RFR: 8210857: Allow retiring TLABs and collecting statistics in parallel

Stefan Johansson stefan.johansson at oracle.com
Thu Sep 20 12:08:52 UTC 2018

On 2018-09-20 12:21, Per Liden wrote:
> Thanks Erik!
> /Per
> On 09/20/2018 12:23 PM, Erik Österlund wrote:
>> Hi Per,
>> Looks good.
>> Thanks,
>> /Erik
>> On 2018-09-20 12:04, Per Liden wrote:
>>> On 09/18/2018 10:16 PM, Per Liden wrote:
>>>> On 09/18/2018 10:13 PM, Per Liden wrote:
>>>>> There is currently no good way of retiring TLABs and collecting 
>>>>> their statistics in parallel. The current code for doing this also 
>>>>> traverses the thread list twice, which is unnecessary. I propose 
>>>>> that we adjust the TLAB API to allow this to be done in parallel, 
>>>>> in a single pass.
>>>>> A few notes on this patch:
>>>>> * The GlobalTLABStats class is restructured into the 
>>>>> ThreadLocalAllocStats class to allow for a more flexible way of 
>>>>> collected statistics when retiring TLABs (flexible in the sense 
>>>>> that it allows for parallel operations).
>>>>> * The TLAB API is slightly adjusted. The "make_parsable(bool 
>>>>> retire_tlab)" is broken into two functions, "make_parsable()" to 
>>>>> just make a TLAB parsable, and "retire(ThreadLocalAllocStats* 
>>>>> stats)" to retire and optionally collects statistics.
>>>>> * GCs are unaffected by this change, in the sense that they 
>>>>> continue to call CollectedHeap::ensure_parsability(). However, all 
>>>>> GCs will benefit from now doing a single pass over the thread list 
>>>>> instead of two to retire and collect stats.
>>>>> * This is the last patch in my series of TLAB related patches. The 
>>>>> remaining patches only touch ZGC to actually retire/resize/remap 
>>>>> TLABs in parallel.
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8210857
>>>>> Webrev: http://cr.openjdk.java.net/~pliden/8210857/webrev.0
>>>> Testing: Passed tier{1,2,3,4,5,6} on linux-x86_64 (product and debug)
>>> Discussed with Erik and Stefan off-line, which resulted in the 
>>> following adjustment. This adjustment actually fixes a long standing 
>>> bug we've had, when make_parsable(false) is called and ZeroTLAB is 
>>> true, in which case we actually retire the TLAB but failed to call 
>>> Thread::incr_allocated_bytes() to update the number of allocated bytes.
>>> Diff: http://cr.openjdk.java.net/~pliden/8210857/webrev.0vs1
>>> Full: http://cr.openjdk.java.net/~pliden/8210857/webrev.1
Nice cleanup, looks good.

>>> /Per

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