[jboss-jira] [JBoss JIRA] (JGRP-815) Scatter/Gather to avoid copying

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13105960#comment-13105960 ] 

Bela Ban commented on JGRP-815:
-------------------------------

Correction: we can connect a *sender's* multicast socket to the mcast_addr, e.g. 
{code:java}
DatagramChannel ch=DatagramChannel.open(StandardProtocolFamily.INET)
          .setOption(StandardSocketOptions.SO_REUSEADDR, true)
          .bind(new InetSocketAddress(7500))
          .setOption(StandardSocketOptions.IP_MULTICAST_IF, ni);
SocketAddress mcast_addr=new InetSocketAddress(addr, 7500);
ch.connect(mcast_addr);
{code}

This allows us to use _gathering writes_ to send multiple buffers in one go. On the receiver side, we cannot connect the sockets, as we don't know the IP addresses of the senders and we'll have multiple senders sending data, so scattering reads cannot be used there. 
This shouldn't be an issue though, as we may not need scattering reads on the receiver side.
See attached programs for an example: bla10 is the receiver and bla9 the sender.


> Scatter/Gather to avoid copying
> -------------------------------
>
>                 Key: JGRP-815
>                 URL: https://issues.jboss.org/browse/JGRP-815
>             Project: JGroups
>          Issue Type: Sub-task
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 4.0
>
>
> When we invoke Channel.send(), we pass a bufffer to JGroups. At the transport level, JGroups marshals the sender and destination address, plus all headers and the buffer into a new byte[] buffer, which is then passed to the socket (DatagramSocket, MulticastSocket, Socket).
> We cannot do gathering writes on a DatagramSocket because DatagramSocket doesn't expose this functionality, contrary to a DatagramChannel.
> We could avoid having to copy the user's buffer by using gathering writes: effectively passing to the socket NIO ByteBuffers containing:
> 1: Src and dest address plus flags, plus possibly size
> 2: The marshalled headers
> 3: The buffer passed to JGroups by the user
> We can obtain a gathering-write channel as follows:
> ByteBuffer[] buffers; // contains the 3 byte buffers above
> DatagramSocket sock;
> DatagramChannel ch=sock.getChannel();
> ch.write(buffers, 0, length); // length is the number of bytes of the total marshalled message
> This is supported by a GatheringByteChannel.
> I don't think there's currently a need to do scattered reads, but this needs to get investigated more. Also investigate whether MulticastSockets support gathering writes (whether they expose the correct DatagramChannel).



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