Re: Optimizing kernel compilation / alignments for network performance

From: "Rafał Miłecki" <zajec5@gmail.com>
To: Andrew Lunn <andrew@lunn.ch>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Alexander Lobakin <alexandr.lobakin@intel.com>,
	Network Development <netdev@vger.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Russell King <linux@armlinux.org.uk>,
	Felix Fietkau <nbd@nbd.name>,
	"openwrt-devel@lists.openwrt.org"
	<openwrt-devel@lists.openwrt.org>,
	Florian Fainelli <f.fainelli@gmail.com>
Subject: Re: Optimizing kernel compilation / alignments for network performance
Date: Fri, 6 May 2022 09:44:35 +0200	[thread overview]
Message-ID: <510bd08b-3d46-2fc8-3974-9d99fd53430e@gmail.com> (raw)
In-Reply-To: <YnP1nOqXI4EO1DLU@lunn.ch>

On 5.05.2022 18:04, Andrew Lunn wrote:
>> you'll see that most used functions are:
>> v7_dma_inv_range
>> __irqentry_text_end
>> l2c210_inv_range
>> v7_dma_clean_range
>> bcma_host_soc_read32
>> __netif_receive_skb_core
>> arch_cpu_idle
>> l2c210_clean_range
>> fib_table_lookup
> 
> There is a lot of cache management functions here. Might sound odd,
> but have you tried disabling SMP? These cache functions need to
> operate across all CPUs, and the communication between CPUs can slow
> them down. If there is only one CPU, these cache functions get simpler
> and faster.
> 
> It just depends on your workload. If you have 1 CPU loaded to 100% and
> the other 3 idle, you might see an improvement. If you actually need
> more than one CPU, it will probably be worse.

It seems to lower my NAT speed from ~362 Mb/s to 320 Mb/s but it feels
more stable now (lower variations). Let me spend some time on more
testing.

FWIW during all my tests I was using:
echo 2 > /sys/class/net/eth0/queues/rx-0/rps_cpus
that is what I need to get similar speeds across iperf sessions

With
echo 0 > /sys/class/net/eth0/queues/rx-0/rps_cpus
my NAT speeds were jumping between 4 speeds:
273 Mbps / 315 Mbps / 353 Mbps / 425 Mbps
(every time I started iperf kernel jumped into one state and kept the
  same iperf speed until stopping it and starting another session)

With
echo 1 > /sys/class/net/eth0/queues/rx-0/rps_cpus
my NAT speeds were jumping between 2 speeds:
284 Mbps / 408 Mbps

> I've also found that some Ethernet drivers invalidate or flush too
> much. If you are sending a 64 byte TCP ACK, all you need to flush is
> 64 bytes, not the full 1500 MTU. If you receive a TCP ACK, and then
> recycle the buffer, all you need to invalidate is the size of the ACK,
> so long as you can guarantee nothing has touched the memory above it.
> But you need to be careful when implementing tricks like this, or you
> can get subtle corruption bugs when you get it wrong.

That was actually bgmac's initial behaviour, see commit 92b9ccd34a90
("bgmac: pass received packet to the netif instead of copying it"):
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=92b9ccd34a9053c628d230fe27a7e0c10179910f

I think it was Felix who suggested me to avoid skb_copy*() and it seems
it improved performance indeed.