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

[Jon Masamitsu]

Mikael,

Thanks.  I'd be interested in seeing the instrumentation you
add.  Might be worth adding as an enhancement in a later
changeset.

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
>>
>