linux-kernel - Re: [PATCH 0/2] NVMe_over_TCP: support specifying the congestion-control

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Date:   Tue, 8 Mar 2022 21:03:20 +0800
From:   Mingbao Sun <sunmingbao@....com>
To:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     tyler.sun@...l.com, ping.gan@...l.com, yanxiu.cai@...l.com,
        libin.zhang@...l.com, ao.sun@...l.com
Subject: Re: [PATCH 0/2] NVMe_over_TCP: support specifying the
 congestion-control

I feel that I'd better address this a little bit more to express the
meaning behind this feature.

You know, InfiniBand/RoCE provides NVMe-oF a lossless network
environment (that is zero packet loss), which is a great advantage
to performance.

In contrast, 'TCP/IP + ethernet' is often used as a lossy network
environment (packet dropping often occurs). 
And once packet dropping occurs, timeout-retransmission would be
triggered. But once timeout-retransmission was triggered, it’s a great
damage to the performance.

So although NVMe/TCP may have a bandwidth competitive to that of
NVMe/RDMA, but the packet dropping of the former is a flaw to
its performance.

However, with the combination of the following conditions, NVMe/TCP
can almost be as competitive as NVMe/RDMA in the data center.

  - Ethernet NICs supporting QoS configuration (support mapping TOS/DSCP
    in IP header into priority, support PFC)

  - Ethernet Switches supporting ECN marking, supporting adjusting
    buffer size of each priority.

  - NVMe/TCP supports specifying the tos for its TCP traffic
    (already implemented)

  - NVMe/TCP supports specifying dctcp as the congestion-control of its
    TCP sockets (the work of this feature)

So this feature is the last item from the software aspect to form up the
above combination.