netdev - Re: high latency with imx8mm compared to imx6q

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Message-ID: <2030061857.147332.1676721783879.JavaMail.zimbra@nod.at>
Date:   Sat, 18 Feb 2023 13:03:03 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     wei fang <wei.fang@....com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        David Laight <David.Laight@...lab.com>,
        netdev <netdev@...r.kernel.org>,
        shenwei wang <shenwei.wang@....com>,
        xiaoning wang <xiaoning.wang@....com>,
        linux-imx <linux-imx@....com>
Subject: Re: high latency with imx8mm compared to imx6q

----- Ursprüngliche Mail -----
> Von: "wei fang" <wei.fang@....com>
>> Hm, I thought my settings are fine (IOW no coalescing at all).
>> Coalesce parameters for eth0:
>> Adaptive RX: n/a  TX: n/a
>> rx-usecs: 0
>> rx-frames: 0
>> tx-usecs: 0
>> tx-frames: 0
>> 
> Unfortunately, the fec driver does not support to set
> rx-usecs/rx-frames/tx-usecs/tx-frames
> to 0 to disable interrupt coalescing. 0 is an invalid parameters. :(

So setting all values to 1 is the most "no coalescing" setting i can get?
 
>> 
>> But I noticed something interesting this morning. When I set rx-usecs, tx-usecs,
>> rx-frames and tx-frames to 1, *sometimes* the RTT is good.
>> 
>> PING 192.168.0.52 (192.168.0.52) 56(84) bytes of data.
>> 64 bytes from 192.168.0.52: icmp_seq=1 ttl=64 time=0.730 ms
>> 64 bytes from 192.168.0.52: icmp_seq=2 ttl=64 time=0.356 ms
>> 64 bytes from 192.168.0.52: icmp_seq=3 ttl=64 time=0.303 ms
>> 64 bytes from 192.168.0.52: icmp_seq=4 ttl=64 time=2.22 ms
>> 64 bytes from 192.168.0.52: icmp_seq=5 ttl=64 time=2.54 ms
>> 64 bytes from 192.168.0.52: icmp_seq=6 ttl=64 time=0.354 ms
>> 64 bytes from 192.168.0.52: icmp_seq=7 ttl=64 time=2.22 ms
>> 64 bytes from 192.168.0.52: icmp_seq=8 ttl=64 time=2.54 ms
>> 64 bytes from 192.168.0.52: icmp_seq=9 ttl=64 time=2.53 ms
>> 
>> So coalescing plays a role but it looks like the ethernet controller does not
>> always obey my settings.
>> I didn't look into the configured registers so far, maybe ethtool does not set
>> them correctly.
>> 
> It look a bit weird. I did the same setting with my i.MX8ULP and didn't have
> this
> issue. I'm not sure whether you network is stable or network node devices also
> enable interrupt coalescing and the relevant parameters are set to a bit high.

I'm pretty sure my network is good, I've tested also different locations.
And as I said, with the imx6q on the very same network everything works as expected.

So, with rx-usecs/rx-frames/tx-usecs/tx-frames set to 1, you see a RTT smaller than 1ms?

Thanks,
//richard