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

[Bob Vandette]

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
>>>>>>