Enabling use of hugepages with java

[David Holmes]

Hi Richard,

Moving to hotspot-dev as the appropriate list.

David

On 1/03/2018 1:20 AM, Richard Achmatowicz wrote:
> Hi
> 
> I hope that I am directing this question to the correct mailing list.
> 
> I have a question concerning the OS setup on Linux required for correct 
> use of the java option -XX:+UseLargePages in JDK 8.
> 
> Official Oracle documentation 
> (http://www.oracle.com/technetwork/java/javase/tech/largememory-jsp-137182.html) 
> suggests that in order to make use of large memory pages, in addition to 
> setting the flag -XX:+UseLargePages, an OS option shmmax needs to be 
> tuned to be larger than the java heap size.
> 
>  From looking at the java documentation, there are various ways of 
> enabling the use of huge pages: -XX:+UseHugeTLBFS, 
> -XX:+UseTransparentHugePages, -XX:+UseSHM and, if I understand 
> correctly, these correspond in part to making use of different OS-level 
> APIs for accessing huge pages (via shared memory, hugetlbfs, and other 
> means).
> 
> My question is this: is setting the shmmax OS value only relevant if we 
> are using -XX:+UseSHM? In other words, if we are using -XX:+UseHugeTLBFS 
> to enable use of hugepages by the JVM, is it the case that setting the 
> shmmax OS setting has no effect on the use of hugepages by the JVM?
> 
> Thanks in advance
> 
> Richard
>