tcp_output starving -- is due to mbuf get delay?

Mattias Pantzare pantzer at
Thu Apr 10 14:58:08 PDT 2003

>>What happens in that case for me is that I run out of CPU resources. Try
>>running "top" in one window and "netstat 1" in another while bashing the
>>net with ttcp.
> This is incredibly bizarre.  It's very hard to saturate the CPU
> at only 1Gbit: in all cases, you are I/O bound, not CPU bound,
> and not memory bandwidth bound
>>IMPORTANT NOTE: Several tests here has shown that this is VERY BADLY
>>affected if You have too much LAN equipment (especially VLAN seems to be
>>harmful) at the edges. My speed of 960 Mbit/sec fell to 165 just by adding
>>10 feet of cable and two switches :-(
> The products that Jeffrey Hsu and I and Alfred and Jon Mini
> worked on at a previous company had no problems at all on a
> 1Gbit/S saturating the link, even through a VLAN trunk through
> Cisco and one other less intelligent switch (i.e. two switches
> and a VLAN trunk).

A key factor here is that the testst where on a link with a 20ms 
round-tip time, and
using a singel TCP connection. So the switches where in addition to a 
few routers
on a 10Gbit/s network.

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