Huge pages on PPC64

[Volker Simonis]

Hi Gustavo,

please find my comments inline:

On Thu, Oct 27, 2016 at 11:12 PM, Gustavo Romero
<gromero at linux.vnet.ibm.com> wrote:
> Hi Volker,
>
> On 25-10-2016 05:13, Volker Simonis wrote:
>> Just a quick question: do you also get the same warnings if you are
>> running without compressed oops (i.e. -XX:-UseCompressedClassPointers
>> -XX:-UseCompressedOops)? If not maybe this is related to the fact that
>> for compressed oops we try to allocate memory in low memory regions.
>
> $ java -XX:+PrintCommandLineFlags -Xms256m -Xmx256m -XX:+UseLargePages -XX:-UseCompressedClassPointers -XX:-UseCompressedOops StrictMath_cos
> -XX:InitialHeapSize=268435456 -XX:MaxHeapSize=268435456 -XX:+PrintCommandLineFlags -XX:-UseCompressedClassPointers -XX:-UseCompressedOops -XX:+UseLargePages -XX:+UseParallelGC
> OpenJDK 64-Bit Server VM warning: Failed to reserve large pages memory req_addr: 0x0000000000000000 bytes: 268435456 (errno = 16).
> 0.1709843554185943
>
> It fails when trying to allocate 256 MiB starting at 0x0, so definitely a low
> memory region. If CompressedClassPointers and CompressedOops are disabled, what
> could be trying to allocate that region?
>

Could you please run the VM in gdb to analyze where the allocation
fails and why?
You can also check that way that you really get large pages with -XX:+UseSHM.

Unfortunately I can't help you much here because I don't have a
machine with your 'huge pages' / 'transparent hug pages' setting. But
I know that setting up huge pages on Linux such that mmap() +
MAP_HUGETLB works is really tricky.

Regards,
Volker

> On the other hand if I use -XX:+UseSHM I see no warnings, so allocation by
> shmget() seems to work fine:
>
> $ java -XX:+PrintCommandLineFlags -Xms256m -Xmx256m -XX:+UseLargePages -XX:+UseSHM -XX:+UseCompressedClassPointers -XX:+UseCompressedOops StrictMath_cos
> -XX:InitialHeapSize=268435456 -XX:MaxHeapSize=268435456 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseLargePages -XX:+UseParallelGC -XX:+UseSHM
> 0.1709843554185943
>
> Could it be an indication that we have an issue with HugeTLBFS (i.e. with
> allocation by mmap() + MAP_HUGETLB flag) but not with UseTransparentHugePages
> and UseSHM?
>
> Thank you!
>
> Regards,
> Gustavo
>