RFR (L) 7133093: Improve system dictionary performance

[coleen.phillimore at oracle.com]

Hi, I have made changes for Ioi's comments below.  The largest change 
since .01 is that PredictedLoadedClassCount is used to size dictionaries 
for class loader data, and should be used to specify the average number 
of classes loaded by each class loader.  Since this is an option 
available via UnlockExperimentalVMOptions, I'm not sure what the process 
is for changing the meaning of this option. We will work with our 
customers who have used this option for specifying the total number of 
classes loaded.   We are also working on the change to allow automatic 
resizing of the dictionaries during safepoints.

See:
open webrev at http://cr.openjdk.java.net/~coleenp/7133093.02/webrev

Tested with the full nightly test set (jdk, nsk, hotspot jtreg tests) on 
linux x64.

Thanks,
Coleen

On 6/30/17 4:45 PM, coleen.phillimore at oracle.com wrote:
>
> Ioi,  Thank you for looking at this change.
>
> On 6/30/17 2:39 PM, Ioi Lam wrote:
>> Hi Coleen,
>>
>> Maybe the bug should be renamed to "Use one Dictionary per class 
>> loader instance"? That way it's more obvious what it is when you look 
>> at the repo history.
>
> I can do that.  I made it One Dictionary per ClassLoaderData
>>
>> 1.
>>
>> Why is assert_locked_or_safepoint(SystemDictionary_lock) necessary in 
>> SystemDictionary::find_class (line 1826), but not necessary 
>> SystemDictionary::find (line 951)? Since you removed 
>> NoSafepointVerifier nosafepoint in the latter, maybe this means it's 
>> safe to remove the assert_locked_or_safepoint in the former?
>
> The call to SystemDictionary::find() is the (I believe) usual lock 
> free lookup and the SystemDictionary::find_class() is used to verify 
> that a class we're about to add or want to add hasn't been added by 
> another thread.   Or certain cases where we already have a lock to do 
> something else, like add loader constraints.  I took out the 
> NoSafepointVerifier because it assumes that system dictionary entries 
> would be moved by GC, which they won't.  The old hash function could 
> safepoint when getting the hash for the class_loader object.
>
>>
>> 2.
>>
>>  455 static ClassLoaderData* _current_loader_data = NULL;
>>  456 static Klass*           _current_class_entry = NULL;
>>  457
>>  458 InstanceKlass* ClassLoaderDataGraph::try_get_next_class() {
>>
>> How about moving the static fields into an iterator object. That way 
>> you don't need to keep track of the globals
>>
>> ClassLoaderDataGraphIterator {
>>    ClassLoaderData* _current_loader_data
>>    Klass*           _current_class_entry;
>>
>>    InstanceKlass* try_get_next_class() { ....}
>> };
>
> Ok, there's a different iterator that iterates over all of the classes 
> for GC.  I will adapt that for this use.   That would be better.
>
>>
>> 3. Double check locking in ClassLoaderData::dictionary() -- someone 
>> else should look at this :-)
>
> I copied code that David Holmes added for modules.
>
>>
>> 4. We may need a better strategy for deciding the size of each 
>> dictionary.
>>
>>  565 const int  _primelist[10] = {1, 107, 1009};
>>  571 Dictionary* ClassLoaderData::dictionary() {
>>  579     if ((dictionary = _dictionary) == NULL) {
>>  580       int size;
>>  581       if (this == the_null_class_loader_data() || 
>> is_system_class_loader_data()) {
>>  582         size = _primelist[2];
>>  583       } else if 
>> (class_loader()->is_a(SystemDictionary::reflect_DelegatingClassLoader_klass())) 
>> {
>>  584         size = _primelist[0];
>>  585       } else {
>>  586         size = _primelist[1];
>>  587       }
>>  588       dictionary = new Dictionary(this, size);
>>
>> I'll do some investigation on this issue and get back to you.
>>
>
> How about if someone uses PredictedLoadedClassCount, then we use that 
> to size all but the reflection and boot class loader?  Then if there's 
> an application that has a class loader with a huge amount of classes 
> loaded in it, that would help this?  It might cost some footprint but 
> an oversized table would simply be a bigger array of pointers, so it 
> might not be that bad to oversize.
>
> I think the long term solution is being able to resize these entries 
> and make the application provide arguments.   Please let me know what 
> you find in your investigation and if that would work.
>> The rest of the changes look good to me.
>
> Thank you!
> Coleen
>>
>> Thanks
>> - Ioi
>>
>>
>> On 6/23/17 4:42 PM, coleen.phillimore at oracle.com wrote:
>>> Summary: Implement one dictionary per ClassLoaderData for faster 
>>> lookup and removal during class unloading
>>>
>>> See RFE for more details.
>>>
>>> open webrev at http://cr.openjdk.java.net/~coleenp/7133093.01/webrev
>>> bug link https://bugs.openjdk.java.net/browse/JDK-7133093
>>>
>>> Tested with full "nightly" run in rbt, plus locally class loading 
>>> and unloading tests:
>>>
>>> jtreg hotspot/test/runtime/ClassUnload
>>>
>>> jtreg hotspot/test/runtime/modules
>>>
>>> jtreg hotspot/test/gc/class_unloading
>>>
>>> make test-hotspot-closed-tonga FILTER=quick TEST_JOBS=4 
>>> TEST=vm.parallel_class_loading
>>>
>>> csh ~/testing/run_jck9  (vm/lang/java_lang)
>>>
>>> runThese -jck - uses class loader isolation to run each jck test and 
>>> unloads tests when done (at -gc:5 intervals)
>>>
>>>
>>> Thanks,
>>> Coleen
>>>
>>>
>>
>