RFR: JDK-8214527 AArch64: ZGC for Aarch64

Stuart Monteith stuart.monteith at linaro.org
Tue May 21 13:44:14 UTC 2019

Hi Per,
   Thanks for taking a look at that.

I'll wait until ZGC on x86_64 isn't experimental. When that happens,
if aarch64 isn't ready for production use, I'd expect we'd make the
definition of "UseZGC" conditional, something like:

#if defined(AARCH64)
  #define USEZGC_COND(type, name, flag, descr)
  #define USEZGC_COND(type, name, flag, descr)

USEZGC_COND(bool, UseZGC, false,                          \
          "Use the Z garbage collector")                              \

That'll need more thought, but I'd like a general solution that could
be applied more widely if, indeed, that were even necessary.

I've done the refactoring of z_aarch64.ad as you suggested here:

Apart from the system calls, I agree with you about the deduplication.
I presume there is no expectation for there to be a SPARC linux port,
or other architectures were there might be a desire for a different


On Tue, 21 May 2019 at 12:05, Per Liden <per.liden at oracle.com> wrote:
> Hi Stuart,
> On 05/17/2019 04:28 PM, Stuart Monteith wrote:
> > Hello,
> >     This patch implements ZGC for AArch64. Unlike the previous set of
> > patches, this duplicates the x86 multimapping scheme rather than using
> > the Aarch64 TBI feature, where the top 8 bits are ignored by the
> > hardware. I didn't measure any conclusive performance problems using
> > multimapping. It would necessitate changes to SA, JNI, and wasn't
> > going to be compatible with Graal or JVMCI in general until they were
> > also able to switch to 64-bit literals dynamically. In addition, there
> > are up and coming features in the AArch64 architecture that exploit
> > the TBI area that we should keep open, such as MTE (Memory Tagging
> > Extensions) that open the door to lightweight hardware ASAN. With MTE
> > using 4 of the 8 bits, and ZGC and MTE perhaps needing more than 4
> > bits, future conflict looks likely.
> >
> > JTreg testing hasn't demonstrated any significant differences from x86
> > ZGC in my testing.
> >
> > The behaviour should be the same as is expected from x86_64, including
> > for the behaviour with command line arguments. Currently ZGC requires
> > -XX:+UnlockExperimentalVMOptions, that should be the case for AArch64
> > this release. If x86_64 were to do have ZGC designated as not
> > experimental, we must ensure it still applies for ZGC on AArch64.
> I don't think there's a good way to declare the a flag product on one
> platform, and experimental on another. But I guess
> zArguments::initialize_platform() on aarch64 could check for and
> warn/exit if UnlockExperimentalVMOptions is false.
> >
> > The associated bug is:
> >      https://bugs.openjdk.java.net/browse/JDK-8214527
> > The patchset is:
> >        http://cr.openjdk.java.net/~smonteith/8214527/webrev.0/
> src/hotspot/cpu/aarch64/gc/z/z_aarch64.ad
> -----------------------------------------
> I'd suggest you break out the duplicate parts of these into a function,
> like we have done in z_x86.ad.
> src/hotspot/os_cpu/linux_aarch64/gc/z/zBackingFile_linux_aarch64.cpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zBackingFile_linux_aarch64.hpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zBackingPath_linux_aarch64.cpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zBackingPath_linux_aarch64.hpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zGlobals_linux_aarch64.cpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zGlobals_linux_aarch64.hpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zPhysicalMemoryBacking_linux_aarch64.cpp
> src/hotspot/os_cpu/linux_aarch64/gc/z/zPhysicalMemoryBacking_linux_aarch64.hpp
> --------------------------------------------------------------------
> I'd would like to deduplicate most of these at some point, but I'm ok
> with doing that after this has been merged, as we might need/want to
> slice this slightly differently (to cater for the different syscall
> numbers etc).
> cheers,
> Per
> >
> > There is a patch contributed by Roland Westerlin required for fixing
> > memory barriers,
> >    http://cr.openjdk.java.net/~smonteith/zgc-mm/20190430/membar/
> > I'll follow up with a separate RFR patch to address that.
> >
> > Thanks,
> >     Stuart
> >

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