Review request: (turn off transparent huge pages) 8024838: Significant slowdown due to transparent huge pages

[Stefan Karlsson]

On 10/11/13 10:32 PM, Andrew Hughes wrote:
> ----- Original Message -----
>> Please review this change to turn off transparent huge pages by default.
>>
>> http://cr.openjdk.java.net/~stefank/8024838/webrev.00/
>>
>> 8024838: Significant slowdown due to transparent huge pages
>> Summary: Don't turn on transparent huge pages
>> (-XX:+UseTransparentHugePages) unless explicitly specified on the
>> command line. This has the effect that large pages are never turned on
>> Linux unless the user has explicitly enabled any of the large pages
>> flags: -XX:+UseLargePages, -XX:+UseTransparentHugePages,
>> -XX:+UseHugeTLBFS, and -XX:+UseSHM.
>> Reviewed-by: TBD1, TBD2
>>
>> thanks,
>> StefanK
>>
> Can you explain where you see this slowdown?

One of the issues are described in the bug:
https://bugs.openjdk.java.net/browse/JDK-8024838

There's another issues we're Nashorn runs 10% slower with CompressedOops 
turned on, most likely due to THP in the Compressed Class Space. That 
bug is marked Confidential, so I can't give you a link. =(

Another issue is the extra memory usage for small work loads. That bug 
is, apparently, also marked Confidential.

See also:
https://blogs.oracle.com/linux/entry/performance_issues_with_transparent_huge

>    I'm a little worried that
> this does not seem to have been questioned in this review.

Turning transparent huge pages on by default was done prematurely. If we 
do more performance evaluations in the future, this decision could be 
reevaluated.

If anyone still wants large pages they are free to use either of:
  -XX:+UseTransparentHugePages
  -XX:+UseHugeTLBFS
  -XX:+UseSHM

thanks,
StefanK
>
> Thanks,