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

[Stefan Karlsson]

On 05/29/2013 12:53 AM, John Cuthbertson wrote:
> Hi Per,
>
> Thanks for looking at the code. Replies inline...
>
> On 5/28/2013 1:15 AM, Per Lidén wrote:
>> Hi John,
>>
>> On 2013-05-25 00:19, 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.
>>
>> A few minor comments/questions:
>>
>> * How about calculating ClaimChunkSize based on number of buckets 
>> that fit into a cacheline? Looking at all the buckets you already got 
>> nicely cached seems tempting. Something like:
>>
>> ClaimChunkSize = X * DEFAULT_CACHE_LINE_SIZE / 
>> sizeof(HashtableBucket<mtSymbol>);
>>
>> Where e.g. X=3 would yield 24 buckets for a release build on x86_64. 
>> Doing 20 (two and a half cachelines) means you would skip buckets you 
>> most likely have nicely cached.
>
> I like this idea.

Don't you have to  cache align  the buckets array to get this to work well?

StefanK

>
>>
>> * There's a debug printout/comment leftover in 
>> possibly_parallel_oops_do().
>
> Oops. It was to verify the claiming and check the distribution.
>
>>
>> * It seems that the worker_id argument passed all the way to 
>> possibly_parallel_oops_do() could be removed, since it's never used.
>
> See my response to Stefan. This is the 3rd time I've had to do such a 
> change (for various reasons). I'll remove it this time but if I have 
> to add it for something else in the future I'll be more insistent that 
> it stays then.
>
>>
>> * Maybe factor out the inner loop and share that between 
>> possibly_parallel_oops_do() and oops_do()?
>
> See my response to Thomas.
>
> Thanks,
>
> JohnC