Supporting allocation of Java object heap on NV-DIMMS

[Kharbas, Kishor]

Hi Per,

We see application of this feature in large scale multi-JVM deployments to increase resource utilization. We could run daemon/service JVMs with heap on slower memory and thus free up DRAM for critical processes.
At Intel we have done internal experiments on large middleware applications and have seen benefit of this feature.

Thanks
Kishor

-----Original Message-----
From: Per Liden [mailto:per.liden at oracle.com] 
Sent: Monday, April 4, 2016 4:37 AM
To: Kharbas, Kishor; hotspot-gc-dev at openjdk.java.net
Subject: Re: Supporting allocation of Java object heap on NV-DIMMS

Hi,

On 2016-04-02 03:13, Kharbas, Kishor wrote:
> Hello all,
>
> I would like to propose adding a capability in the JVM to allocate 
> object heap on new kinds of heterogeneous memories which are exposed 
> through a file system. This could be achieved by adding a command line 
> option to specify a path in the filesystem which would be used for
> mmap() system call.
>
> Details:
>
> Linux kernel 4.0 onwards supports DAX mode[1] in Ext4 file system [2] 
> which is designed to support new heterogeneous memory devices such as 
> Intel's 3D XPoint memory [3]. DAX mode removes the need for page cache 
> and provides a direct mapping of process virtual space to storage space.
>
> We would like to add a command line option called "FSDirForHeap" which 
> will - 1) Enable using a file system as backing store for Java object 
> heap 2) Use the provided path to create a temporary file for mmap() 
> system call.

The new kinds of memories opens up for some new and interesting possibilities. However, as long as DRAM is faster (which I assume they will continue be for at least some time), I have a hard time seeing that you want to place your whole heap onto the NV-DIMMs. I can see that some applications might want to place some large subset of their data there (e.g. an in memory-database), but I'm also guessing that placing the whole heap there would come with a noticeable impact on performance? Any thoughts on that?

Do you have any data to share with regards to performance difference on some relevant workloads? That would help in getting a better understanding of the feasibility of this mode of operation.

cheers,
Per

>
> The description of the feature can be found at
> https://bugs.openjdk.java.net/browse/JDK-8153327
>
> and the webrev is uploaded at
> http://cr.openjdk.java.net/~vdeshpande/8153327/webrev.00/
>
> References:
>
> [1] https://www.kernel.org/doc/Documentation/filesystems/dax.txt
>
> [2] https://www.kernel.org/doc/Documentation/filesystems/ext4.txt
>
> [3]
> https://newsroom.intel.com/press-kits/introducing-intel-optane-technol
> ogy-bringing-3d-xpoint-memory-to-storage-and-memory-products/
>
> Much appreciated!
>
> Thanks you.
>
> Kishor Kharbas
>