Reduced performance in Java 9.0.1 (vs 8u152)

[Andrew Haley]

On 02/01/18 09:37, Dawid Weiss wrote:
>> do with a different unrolling strategy which perhaps leads to more
>> spilling.  8u152 has a tighter inner loop. It's just one of those
>> things which happens sometimes with JIT compilers, IMO.  G1 is a red
>> herring.
> 
> I don't have enough knowledge of JIT internals to argue, but my
> experience and real-life software
> runs clearly show the difference Martin mentioned within a single JVM version.
> 
> JDK, GC, Time
> 8, g1, 3h 25m
> 8, par (default), 3h 0m
> 
> Again -- this is a particular result, but they're very repeatable and
> the average/ variance is definitely
> not accidental.

I'm only looking at the posted assembler code.  I can't see any
barriers.  However, the fact that there are barriers in the code we're
not looking at will affect register allocation, especially on a
machine as register starved as x86, so changing the GC will affect
even code which doesn't do anything GC related.  All it has to do is
use one more register, and that can cause a storm of spilling.

It would help a lot of someone posted the assembler code from a debug
build.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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