Subject: Re: [QA-TCP] How to send tcp small packages immediately?

From: Rick Jones <[email-masked]>
Date: Fri, 24 Oct 2014 08:19:46 -0700

On 10/24/2014 12:41 AM, Zhangjie (HZ) wrote:
> Hi,
>
> I use netperf to test the performance of small tcp package, with TCP_NODELAY set :
>
> netperf -H 129.9.7.164 -l 100 -- -m 512 -D
>
> Among the packages I got by tcpdump, there is not only small packages, also lost of
> big ones (skb->len=65160).
>
> IP 129.9.7.186.60840 > 129.9.7.164.34607: tcp 65160
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.186.60840 > 129.9.7.164.34607: tcp 65160
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.164.34607 > 129.9.7.186.60840: tcp 0
> IP 129.9.7.186.60840 > 129.9.7.164.34607: tcp 80
> IP 129.9.7.186.60840 > 129.9.7.164.34607: tcp 512
> IP 129.9.7.186.60840 > 129.9.7.164.34607: tcp 512
>
> SO, how to test small tcp packages? Including TCP_NODELAY, What else should be set?

Well, I don't think there is anything else you can set.  Even with 
TCP_NODELAY set, segment size with TCP will still be controlled by 
factors such as congestion window.

I am ass-u-me-ing your packet trace is at the sender.  I suppose if your 
sender were fast enough compared to the path that might combine with 
congestion window to result in the very large segments.

Not to say there cannot be a bug somewhere with TSO overriding 
TCP_NODELAY, but in broad terms, even TCP_NODELAY does not guarantee 
small TCP segments.  That has been something of a bane on my attempts to 
use TCP for aggregate small-packet performance measurements via netperf 
for quite some time.

And since you seem to have included a virtualization mailing list I 
would also ass-u-me that virtualization is involved somehow.  Knuth only 
knows how that will affect the timing of events, which will be very much 
involved in matters of congestion window and such.  I suppose it is even 
possible that if the packet trace is on a VM receiver that some delays 
in getting the VM running could mean that GRO would end-up making large 
segments being pushed up the stack.

happy benchmarking,

rick jones