RFR(S): 8015237: Parallelize string table scanning during strong root processing

Stefan Karlsson stefan.karlsson at oracle.com
Wed Jun 5 11:10:14 UTC 2013


On 06/04/2013 09:00 PM, John Cuthbertson wrote:
> Hi Everyone,
>
> Here's a new webrev for this change: 
> http://cr.openjdk.java.net/~johnc/8015237/webrev.1

Looks good. Thanks for doing all the cleanups.

>
> Changes from before:
> * Refactored the code that loops over the buckets into its own routine.
> * Removed the commented out instrumentation (oops).
> * Changed the types to int to be consistent with the rest of 
> symbolTable and allow removal of the casts.
> * Increase the number of buckets per claim to 32 based upon a verbal 
> comment from John Coomes.

Care to describe the reasoning why 32 should be better?

> * Removed the additional worker ID parameter for the sake of peace and 
> harmony.

Thanks.

StefanK

>
> Testing: jprt.
>
> Thanks,
>
> JohnC
>
> On 5/24/2013 3:19 PM, John Cuthbertson wrote:
>> Hi Everyone,
>>
>> Can I have a couple of reviewers look over these changes - the webrev 
>> is: http://cr.openjdk.java.net/~johnc/8015237/webrev.0/
>>
>> Summary:
>> On some workloads we are seeing that the scan of the intern string 
>> table (among others) can sometimes take quite a while. This showed up 
>> on some FMW workloads with G1 where the scan of the string table 
>> dominated the pause time for some pauses. G1 was particularly 
>> affected since it doesn't do class unloading (and hence pruning of 
>> the string table) except at full GCs. The solution was to change the 
>> string table from being considered a single root task and treat 
>> similarly to the Java thread stacks: each GC worker claims a given 
>> number of buckets and scans the entries in those buckets.
>>
>> Testing
>> Kitchensink; jprt; GC test suite. With all collectors.
>>
>> Overhead:
>> Not real performance numbers but I did some measurement of the 
>> synchronization overhead of using 1 GC worker thread. They are 
>> summarized here:
>>
>>
>> 	0-threads (ms)
>> 	1-thread-chunked (ms)
>> Min 	0.200
>> 	0.300
>> Max 	6.900
>> 	8.800
>> Avg 	0.658
>> 	0.794
>>
>>
>> These were from 1 hour long runs of Kitchensink with around ~2800 GCs 
>> in each run.
>>
>> Thanks,
>>
>> JohnC
>

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