Taking advantage of TCP Loopback fast path in Windows

[Michael McMahon]

On 24/09/14 10:43, Michael McMahon wrote:
> 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

As a matter of interest. Is this effectively an enhancement for code 
that is already using the loopback
interface, or does it apply to all socket connections where both peers 
are on same system (and where the
option has been set by both peers)?

If the latter, does it alter the local or remote addresses reported by 
the socket, since the connection
is now effectively using the loopback interface, or does it pretend to 
be still using the original interfaces?

- Michael.
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