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

[Mikael Gerdin]

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
>