Subject: Re: high latency with TCP connections

From: Rick Jones <[email-masked]>
Date: Thu, 31 Aug 2006 12:40:46 -0700

Kelly Burkhart wrote:
> On 8/30/06, David Miller <[email-masked]> wrote:
> 
>> From: Stephen Hemminger <[email-masked]>
>> > Expecting any performance with one byte write's is silly.
>>
>> This is absolutely true.  TCP_NODELAY can only save you when you are
>> sending a small amount of data "in aggregate", such as in an SSH or
>> telnet session, whereas in the case being shown here a large amount of
>> data is being sent in small chunks which will always get bad
>> performance.
> 
> 
> 
> The word performance in this list seems to always mean 'throughput'.
> It seems though that there could be some knob to tweak for those of us
> who don't care so much about throughput but care a great deal about
> latency.

IIRC Apart from interactions with Nagle (TCP_NODELAY) or the mixing of 
packet and byte-based congestion control and avoidance heuristics, there 
really isn't much of anything else to tweak in TCP.  If it can send 
data, it sends data.

Where there _is_ a knob to turn these days might be down with the 
drivers and their NICs' use of interrupt coalescing:

ftp://ftp.cup.hp.com/dist/networking/briefs/nic_latency_vs_tput.txt

rick jones