netdev - RE: high latency with imx8mm compared to imx6q

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Date:   Sat, 18 Feb 2023 01:27:21 +0000
From:   Wei Fang <wei.fang@....com>
To:     Andrew Lunn <andrew@...n.ch>,
        David Laight <David.Laight@...lab.com>
CC:     'Richard Weinberger' <richard@....at>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Shenwei Wang <shenwei.wang@....com>,
        Clark Wang <xiaoning.wang@....com>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: high latency with imx8mm compared to imx6q



> -----Original Message-----
> From: Andrew Lunn <andrew@...n.ch>
> Sent: 2023年2月18日 9:05
> To: David Laight <David.Laight@...lab.com>
> Cc: 'Richard Weinberger' <richard@....at>; netdev@...r.kernel.org; Wei Fang
> <wei.fang@....com>; Shenwei Wang <shenwei.wang@....com>; Clark Wang
> <xiaoning.wang@....com>; dl-linux-imx <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.
> 
Yes, I agree with Andrew, the interrupt coalescence feature default to be enabled
on i.MX8MM platforms. The purpose of the interrupt coalescing is to reduce the
number of interrupts generated by the MAC so as to reduce the CPU loading. 
As Andrew said, you can turn down rx-usecs and tx-usecs, and then try again.