RFR: 2178143: VM crashes if the number of bound CPUs changed during runtime

[Yumin Qi]

David,

On 3/25/2013 5:44 PM, David Holmes wrote:
> Hi Yumin,
>
> I have a few suggested changes.
>
> First in os.hpp I suggest
>
>  return _processor_count > 1 || AssumeMP ;
>
> That way we don't bypass the assertion and don't encounter unnecessary 
> overhead on the common path.
>
Yes, will change.
> In arguments.cpp:
>
> +   if (AssumeMP && FLAG_IS_DEFAULT(ParallelGCThreads)) {
> +     warning("With AssumeMP, recommend run with 
> -XX:ParallelGCThreads=<num_of_gc_threads>, where"
> +             "  num_of_gc_threads can be set to number of cores");
> +   }
>
> I'm not convinced issuing a warning is necessary or desirable here. 
> With a uniprocessor startup will we even default to using parallel GC? 
> The above should only be issued if using parallel GC - so better to 
> move it into set_parallel_gc_flags().
>
> In any case it isn't clear what the user should supply here. If they 
> use the maximum expected number of cores initially then while they 
> only have 1 core their VM may not even function adequately due to GC 
> overhead. I think all you can say here is something like:
>
> warning("If the number of processors is expected to increase from one, 
> then you should configure the number of parallel GC threads 
> appropriately using -XX:ParallelGCThreads=N");
>
I am giving a hint here to suggest about number of gc threads based on 
code. If customer want to use this flag, they should know it will affect 
GC. I will move this to set_parallel_gc_flags, which covers parnew and 
parallel. Thanks for pointing this, I did not notice serial gc not using 
ParallelGCThreads.
>
> In globals.hpp:
>
> +   product(bool, AssumeMP, false,       \
> +           "Assume run on multiple processors always")       \
>
> Was there any discussion on making this default to true instead?
>
No discussion for it. I like to have false as default because if (in 
fact rarely) user run on a single cpu machine, keep it as used to be is 
better.


> Suggested re-wording:
>
>  "Instruct the VM to assume multiple processors are available"
>

Thanks
Yumin
> Thanks,
> David
>
> On 26/03/2013 9:15 AM, Yumin Qi wrote:
>> It should be "AssumeMP".
>>
>> /Yumin
>>
>> On 3/25/2013 3:32 PM, Yumin Qi wrote:
>>> Hi,
>>>
>>>   New webrev to use "-XX:+AssumMP" flag. Also add warning to recommend
>>> use this flag with "-XX:ParallelGCThreads" to remind user to avoid
>>> running with only one GC thread. This is verified by Oleksandr with
>>> the test case running on Linux which is not Zone configured.
>>>
>>>   Same link.
>>>
>>> Thanks
>>> Yumin
>>>
>>> On 3/20/2013 2:27 PM, Yumin Qi wrote:
>>>>
>>>> 2178143:  VM crashes if the number of bound CPUs changed during 
>>>> runtime.
>>>>
>>>> Situation: Customer first configure only one CPU online and turn
>>>> others offline to run java application, after java program started,
>>>> bring more CPUs back online. Since VM started on a single CPU,
>>>> os::is_MP() will return false, but after more CPUs available, OS will
>>>> schedule the app run on multiple CPUs, this caused SEGV in various
>>>> places where data consistency was broken. The solution is supply a
>>>> flag to assume it is running on MP, so lock is forced to be called.
>>>>
>>>> http://cr.openjdk.java.net/~minqi/2178143/
>>>>
>>>> Thanks
>>>> Yumin
>>>
>>