CPU utilisation cap?

[Sean Chittenden]

> I am measuring idle time using a CPU soaker process which runs at a 
> very low priority. Top seems to confirm the output it gives.
>
> What I see is strange. CPU utilisation always peaks (and stays) at 
> between 80 & 85%. If I increase the amount of work done by the UDP 
> echo program (by inserting additional packet copies), CPU utilisation 
> does not rise, but rather, throughput declines. The 80% figure is 
> common to both the slow and fast PCI cards as well.
>
> This is rather confusing, as I cannot tell if the system is IO bound 
> or CPU bound. Certainly I would not have expected the 133/64 PCI bus 
> to be saturated given that peak throughput is around 550Mbit/s with 
> 1024-byte packets. (Such a low figure is not unexpected given there 
> are 2 syscalls per packet).

There are two things that come to mind.  The first being a patch that 
should have been applied in time for 5.2, but I forget the timing of 
the releases.

http://lists.freebsd.org/mailman/htdig/cvs-src/2003-October/012628.html

IIRC, there was another commit that made a similar change specifically 
in the handling of UDP packets, such that it used a TAILQ append 
instead of traversing a linked list.  For some reason I think this 
happened after 5.2, but I'm not able to find high nor low of the commit 
and could be pulling said memory into existence.  Too many commits to 
keep track of.

Recently rwatson has been doing a bunch of testing of our UDP stack and 
has also committed a fair number of fixes.  He likely has many words of 
wisdom on what's changed or not right in 5.2.1.  Regardless, I wager 
that if you give RELENG_5_3 a shot, I bet you'd find performance to be 
less surprising.  -sc

-- 
Sean Chittenden