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

[Poonam Parhar]

Updated the webrev with comments:
http://cr.openjdk.java.net/~poonam/8185572/webrev.01/

Thanks,
Poonam


> -----Original Message-----
> From: Vladimir Kozlov
> Sent: Thursday, August 10, 2017 11:24 AM
> To: hotspot-compiler-dev at openjdk.java.net; Poonam Bajaj Parhar
> Cc: hotspot-runtime-dev at openjdk.java.net runtime
> Subject: Re: [10] RFR(S): 8185572: Enable AssumeMP by default on SPARC
> machines
> 
> 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
> >>>