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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
