RFR(XL): 8167108 - SMR and JavaThread Lifecycle

Daniel D. Daugherty daniel.daugherty at oracle.com
Wed Nov 22 12:53:16 UTC 2017


On 11/22/17 4:07 AM, Erik Österlund wrote:
> Hi,
>
> Some replies...
>
> On 2017-11-21 17:28, Daniel D. Daugherty wrote:
>> Hi Coleen!
>>
>> Thanks for making time to review the Thread-SMR stuff again!!
>>
>> I have added back the other three OpenJDK aliases... This review is
>> being done on _four_ different OpenJDK aliases.
>>
>> As always, replies are embedded below...
>>
>>
>> On 11/20/17 3:12 PM, coleen.phillimore at oracle.com wrote:
>>> http://cr.openjdk.java.net/~dcubed/8167108-webrev/jdk10-09-full/src/hotspot/share/runtime/threadSMR.cpp.html 
>>>
>>>
>>>   32 ThreadsList::ThreadsList(int entries) : _length(entries), 
>>> _threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtGC)), 
>>> _next_list(NULL) {
>>>
>>> Seems like it should be mtThread rather than mtGC.
>>
>> Fixed. Definitely an artifact of Erik's original prototype when he
>> extracted Thread-SMR from his GC work... Thanks for catching it.
>>
>
> Confirmed. At the time I considered the Threads list overheads GC 
> overheads, but I agree mtThread is a better fit today.
>
>>
>>>
>>> + return (unsigned int)(((uint32_t)(uintptr_t)s1) * 2654435761u);
>>>
>>> Can you add a comment about where this number came from?
>>
>> I'll have to get that from Erik...
>
> Wow, that looks like code I wrote a *very* long time ago. :) That is a 
> variation of Knuth's multiplicative hash which is outlined in a 
> comment in synchronizer.cpp and referred to in that comment as a 
> phi-based scheme. Basically the magic number is 2^32 * Phi (the golden 
> ratio), which happens to be a useful value for building a reasonably 
> simple yet pretty good hash function. It is not the optimal hash 
> function, but seemed to definitely be good enough for our purposes.

So a reasonable comment would be:

// The literal value is 2^32 * Phi (the golden ratio).

If so, then I'll add it in my wrap up round...

Dan

>
> Thanks,
> /Erik



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