Re: Gigabit/SMP performance problem

[Avery Fay]

Hmm. That paper is actually very interesting. I'm thinking maybe with the 
P4 I'm better off with only 1 cpu. WRT hyperthreading, I actually disabled 
it because it make performance worse (wasn't clear in the original email).

Avery Fay

Anton Blanchard <[e-mail]>
01/03/2003 10:33 PM

 
        To:     Avery Fay <[e-mail]>
        cc:     [e-mail]
        Subject:        Re: Gigabit/SMP performance problem

 
> I'm working with a dual xeon platform with 4 dual e1000 cards on 
different 
> pci-x buses. I'm having trouble getting better performance with the 
second 
> cpu enabled (ht disabled). With a UP kernel (redhat's 2.4.18), I can 
route 
> about 2.9 gigabits/s at around 90% cpu utilization. With a SMP kernel 
> (redhat's 2.4.18), I can route about 2.8 gigabits/s with both cpus at 
> around 90% utilization. This suggests to me that the network code is 
> serialized. I would expect one of two things from my understanding of 
the 
> 2.4.x networking improvements (softirqs allowing execution on more than 
> one cpu):

The Fujitsu guys have a nice summary of this:

http://www.labs.fujitsu.com/en/techinfo/linux/lse-0211/index.html

Skip forward to page 8.

Dont blame the networking code just yet :) Notice how worse UP vs SMP
performance is on the P4 compared to the P3?

This brings up another point, is a single CPU with hyperthreading worth
it? As Rusty will tell you, you need to compare it with a UP kernel
since it avoids all the locking overhead. I suspect for a lot of cases
HT will be a loss (imagine your case, comparing UP and one CPU HT)

Anton