Huge pages on PPC64

[Thomas Stüfe]

Hi guys,

On Fri, Oct 28, 2016 at 9:39 AM, Volker Simonis <volker.simonis at gmail.com>
wrote:

> 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.


Just a quick pipe-in, req_addr=NULL means "attach to any address, no
preference", so this has nothing to do with attaching to low memory regions.

Cheers, Thomas


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
> >
>
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