Taking advantage of TCP Loopback fast path in Windows

[Michael McMahon]

On 24/09/14 08:50, Alan Bateman wrote:
> On 24/09/2014 02:19, Martin Sawicki (MS OPEN TECH) wrote:
>>
>> Hello
>>
>> We’re proposing an improvement to the OpenJDK which enables users to 
>> take advantage of the TCP loopback fast path mechanism in Windows for 
>> significantly higher performance of sockets whose both end points are 
>> on the same machine.  This is especially relevant in distributed 
>> server-side/cloud scenarios, such as Hadoop.
>>
>> We have the code figured out and tested internally. I’ve uploaded our 
>> webrev package here (too big to send as an attachment):
>>
>> https://openjdkcontrib.blob.core.windows.net/tcploopback/webrev-20140918.zip 
>>
>>
>> The crux of the change lies in enabling the SIO_LOOPBACK_FAST_PATH 
>> IOCTL flag on each socket creation call in Windows.
>>
>> As for activating this optimization, we’re proposing the conservative 
>> approach of keeping it off by default and adding a command line 
>> argument to activate it:
>>
>> -Dwindows.enableFastLocalTcpLoopback=true | false
>>
>> (IMHO though, it’d be great to eventually just have it on by default).
>>
>> We’d appreciate a review and acceptance of this improvement.
>>
>> And, as we are new in the OpenJDK community and this may as well be 
>> our first contribution to Java, I apologize for any steps in the 
>> submission process that I may have missed here and would appreciate 
>> guidance as needed.
>>
>>
> Welcome, it's good to see Microsoft engineers on OpenJDK lists.
>

Yes, this looks like a useful contribution.

> If I understand this ioctl correctly then it should only need to be 
> set once on a SOCKET. I'm curious about the listener oriented channels 
> (ServerSocketChannel etc.) where it is being set prior to each call to 
> accept. Is this needed? If not then I assume that we can just set it 
> when creating the SOCKET, sun.nio.ch.Net.socket0 of SocketChannel, 
> ServerSocketChannel, etc. You'll see that it already goes some Windows 
> specific setup and that might be the place to put it (and I think 
> should eliminate the need for most of the changes to the NIO code).
>

Right. The technet article suggests that it only needs to be set on the 
listening socket (ie prior to calling accept()). So, the same
comment applies to the DualStackPlainSocketImpl change (could be moved 
to socketListen() from socketAccept()).
The DualStack/TwoStacks code is being reorganised at the moment. So, I 
think the patch will need to be rebased
against that work. But, the idea seems pretty reasonable to me.

- Michael
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