RFR (XS): Optimize branch frequency of G1's write post-barrier in C2
dean.long at oracle.com
dean.long at oracle.com
Mon Aug 5 20:04:09 UTC 2019
Looks OK to me
On 8/3/19 12:27 PM, Thomas Schatzl wrote:
> ping at compiler team to have a quick look.
> On 11.07.19 16:35, Man Cao wrote:
>> Thanks Thomas for the review and running experiments!
>> > - can you share the code changes to generate the statistics? It would
>> > be nice to confirm these on a few more applications and play around
>> > with them a bit :)
>> > I would like to confirm some very old numbers we have for other older
>> > benchmarks that this is indeed the best probabibility distribution.
>> > Particularly I do not understand that from these numbers we did not
>> > change the probabilities as you suggested :( There were other changes
>> > mostly related to barrier elision in that time frame, but it seems
>> > likelihood changes were not attempted.
>> It is here: http://cr.openjdk.java.net/~manc/8225776/branch_profiling/
>> I also added a comment in
>> https://bugs.openjdk.java.net/browse/JDK-8225776 to clarify the
>> > - these numbers (and yours) also indicate that the not-young check is
>> > very likely to be not taken (i.e. you jump over the storeload).
>> Did you
>> > also perform some experiments changing the order a bit?
>> > It might be detrimental for this particular case where the
>> StoreLoad is
>> > expensive, and the xor/non-null filter out at least some
>> additional of
>> > those, but maybe
>> > if (young) -> exit
>> > if (different-region) -> exit
>> > if (non-null) -> exit
>> > StoreLoad
>> > ...
>> > may be better to do? I am aware that the "young" check adds a load,
>> > which is also expensive (but not as much as the StoreLoad), but it
>> > seems to be an interesting case to look at.
>> > In our old results (as far as I can interpret them) it did not
>> seem to
>> > have any advantage/disadvantage, so I am just curious whether you did
>> > such tests and their conclusion.
>> Yes, I did this experiment. The load from card table on the fast path
>> turns out to be expensive for several benchmarks:
>> For this experiment, I was setting 4G heap with -XX:NewRatio=1, so
>> most writes happen to young object, and GC happens very infrequently.
>> The implementation had some bug that some benchmarks crashed while
>> running. I didn't look into fixing the bug, as this direction does
>> not seem worthwhile.
>> > - internal (quick) perf testing showed no overall score changes,
>> > that maxJOPS on SpecJBB2015 seemed to improve by ~1.2% (only had time
>> > for very few experiments at this time, will rerun, so there is some
>> > chance that this has been a fluke) which is definitely nice.
>> Good to hear that!
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