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

[Bob Vandette]

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
>>>>