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

[Vladimir Kozlov]

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
>>>