RFR(XS): JDK-8010941: MinJumpTableSize is set to 18, investigate if that's still optimal

Niclas Adlertz niclas.adlertz at oracle.com
Tue Sep 10 01:49:40 PDT 2013

On 2013-09-09 11:33, Aleksey Shipilev wrote:
> On 09/09/2013 01:16 PM, Niclas Adlertz wrote:
>> On 2013-09-06 14:02, Aleksey Shipilev wrote:
>>> On 09/06/2013 03:54 PM, Niclas Adlertz wrote:
>>>>> It is customary for us (perf guys) to create multiple
>>>>> @GMB methods with different problems sizes, and run them in single JMH
>>>>> session.
>>>> That would have been smarter, yes. I'll do that next time. Thanks.
>>>> And thank you for helping me with JMH.
>>> Sure, any time.
>>>>> Yes, "10" seems the turn-point for X64; for SPARC, I'd set it to "5".
>>>> Ok, I'll have one value for each platform instead.
>>>> WEBREV: http://cr.openjdk.java.net/~adlertz/JDK-8010941/webrev02/
>>> Good!
>>> I worry about the 32-bit x86 though. It seems the register pressure with
>>> jump tables is higher? If so, the turn-point for x86 can be even larger.
>>> Can you double-check the 32-bit x86 VM?
>>> -Aleksey.
>> Hi Aleksey.
>> I don't think the register pressure is higher using a jump table since
>> it's just a jump to an address with an offset.
>> In fact, tests seem to show the opposite; with a lookup table
>> (implemented as a binary search) fewer registers seem to have a negative
>> impact.
>> When we have 6 cases, it's actually faster to use a jump table on 32-bit
>> x86 instead of a lookup table (compared to 10 on x64).
> Thumbs up. Let's go with your change then.
> -Aleksey.

Thank you Aleksey.

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