Request for review: JDK-8009561 NPG: Metaspace fragmentation when retiring a Metachunk

[Mikael Gerdin]

Jon,


On 2013-06-06 16:50, Jon Masamitsu wrote:
> Mikael,
>
> Thanks.  I'd be interested in seeing the instrumentation you
> add.  Might be worth adding as an enhancement in a later
> changeset.

I did a 1hr KS run today with and without block splitting, here's what I 
came up with (in an entirely non-scientific way)

http://cr.openjdk.java.net/~mgerdin/8009561/splitting.txt
http://cr.openjdk.java.net/~mgerdin/8009561/splitting.png

We hit the HWM 4 times with splitting and 5 times without splitting.
On the other hand: splitting did lead us with more metaspace memory 
committed in the end.

I put up the very simple instrumentation at:
http://cr.openjdk.java.net/~mgerdin/8009561/instrument/webrev

I also changed the allocation_from_dictionary_limit to 4k to force us to 
make more freelist allocations.

/Mikael

>
> Jon
>
>
> On 6/6/13 2:22 AM, Mikael Gerdin wrote:
>> Jon,
>>
>> On 2013-06-06 04:41, Jon Masamitsu wrote:
>>>
>>> On 6/5/2013 7:04 AM, Mikael Gerdin wrote:
>>>> Hi,
>>>>
>>>> Can I have some reviews of this small fix to the Metaspace memory
>>>> allocation path.
>>>>
>>>> Problem:
>>>> When a Metaspace allocation request cannot be satisfied by the current
>>>> chunk the chunk is retired and a new chunk is requested. This causes
>>>> whatever is left in the chunk to be effectively leaked.
>>>>
>>>> Suggested fix:
>>>> Put the remaining memory in each chunk on the Metablock freelist so it
>>>> can be used to satisfy future allocations.
>>>>
>>>> Possible addition:
>>>> When allocating from the block free list, use
>>>> FreeBlockDictionary<Metablock>::atLeast instead of
>>>> FreeBlockDictionary<Metablock>::exactly and split the Metablock if
>>>> it's large enough.
>>>>
>>>> One might argue that this increases the fragmentation of the memory on
>>>> the block free list but I think that we primarily want to use the
>>>> block free list for small allocations and allocate from chunks for
>>>> large allocations.
>>>>
>>>> Webrev:
>>>> Only fix:
>>>> http://cr.openjdk.java.net/~mgerdin/8009561/webrev.0/
>>>
>>> The "Only fix" looks good.  Did you test with
>>> metaspace_slow_verify=true?
>>>
>>>>
>>>> Incremental webrev for splitting blocks:
>>>> http://cr.openjdk.java.net/~mgerdin/8009561/webrev.0%2b/
>>>
>>> Change looks good.
>>>
>>> Did you do any long running tests with the block splitting? Such as
>>> 24hours with kitchensink?  Something that would reuse Metablocks
>>> so that we can see if we are fragmenting instead of reusing?
>>>
>>
>> I did some runs earlier but I don't have any data from them.
>> I can try to get an instrumented build together and run KS over the
>> weekend.
>>
>> /Mikael
>>
>>> Jon
>>>
>>>
>>>>
>>>> Bug links:
>>>> https://jbs.oracle.com/bugs/browse/JDK-8009561
>>>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8009561
>>>>
>>>> Thanks
>>>> /Mikael
>>>
>>
>