Subject: Re: >10% performance degradation since 2.6.18

From: Jeff Garzik <[email-masked]>
Date: Sun, 05 Jul 2009 16:44:41 -0400

Herbert Xu wrote:
> Jeff Garzik <[email-masked]> wrote:
>> What's the best setup for power usage?
>> What's the best setup for performance?
>> Are they the same?
> 
> Yes.

Is this a blind guess, or is there real world testing across multiple 
setups behind this answer?

Consider a 2-package, quad-core system with 3 userland threads actively 
performing network communication, plus periodic, low levels of network 
activity from OS utilities (such as nightly 'yum upgrade').

That is essentially an under-utilized 8-CPU system.  For such a case, it 
seems like a power win to idle or power down a few cores, or maybe even 
an entire package.

Efficient power usage means scaling _down_ when activity decreases.  A 
blind "distribute network activity across all CPUs" policy does not 
appear to be responsive to that type of situation.

>> Is it most optimal to have the interrupt for socket $X occur on the same 
>> CPU as where the app is running?
> 
> Yes.

Same question:  blind guess, or do you have numbers?

Consider two competing CPU hogs:  a kernel with tons of netfilter tables 
and rules, plus an application that uses a lot of CPU.

Can you not reach a threshold where it makes more sense to split kernel 
and userland work onto different CPUs?

>> If yes, how to best handle when the scheduler moves app to another CPU?
>> Should we reprogram the NIC hardware flow steering mechanism at that point?
> 
> Not really.  For now the best thing to do is to pin everything
> down and not move at all, because we can't afford to move.
> 
> The only way for moving to work is if we had the ability to get
> the sockets to follow the processes.  That means, we must have
> one RX queue per socket.

That seems to presume it is impossible to reprogram the NIC RX queue 
selection rules?

If you can add a new 'flow' to a NIC hardware RX queue, surely you can 
imagine a remove + add operation for a migrated 'flow'...  and thus, at 
least on the NIC hardware level, flows can follow processes.

	Jeff