NUMA-Aware Java Heaps for in-memory databases

[Volker Simonis]

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


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