[10] RFR(S): 8185572: Enable AssumeMP by default on SPARC machines

[Poonam Parhar]

We have customer reports of crashes with 8u on T7 sparc systems. So the change needs to be made in 8, and possibly in 7 too.

Thanks,
Poonam

> -----Original Message-----
> From: Bob Vandette
> Sent: Thursday, August 10, 2017 12:14 PM
> To: Vladimir Kozlov
> Cc: hotspot-compiler-dev at openjdk.java.net; Poonam Bajaj Parhar;
> hotspot-runtime-dev at openjdk.java.net runtime
> Subject: Re: [10] RFR(S): 8185572: Enable AssumeMP by default on SPARC
> machines
> 
> I don’t think that we should backport it to JDK 7 and 8 since older
> single CPU systems may get a security update for these older releases
> and see a performance regression.
> Perhaps JDK9 and 10 would be ok.
> 
> Bob.
> 
> 
> > On Aug 10, 2017, at 3:10 PM, Vladimir Kozlov
> <vladimir.kozlov at oracle.com> wrote:
> >
> > Bob, we have JDK-8185062 for that:
> >
> > https://bugs.openjdk.java.net/browse/JDK-8185062
> >
> > IMHO this fix is intended for backports, should be simple and don't
> cause regression, for example on embedded platforms.
> >
> > But I am fine if runtime group think it is fine to enable it on all
> platforms in jdk 7, 8 and 9.
> >
> > I agree that due to problem with dynamic cpus configuration in
> containers it may be good to enable it on all platforms in previous
> releases too.
> >
> > Thanks,
> > Vladimir
> >
> > On 8/10/17 11:53 AM, Bob Vandette wrote:
> >> Can we just always run with AssumeMP true for all platforms these
> days?
> >> Surely single CPU systems are rare now.
> >> We might have issues with Docker containers that have a limit 1 CPU
> >> on a large mp system which may cause issues.
> >> Bob.
> >>> On Aug 10, 2017, at 2:23 PM, Vladimir Kozlov
> <vladimir.kozlov at oracle.com> wrote:
> >>>
> >>> Poonam,
> >>>
> >>> I mean to add a small (one or two sentences) comment to the code.
> Some thing like next but may better wording:
> >>>
> >>> +  if (FLAG_IS_DEFAULT(AssumeMP)) {
> >>> +    // BIS instructions require 'membar' instruction regardless
> number of CPU.
> >>> +    // Otherwise in virtualized/container environments which use
> only 1 cpu BIS instructions may produce incorrect results.
> >>> +    FLAG_SET_DEFAULT(AssumeMP, true);
> >>>
> >>> Thanks,
> >>> Vladimir
> >>>
> >>> On 8/10/17 10:44 AM, Poonam Parhar wrote:
> >>>> Thanks Vladimir.
> >>>> Since the SPARC machines are always multi-cores, we can safely set
> AssumeMP to true on these.
> >>>> Adding my comments from the previous mail here again for better
> readability:
> >>>> -------------------------------------
> >>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8185572: Enable
> >>>> AssumeMP by default on SPARC machines
> >>>> Webrev: http://cr.openjdk.java.net/~poonam/8185572/webrev.00/
> >>>> This change enables AssumeMP by default on SPARC machines. On
> Sparc T7, to finalize BIS instructions the server compiler needs to add
> a 'membar' instruction at the end. But the generation of 'membar' is
> guarded by os::is_MP(), and os::is_MP() returns false when there is a
> single cpu available on the system. Now, in virtualized/container
> environments, the number of processors allocated to a virtual machine
> can dynamically change during the application runtime. That could lead
> to incorrect generation of BIS instructions and can cause JVM crashes.
> Enabling AssumeMP makes is_MP() always return true on SPARC systems.
> >>>> In future, we may consider making generation of 'membar'
> unconditional with the enhancement request:
> https://bugs.openjdk.java.net/browse/JDK-8150715.
> >>>> Thanks,
> >>>> Poonam
> >>>>> -----Original Message-----
> >>>>> From: Vladimir Kozlov
> >>>>> Sent: Thursday, August 10, 2017 9:47 AM
> >>>>> To: Poonam Parhar; hotspot-compiler-dev at openjdk.java.net
> >>>>> Cc: hotspot-runtime-dev at openjdk.java.net runtime
> >>>>> Subject: Re: [10] RFR(S): 8185572: Enable AssumeMP by default on
> >>>>> SPARC machines
> >>>>>
> >>>>> CCing to Runtime.
> >>>>>
> >>>>> Can you add comment explaining why it set to true on SPARC?
> >>>>>
> >>>>> Thanks,
> >>>>> Vladimir
> >>>>>
> >>>>> On 8/10/17 6:26 AM, Poonam Parhar wrote:
> >>>>>> Hello,
> >>>>>>
> >>>>>> Please review this simple patch:
> >>>>>>
> >>>>>> Bug:_JDK-8185572_<https://bugs.openjdk.java.net/browse/JDK-
> >>>>> 8185572>:En
> >>>>>> able
> >>>>>> AssumeMP by default on SPARC machines
> >>>>>>
> >>>>>> Webrev:http://cr.openjdk.java.net/~poonam/8185572/webrev.00/
> >>>>>>
> >>>>>> This change enables AssumeMP by default on SPARC machines. On
> >>>>>> Sparc T7, to finalize BIS instructions the server compiler needs
> >>>>>> toadd a'membar'instruction at the end.But the generation
> >>>>>> of'membar'is guarded byos::is_MP(), andos::is_MP()returns false
> >>>>>> when there isa singlecpu available on the system.
> >>>>>> Now,invirtualized/containerenvironments, the number
> >>>>>> ofprocessorsallocated to a virtual machine can dynamically
> change
> >>>>>> during the application runtime.That could lead to incorrect
> >>>>> generation
> >>>>>> of BIS instructions and can cause JVM crashes.Enabling AssumeMP
> >>>>>> makes
> >>>>>> is_MP() always return true on SPARC systems.
> >>>>>>
> >>>>>> In future, we may consider makinggeneration
> >>>>>> of'membar'unconditional
> >>>>>> withtheenhancementrequest:_JDK-
> >>>>> 8150715_<https://bugs.openjdk.java.net/browse/JDK-8150715>.
> >>>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>> Poonam
> >>>>>>
>