netdev - Re: [PATCH net-next] tcp: optimise receiver buffer autotuning initialisation for high latency connections

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Date:   Mon, 7 Dec 2020 16:25:17 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "ycheng@...gle.com" <ycheng@...gle.com>,
        "ncardwell@...gle.com" <ncardwell@...gle.com>,
        "weiwan@...gle.com" <weiwan@...gle.com>,
        "Strohman, Andy" <astroh@...zon.com>,
        "Herrenschmidt, Benjamin" <benh@...zon.com>
Subject: Re: [PATCH net-next] tcp: optimise receiver buffer autotuning
 initialisation for high latency connections

On Sat, Dec 5, 2020 at 1:03 PM Mohamed Abuelfotoh, Hazem
<abuehaze@...zon.com> wrote:
>
> Unfortunately few things are missing in this report.
>
>     What is the RTT between hosts in your test ?
>      >>>>>RTT in my test is 162 msec, but I am able to reproduce it with lower RTTs for example I could see the issue downloading from google   endpoint with RTT of 16.7 msec, as mentioned in my previous e-mail the issue is reproducible whenever RTT exceeded 12msec given that    the sender is using bbr.
>
>         RTT between hosts where I run the iperf test.
>         # ping 54.199.163.187
>         PING 54.199.163.187 (54.199.163.187) 56(84) bytes of data.
>         64 bytes from 54.199.163.187: icmp_seq=1 ttl=33 time=162 ms
>         64 bytes from 54.199.163.187: icmp_seq=2 ttl=33 time=162 ms
>         64 bytes from 54.199.163.187: icmp_seq=3 ttl=33 time=162 ms
>         64 bytes from 54.199.163.187: icmp_seq=4 ttl=33 time=162 ms
>
>         RTT between my EC2 instances and google endpoint.
>         # ping 172.217.4.240
>         PING 172.217.4.240 (172.217.4.240) 56(84) bytes of data.
>         64 bytes from 172.217.4.240: icmp_seq=1 ttl=101 time=16.7 ms
>         64 bytes from 172.217.4.240: icmp_seq=2 ttl=101 time=16.7 ms
>         64 bytes from 172.217.4.240: icmp_seq=3 ttl=101 time=16.7 ms
>         64 bytes from 172.217.4.240: icmp_seq=4 ttl=101 time=16.7 ms
>
>     What driver is used at the receiving side ?
>       >>>>>>I am using ENA driver version version: 2.2.10g on the receiver with scatter gathering enabled.
>
>         # ethtool -k eth0 | grep scatter-gather
>         scatter-gather: on
>                 tx-scatter-gather: on
>                 tx-scatter-gather-fraglist: off [fixed]

This ethtool output refers to TX scatter gather, which is not relevant
for this bug.

I see ENA driver might use 16 KB per incoming packet (if ENA_PAGE_SIZE is 16 KB)

Since I can not reproduce this problem with another NIC on x86, I
really wonder if this is not an issue with ENA driver on PowerPC
perhaps ?