PM, Zhengyu Gu <zgu at redhat.com> wrote:
>>
>>>> Updated webrev:
>>>>
>>>> http://cr.openjdk.java.net/~zgu/8205921/webrev.01/index.html
>>> src/hotspot/share/gc/shared/taskqueue.inline.hpp
>>>   255     if (sz2 > sz1) {
>>>   256       sel_k = k2;
>>>   257       suc = _queues[k2]->pop_global(t);
>>>   258     } else {
>>>   259       sel_k = k1;
>>>   260       suc = _queues[k1]->pop_global(t);
>>>   261     }
>>> The paper avoids the steal attempt when both potential victims have a
>>> size of zero, e.g. insert another clause:
>>>    } else if (sz1 == 0) {
>>>      sel_k = k1;  // Might be needed to avoid uninitialized variable warnings?
>>>      suc = false;
>>>    } else {
>>>      ...
>>> There is a race condition between obtaining the size and checking it
>>> here, but I don't think that's important.  The point is to avoid an
>>> expensive steal attempt when it is very likely to fail.
>>
>> Yes, I missed this.
>>
>> http://cr.openjdk.java.net/~zgu/8205921/webrev.02/index.html
>>
>> Thanks,
>>
>> -Zhengyu
> 
> I think that makes the change accurately reflect the paper.
> 
> Just one minor nit: extraneous whitespace in “0 )”:
>   258     } else if (sz1 > 0 ) {
> 
I will fix it before push.

Thanks a lot!

-Zhengyu

>