Subject: Re: [PATCH 0/2] HTB scheduler HTB_HYSTERESIS modifications

From: Martin Devera <[email-masked]>
Date: Wed, 04 Jun 2008 17:31:41 +0200

>> I'm thinking about removing HYSTERESIS code altogether, but
>> (and it holds for setting HYSTERESIS=off as default) 
> 
> I prefer a turn knob, where the default is off.

Ok. As seen from interrupt load below, it is reasonable.

> With current testing (on 2.6.25.4) I could not see any changes with CPU
> usage, but thats probably caused by the fast CPU on the test system
> (Dual Xeon). BUT an interesting observation was that the number of
> interrupts increased significantly, from approx 1300 to 2000 per sec.
> The other observation was that I got significantly less jitter.

Interesting. It is probably result of merging of subsequent yellow/red
states without going to green for while. The traffic is more bursty then
(it is no longer only timer-granular, but {c,}burst sized bursts are
FORCED in output) but NIC driver can emit more outgoing packets
per in single "buffer free" interrupt.

>> Also, I'm a bit busy for a few weeks, would you be able/interested
>> to do such testing ?
> 
> Okay, lets wait until I get this kernel into our real-life production
> system.  Then I can report on the real-life impact on CPU performance.

Thanks, I'll try to set some test up later. With large class-count as
state changes will cost more then (RB tree deletes/inserts).

Regarding your intr/sec count, what about to keep default value
of hysteresis switch as "on" (just to be on safe side) ?
I'm afraid what that 40% larger interrupt load might do with bigger
routers (I already got some performance related questions from ppl
with hundreds classes on 100's Mb links).

m.