[
https://issues.apache.org/jira/browse/ARROW-1163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16242298#comment-16242298
]
Philipp Moritz commented on ARROW-1163:
---------------------------------------
Hey Lu Qi,
I have very limited experience with Java, here are some thoughts that are I
hope are helpful:
You can do zero copy reads in Java using an off-heap method like
http://xcorpion.tech/2016/09/10/It-s-all-about-buffers-zero-copy-mmap-and-Java-NIO/.
Given the data already lives in (in-memory) memory-mapped files, this might be
the best way to go forward here.
We would essentially define our own Tensor class and then use code like
https://github.com/apache/spark/tree/50ada2a4d31609b6c828158cad8e128c2f605b8d/common/unsafe/src/main/java/org/apache/spark/unsafe
(see for example
https://github.com/apache/spark/blob/50ada2a4d31609b6c828158cad8e128c2f605b8d/common/unsafe/src/main/java/org/apache/spark/unsafe/array/LongArray.java)
to access the data without copies.
Arrow already has a Tensor class in C++ that does similar things and the the
current Python serialization code uses that to read Tensors in a zero copy way
from the object store and expose them as numpy arrays to the user. On the Java
side I think not much is available yet for reading tensors; as a point to get
started, the code for parsing Tensor metadata is generated here:
https://github.com/apache/arrow/blob/82eea49b3eea6047f53478113ab3ff9a38f0d344/java/format/pom.xml#L108
If you look at the code for reading C++ Tensors, you should be able to get a
prototype of this working. I'm also cc'ing some of the people who have done
most work on the Java implementation for more input.
[~bryanc] [~siddteotia] [~jnadeau]
-- Philipp.
> [Plasma] Java client for Plasma
> -------------------------------
>
> Key: ARROW-1163
> URL: https://issues.apache.org/jira/browse/ARROW-1163
> Project: Apache Arrow
> Issue Type: New Feature
> Reporter: Philipp Moritz
>
> We should start thinking about how a Java client for plasma would look like.
> Given the focus of arrow to support Python, C++ and Java really well, it is
> the next important target after Python and C++.
> My preliminary thoughts on it are the following ones: We can either go with
> JNI and wrap the C++ client or (in my opinion preferable) write a pure Java
> client. It would communicate with the Plasma store via Java flatbuffers over
> sockets.
> It seems that the only thing blocking a pure Java client at the moment is the
> way we ship file descriptors for the memory mapped files between store and
> client (see the file fling.cc in the Plasma repo). We would need to get rid
> of that because there is no pure Java API that allows transferring file
> descriptors over a process boundary. So the way to transfer memory mapped
> files over process boundaries then is probably to use the file system and
> keep the memory mapped files in the file system instead of unlinking them
> immediately (as we do at the moment), so they can be opened by the client
> process via their path.
> The challenge in this case is how to clean the files up and make sure they
> are not lying around if the plasma store crashes. One option is to store the
> plasma store PID with the file (i.e. as part of the file name) and let the
> plasma store clean them up the next time it is started); maybe there is OS
> level support for temporary files we can reuse.
> I probably won't get to this for a while, so if anybody needs this or has
> free cycles, they should feel free to chime in. Also opinions on the design
> are appreciated!
> -- Philipp.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
