Subject: Re: [PATCH v2 2/3] nvme-tcp: support specifying the congestion-control

From: Mingbao Sun <[email-masked]>
Date: Tue, 29 Mar 2022 10:48:06 +0800

> As I said, TCP can be tuned in various ways, congestion being just one
> of them. I'm sure you can find a workload where rmem/wmem will make
> a difference.

agree.
but the difference for the knob of rmem/wmem is:
we could enlarge rmem/wmem for NVMe/TCP via sysctl,
and it would not bring downside to any other sockets whose
rmem/wmem are not explicitly specified.

> In addition, based on my knowledge, application specific TCP level
> tuning (like congestion) is not really a common thing to do. So why in
> nvme-tcp?
> 
> So to me at least, it is not clear why we should add it to the driver.

As mentioned in the commit message, though we can specify the
congestion-control of NVMe_over_TCP via sysctl or writing
'/proc/sys/net/ipv4/tcp_congestion_control', but this also
changes the congestion-control of all the future TCP sockets on
the same host that have not been explicitly assigned the
congestion-control, thus bringing potential impaction on their
performance.

For example:

A server in a data-center with the following 2 NICs:

    - NIC_fron-end, for interacting with clients through WAN
      (high latency, ms-level)

    - NIC_back-end, for interacting with NVMe/TCP target through LAN
      (low latency, ECN-enabled, ideal for dctcp)

This server interacts with clients (handling requests) via the fron-end
network and accesses the NVMe/TCP storage via the back-end network.
This is a normal use case, right?

For the client devices, we can’t determine their congestion-control.
But normally it’s cubic by default (per the CONFIG_DEFAULT_TCP_CONG).
So if we change the default congestion control on the server to dctcp
on behalf of the NVMe/TCP traffic of the LAN side, it could at the
same time change the congestion-control of the front-end sockets
to dctcp while the congestion-control of the client-side is cubic.
So this is an unexpected scenario.

In addition, distributed storage products like the following also have
the above problem:

    - The product consists of a cluster of servers.

    - Each server serves clients via its front-end NIC
     (WAN, high latency).

    - All servers interact with each other via NVMe/TCP via back-end NIC
     (LAN, low latency, ECN-enabled, ideal for dctcp).