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

Yumin Qi yumin.qi at oracle.com
Tue Mar 26 14:18:17 PDT 2013


Thanks Jon.

Yumin
On 3/26/2013 2:16 PM, Jon Masamitsu wrote:
> Yumin,
>
> Looks good.  Thanks.
>
> Jon
>
> On 03/26/13 12:44, Yumin Qi wrote:
>> Sorry, copied wrong link. It is at same link.
>>
>> http://cr.openjdk.java.net/~minqi/2178143/
>>
>> Thanks
>> Yumin
>>
>> On 3/26/2013 12:42 PM, Daniel D. Daugherty wrote:
>>>
>>> On 3/26/13 1:17 PM, Yumin Qi wrote:
>>>> Hi, Jon
>>>>
>>>>   I made a change to the warning condition and move it after GC 
>>>> flags are set. I think at this time, ParallelGCThreads has a value 
>>>> != 0. Please take a look if it is right.  The This is the new webrev:
>>>>
>>>>   http://javaweb.us.oracle.com/~yqi/webrev/2178143/
>>>
>>> This webrev isn't accessible outside Oracle...
>>>
>>> Dan
>>>
>>>
>>>>
>>>>   Tested with platform of ncpus > 1 and no ParallelGCThreads set 
>>>> with turning on AssumeMP.  The warning is gone.
>>>>    Also tested with ncpus = 1 and the warning is on too.
>>>>
>>>> Thanks
>>>> Yumin
>>>>
>>>> On 3/26/2013 8:03 AM, Jon Masamitsu wrote:
>>>>> If I set AssumeMP on the command line and nothing else
>>>>> and am running on a platform where I get ParallelGCThreads > 1,
>>>>> am I going to see the warning?    If yes, that seems too intrusive.
>>>>> I can see users hearing about the flag and deciding to include
>>>>> it always just to be safe.
>>>>>
>>>>> Jon
>>>>>
>>>>> On 3/25/13 11:21 PM, Yumin Qi wrote:
>>>>>> You are right, thanks!  I misread the INCLUDE.
>>>>>> new web in the same link.
>>>>>>
>>>>>> Thanks
>>>>>> Yumin
>>>>>>
>>>>>> On 3/25/2013 11:00 PM, David Holmes wrote:
>>>>>>> On 26/03/2013 3:56 PM, Yumin Qi wrote:
>>>>>>>> David,
>>>>>>>>
>>>>>>>> On 3/25/2013 10:41 PM, David Holmes wrote:
>>>>>>>>> On 26/03/2013 3:30 PM, Yumin Qi wrote:
>>>>>>>>>> David and All,
>>>>>>>>>>
>>>>>>>>>> Changed as suggested by David.
>>>>>>>>>>
>>>>>>>>>> David, I did not move the warning into 
>>>>>>>>>> set_parallel_gc_flags() since
>>>>>>>>>> except for serial gc, all other GCs will check this flag, so 
>>>>>>>>>> I moved it
>>>>>>>>>> into
>>>>>>>>>> #if INCLUDE_ALL_GCS
>>>>>>>>>>
>>>>>>>>>> before call set GC flags.
>>>>>>>>>
>>>>>>>>> I didn't realize all the other GCs would utilize 
>>>>>>>>> ParallelGCThreads.
>>>>>>>>> Can we at least exclude it for serial GC case?
>>>>>>>>>
>>>>>>>>
>>>>>>>> Serial GC is set here:
>>>>>>>
>>>>>>> INCLUDE_ALL_GCS allows all GCs including Serial GC. Otherwise we 
>>>>>>> only have SerialGC. So you have excluded it for the case where 
>>>>>>> SerialGC is the only GC built into the VM, but not for the case 
>>>>>>> where all GCs are available and the user requests UseSerialGC.
>>>>>>>
>>>>>>> David
>>>>>>> -----
>>>>>>>
>>>>>>>
>>>>>>>> 3253 #if !INCLUDE_ALL_GCS
>>>>>>>> 3254   force_serial_gc();
>>>>>>>> 3255 #endif // INCLUDE_ALL_GCS
>>>>>>>>
>>>>>>>> So after it moved into
>>>>>>>> 3283 #if INCLUDE_ALL_GCS
>>>>>>>> 3284   if (AssumeMP && FLAG_IS_DEFAULT(ParallelGCThreads)) {
>>>>>>>> 3285     warning("If the number of processors is expected to 
>>>>>>>> increase
>>>>>>>> from one, then"
>>>>>>>> 3286             " you should configure the number of parallel GC
>>>>>>>> threads appropriately"
>>>>>>>> 3287             " using -XX:ParallelGCThreads=N");
>>>>>>>> 3288   }
>>>>>>>> 3289   // Set per-collector flags
>>>>>>>> 3290   if (UseParallelGC || UseParallelOldGC) {
>>>>>>>> 3291     set_parallel_gc_flags();
>>>>>>>> 3292   } else if (UseConcMarkSweepGC) { // should be done 
>>>>>>>> before ParNew
>>>>>>>> check below
>>>>>>>> 3293     set_cms_and_parnew_gc_flags();
>>>>>>>> 3294   } else if (UseParNewGC) {  // skipped if CMS is set above
>>>>>>>> 3295     set_parnew_gc_flags();
>>>>>>>> 3296   } else if (UseG1GC) {
>>>>>>>> 3297     set_g1_gc_flags();
>>>>>>>> 3298   }
>>>>>>>> 3299   check_deprecated_gcs();
>>>>>>>> 3300 #else // INCLUDE_ALL_GCS
>>>>>>>> 3301   assert(verify_serial_gc_flags(), "SerialGC unset");
>>>>>>>> 3302 #endif // INCLUDE_ALL_GCS
>>>>>>>>
>>>>>>>> It is excluded for serial GC.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>> Yumin
>>>>>>>>
>>>>>>>>> Otherwise looks good.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> David
>>>>>>>>>
>>>>>>>>>> new webrev: http://cr.openjdk.java.net/~minqi/2178143/
>>>>>>>>>>
>>>>>>>>>> Thanks
>>>>>>>>>> Yumin
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> 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.
>>>>>>>>>>>
>>>>>>>>>>> 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");
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> In globals.hpp:
>>>>>>>>>>>
>>>>>>>>>>> +   product(bool, AssumeMP, false,       \
>>>>>>>>>>> +           "Assume run on multiple processors always") \
>>>>>>>>>>>
>>>>>>>>>>> Was there any discussion on making this default to true 
>>>>>>>>>>> instead?
>>>>>>>>>>>
>>>>>>>>>>> Suggested re-wording:
>>>>>>>>>>>
>>>>>>>>>>>  "Instruct the VM to assume multiple processors are available"
>>>>>>>>>>>
>>>>>>>>>>> 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
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>



More information about the hotspot-gc-dev mailing list