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
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