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

[Vladimir Kozlov]

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
>>>>>
>