netdev - Re: [PATCH net-next 0/2] net: Use SMP threads for backlog NAPI (or optional).

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Message-ID: <20231031101424.I2hTisNY@linutronix.de>
Date: Tue, 31 Oct 2023 11:14:24 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jakub Kicinski <kuba@...nel.org>,
	Wander Lairson Costa <wander@...hat.com>, juri.lelli@...hat.com
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, Come On Now <hawk@...nel.org>
Subject: Re: [PATCH net-next 0/2] net: Use SMP threads for backlog NAPI (or
 optional).

On 2023-10-16 16:53:39 [+0200], To Jakub Kicinski wrote:
> On 2023-10-16 07:17:56 [-0700], Jakub Kicinski wrote:
> > On Mon, 16 Oct 2023 11:53:21 +0200 Sebastian Andrzej Siewior wrote:
> > > > Do we have reason to believe nobody uses RPS?  
> > > 
> > > Not sure what you relate to. I would assume that RPS is used in general
> > > on actual devices and not on loopback where backlog is used. But it is
> > > just an assumption.
> > > The performance drop, which I observed with RPS and stress-ng --udp, is
> > > within the same range with threads and IPIs (based on memory). I can
> > > re-run the test and provide actual numbers if you want.
> > 
> > I was asking about RPS because with your current series RPS processing
> > is forced into threads. IDK how well you can simulate the kind of
> > workload which requires RPS. I've seen it used mostly on proxyies 
> > and gateways. For proxies Meta's experiments with threaded NAPI show
> > regressions across the board. So "force-threading" RPS will most likely
> > also cause regressions.
> 
> Understood.
> 
> Wandere/ Juri: Do you have any benchmark/ workload where you would see
> whether RPS with IPI (now) vs RPS (this patch) shows any regression?

So I poked offlist other RH people and I've been told that they hardly
ever test RPS since the NICs these days have RSS in hardware.

Sebastian