Subject: Re: NAPI, e100, and system performance problem

From: Andi Kleen <[email-masked]>
Date: 22 Apr 2005 19:21:08 +0200

On Fri, Apr 22, 2005 at 08:33:15AM -0400, jamal wrote:
> On Fri, 2005-22-04 at 13:36 +0200, Andi Kleen wrote:
> > Greg Banks <[email-masked]> writes:
> > >
> > > An inordinate amount of CPU is being spent running around polling the
> > > device instead of dealing with the packets in IP, TCP and NFS land.
> > > By inordinate, we mean twice as much or more cpu% than a MIPS/Irix
> > > box with slower CPUs.
> > 
> > We have seen similar behaviour. With NAPI some benchmarks run
> > a lot slower than on a driver on the same hardware/NIC without NAPI.
> 
> They should not run slower - but they may consume more CPU.

They actually run slower.

Now before David complains this was with old 2.6 kernels and I dont have
time right now to rerun the benchmarks, but at least I dont think
there was ever any patch addressing these issues.

> 
> > This can be even observed with simple tests like netperf single stream
> > between two boxes.
> > 
> 
> Yes, slow traffic coming into the system would chew more CPU if you have
> a fast CPU ;-> You should know this Andi, but let me explain the reason
> for about the 100th time:

No, the performance of the data transfer was actually slower. CPU time
was not the problem, Opterons have enough of that ...

> this is a design choice - a solution could be created to "fix" this but
> hasnt happened because there has not been a good reason to complicate
> things. The people who are bitching about this are benchmarkers who want
> to win at both high and low rates and they are not happy because while
> they can win at high rates, they cant at low rates.

My impression is that NAPI seems to be more optimized for a rather
obscure work load (routing), while it does not seem to be that 
great on the far more common server/client type workloads.
If that was a design choice then it was a bad design.

-Andi