VM warning: Performance bug

[Tom Rodriguez]

On Dec 15, 2011, at 1:10 PM, Tom Rodriguez wrote:

> 
> On Dec 15, 2011, at 1:05 PM, Karen Kinnear wrote:
> 
>> Actually I am working on a fix for it - see 7105270.
> 
> Great.  Will this be something that other subclasses of BasicHashtable can take advantage of?  The interned string table has similar issues that we've papered over.  Having more general support for rehashing would help.
> 
> tom
> 
>> 
>> It is "benign" in the sense of correct operation. However, we have an internal customer who has
>> seen measurable performance loss when they get this warning. So I am working on resizing the
>> system dictionary if the lookups take "too long".

Also I didn't mean to imply that it was always benign.  The message itself doesn't provide enough information to conclude anything since lots of different patterns could give you lookup statistics like that.  It would need to report some structural statistics around occupancy and chain length to make sense of those numbers.  Those particular numbers seem pretty benign though.

tom

>> 
>> If you have a way to duplicate it that doesn't require a database - that would make it easier for
>> me to debug.
>> 
>> thanks,
>> Karen
>> 
>> On Dec 15, 2011, at 3:38 PM, Tom Rodriguez wrote:
>> 
>>> 
>>> On Dec 15, 2011, at 12:02 PM, Kelly O'Hair wrote:
>>> 
>>>> 
>>>> Anyone seen this message before:
>>>> 
>>>> OpenJDK 64-Bit Server VM warning: Performance bug: SystemDictionary lookup_count=22915 lookup_length=24212 average=1.056600 load=0.528246
>>>> 
>>>> Maybe from jdk7u2?
>>> 
>>> I think it's fairly benign.  It just means that the hashtable for the SystemDictionary isn't performing quite as well as we'd like.  It's a little hard to evaluate without knowing how many entries are in the table.  I would assume that it just has a lot of classes loaded.  It's a somewhat useless message but it's debug only so I guess I wouldn't worry about it.
>>> 
>>> tom
>>> 
>>>> 
>>>> -kto
>>>> 
>>> 
>> 
>