NUMA-Aware Java Heaps for in-memory databases

[Remi Forax]

On 02/20/2013 02:28 PM, Volker Simonis wrote:
> The limitations you describe are the reason why databases implemented
> in Java usually use external memory (to a greater or lesser extent) to
> work around (mostly performance) problems of Java heap memory. There
> was a very interesting talk at JavaOne about how these problems are
> solved in Cassandra: "Dealing with JVM Limitations in Apache
> Cassandra" (https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=3586)
>
> Regards,
> Volker

but as far as I know, Cassandra only stores primitive values no object.

cheers,
Rémi

>
>
> On Wed, Feb 13, 2013 at 2:42 PM, Antoine Chambille <ach at quartetfs.com> wrote:
>> We are developing a Java in-memory analytical database (it's called
>> "ActivePivot") that our customers deploy on ever larger datasets. Some
>> ActivePivot instances are deployed on java heaps close to 1TB, on NUMA
>> servers (typically 4 Xeon processors and 4 NUMA nodes). This is becoming a
>> trend, and we are researching solutions to improve our performance on NUMA
>> configurations.
>>
>>
>> We understand that in the current state of things (and including JDK8) the
>> support for NUMA in hotspot is the following:
>> * The young generation heap layout can be NUMA-Aware (partitioned per NUMA
>> node, objects allocated in the same node than the running thread)
>> * The old generation heap layout is not optimized for NUMA (at best the old
>> generation is interleaved among nodes which at least makes memory accesses
>> somewhat uniform)
>> * The parallel garbage collector is NUMA optimized, the GC threads focusing
>> on objects in their node.
>>
>>
>> Yet activating -XX:+UseNUMA option has almost no impact on the performance
>> of our in-memory database. It is not surprising, the pattern for a database
>> is to load the data in the memory and then make queries on it. The data
>> goes and stays in the old generation, and it is read from there by queries.
>> Most memory accesses are in the old gen and most of those are not local.
>>
>> I guess there is a reason hotspot does not yet optimize the old generation
>> for NUMA. It must be very difficult to do it in the general case, when you
>> have no idea what thread from what node will read data and interleaving is.
>> But for an in-memory database this is frustrating because we know very well
>> which threads will access which piece of data. At least in ActivePivot data
>> structures are partitioned, partitions are each assigned a thread pool so
>> the threads that allocated the data in a partition are also the threads
>> that perform sub-queries on that partition. We are a few lines of code away
>> from binding thread pools to NUMA nodes, and if the garbage collector would
>> leave objects promoted to the old generation on their original NUMA node
>> memory accesses would be close to optimal.
>>
>> We have not been able to do that. But that being said I read an inspiring
>> 2005 article from Mustafa M. Tikir and Jeffrey K. Hollingsworth that did
>> experiment on NUMA layouts for the old generation. ("NUMA-aware Java heaps
>> for server applications"
>> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.92.6587&rep=rep1&type=pdf).
>> That motivated me to ask questions in this mailing list:
>>
>>
>> * Are there hidden or experimental hotspot options that allow NUMA-Aware
>> partitioning of the old generation?
>> * Do you know why there isn't much (visible, generally available) progress
>> on NUMA optimizations for the old gen? Is the Java in-memory database use
>> case considered a rare one?
>> * Maybe we at Quartet FS should experiment and even contribute new heap
>> layouts to the open-jdk project. Can you comment on the difficulty of that?
>>
>>
>> Thanks for reading, and Best Regards,
>>
>> --
>> Antoine CHAMBILLE
>> Director Research & Development
>> Quartet FS