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

[David Holmes]

On 11/08/2017 7:57 AM, David Holmes wrote:
> Hi Poonam,
> 
> On 11/08/2017 3:44 AM, Poonam Parhar wrote:
>> Thanks Vladimir.
>>
>> Since the SPARC machines are always multi-cores, we can safely set 
>> AssumeMP to true on these.
> 
> I'm still unclear about the reported problem here. As Vladimir pointed 
> out in the bug report the is_MP checks uses _processor_count which is 
> set to the number of cpus on the machine _not_ the number of cpus 
> currently available to the VM.
> 
> void os::Solaris::initialize_system_info() {
>    set_processor_count(sysconf(_SC_NPROCESSORS_CONF));
> 
> So all this discussion about containers and dynamic changes to available 
> cpus should be moot. So the only way this can fail is if the number of 
> configured processors on the machine dynamically changed _and_ it was 
> initially 1 - which seems to me to be impossible with sparc unless the 
> hardware info is being incorrectly reported (ie virtualization bug?)

No it isn't impossible as a LDOM can be configured with 1 "cpu", and 1 
is what sysconf(_SC_NPROCESSORS_CONF) will return.

A change to the number of cpus is not the issue here, but the simple 
fact that the BIS instructions have to have a membar issued, and that is 
elided when there is only 1 processor - unless AssumeMP is set true.

Sorry for the noise.

David

> That aside I have no issue with the fix as we will likely be assuming MP 
> always in the future.
> 
> Thanks,
> David
> 
>> 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
>>>>