netdev - Re: high latency with imx8mm compared to imx6q

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Message-ID: <Y/AkI7DUYKbToEpj@lunn.ch>
Date:   Sat, 18 Feb 2023 02:04:35 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Richard Weinberger' <richard@....at>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "wei.fang@....com" <wei.fang@....com>,
        "shenwei.wang@....com" <shenwei.wang@....com>,
        "xiaoning.wang@....com" <xiaoning.wang@....com>,
        "linux-imx@....com" <linux-imx@....com>
Subject: Re: high latency with imx8mm compared to imx6q

On Fri, Feb 17, 2023 at 08:49:23PM +0000, David Laight wrote:
> From: Richard Weinberger
> > Sent: 17 February 2023 16:53
> ...
> > I'm investigating into latency issues on an imx8mm system after
> > migrating from imx6q.
> > A regression test showed massive latency increases when single/small packets
> > are exchanged.
> > 
> > A simple test using ping exhibits the problem.
> > Pinging the very same host from the imx8mm has a way higher RTT than from the imx6.
> > 
> > Ping, 100 packets each, from imx6q:
> > rtt min/avg/max/mdev = 0.689/0.851/1.027/0.088 ms
> > 
> > Ping, 100 packets each, from imx8mm:
> > rtt min/avg/max/mdev = 1.073/2.064/2.189/0.330 ms
> > 
> > You can see that the average RTT has more than doubled.
> ...
> 
> Is it just interrupt latency caused by interrupt coalescing
> to avoid excessive interrupts?

Just adding to this, it appears imx6q does not have support for
changing the interrupt coalescing. imx8m does appear to support it. So
try playing with ethtool -c/-C.

    Andrew