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

[Christian Thalinger]

Since I'm messing around in the same code right now I took a look.
+volatile int StringTable::_par_claimed_idx = 0;
Can't we just say parallel instead of par?
+  for (int i = start_idx; i < end_idx; i += 1) {
The += 1 is perfectly sane but it looks odd :-)

-- Chris

On Jun 4, 2013, at 12:00 PM, John Cuthbertson <john.cuthbertson at oracle.com> wrote:

> Hi Everyone,
> 
> Here's a new webrev for this change: http://cr.openjdk.java.net/~johnc/8015237/webrev.1
> 
> 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.
> * Removed the additional worker ID parameter for the sake of peace and harmony.
> 
> 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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