netdev - Re: RFC: Nagle latency tuning

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Date:	Tue, 9 Sep 2008 14:21:30 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Chuck Lever <chuck.lever@...cle.com>
Cc:	Rick Jones <rick.jones2@...com>, Chris Snook <csnook@...hat.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: RFC: Nagle latency tuning

Em Tue, Sep 09, 2008 at 12:54:31PM -0400, Chuck Lever escreveu:
> On Sep 9, 2008, at Sep 9, 2008, 12:33 PM, Rick Jones wrote:
>>> Most of the apps where people care about this enough to complain to  
>>> their vendor (the cases I hear about) are in messaging apps, where  
>>> they're relaying a stream of events that have little to do with each 
>>> other, and they want TCP to maintain the integrity of the connection 
>>> and do a modicum of bandwidth management, but 40 ms stalls greatly 
>>> exceed their latency tolerances.
>>
>> What _are_ their latency tolerances?  How often are they willing to  
>> tolerate a modicum of TCP bandwidth management?  Do they go ape when  
>> TCP sits waiting not just for 40ms, but for an entire RTO timer?
>>
>>> Using TCP_NODELAY is often the least bad option, but sometimes it's
>>> infeasible because of its effect on the network, and it certainly
>>> adds to the network stack overhead.  A more tunable Nagle delay would
>>> probably serve many of these apps much better.
>>
>> If the applications are sending streams of logically unrelated sends  
>> down the same socket, then setting TCP_NODELAY is IMO fine.  Where it 
>> isn't fine is where these applications are generating their logically 
>> associated data in two or more small sends.  One send per message good. 
>> Two sends per message bad.
>
> Can the same be said of the Linux kernel's RPC client, which uses  
> MSG_MORE and multiple sends to construct a single RPC request on a TCP  
> socket?
>
> See net/sunrpc/xprtsock.c:xs_send_pagedata() for details.

That is not a problem, it should be equivalent to corking the socket.
I.e. the uncorking operation will be the last part of the buffer, where
'more' will be 0.

- Arnaldo
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