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

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Date:   Tue, 29 Mar 2022 10:46:08 +0300
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Mingbao Sun <sunmingbao@....com>
Cc:     Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        tyler.sun@...l.com, ping.gan@...l.com, yanxiu.cai@...l.com,
        libin.zhang@...l.com, ao.sun@...l.com
Subject: Re: [PATCH v2 2/3] nvme-tcp: support specifying the
 congestion-control


>> 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.

It can most certainly affect them, positively or negatively, depends
on the use-case.

>> 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).

Separate networks are still not application (nvme-tcp) specific and as
mentioned, we have a way to control that. IMO, this still does not
qualify as solid justification to add this to nvme-tcp.

What do others think?