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

Erik Österlund erik.osterlund at oracle.com
Thu Sep 20 10:23:26 UTC 2018


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
>
> /Per



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