RFR: Trim the TLAB sizes to avoid wasteful retirement under TLAB races

[Roman Kennke]

Am 21.09.2017 um 09:31 schrieb Aleksey Shipilev:
> On 09/20/2017 09:17 PM, Aleksey Shipilev wrote:
>> http://cr.openjdk.java.net/~shade/shenandoah/tlab-race-trim/webrev.01/
>>
>> This implements TLAB trimming to avoid wasteful retirement. The rationale is given in the comment
>> right in the code. It does not seem to make allocation significantly more costly. To make it less
>> overheady, changed the counter update code to be called on new region issuance, not on TLAB issuance.
> Added more discussion:
>    http://cr.openjdk.java.net/~shade/shenandoah/tlab-race-trim/webrev.02/
>
> -Aleksey
>
The number 8 is hardwired. Does this make sense? Should we allow this to 
be configured, if only for our own experimentation?

Otherwise looks good to me.

Roman