RFR: 2178143: VM crashes if the number of bound CPUs changed during runtime
Daniel D. Daugherty
daniel.daugherty at oracle.com
Tue Mar 26 19:42:13 UTC 2013
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
>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>
>>>
>>
>
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