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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?
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