On Sep 9, 2008, at Sep 9, 2008, 12:33 PM, Rick Jones wrote:
quoted text
>> 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.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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