Tuesday, June 2, 2015

TCP Offloads: more harm than good

+Dave Taht has been saying for a while that TCP offloads do more harm than good, especially when mixed with fq_codel, and the ingress rate limiter that the sqm_scripts package uses to replace the large inbound buffers in the modem and CMTS with a much smaller buffer (but nearly as fast bandwidth), under the control of the router.

I finally put some numbers on that tonight.

The first dataset (green plots) are without gro, tso, and gso.  The second plots are with those offloads all re-enabled.  So enabling offloads:

1) slows it down
2) increases latency


Yeah, I'm keeping all the offloads turned off (and adjusted my router startup scripts to keep them off each time the simple.qos script runs).