netdev - Re: [RFC PATCH net-next] net/smc: Introduce receive queue flow control support

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Message-ID: <20220120095130.GB41938@linux.alibaba.com>
Date:   Thu, 20 Jan 2022 17:51:30 +0800
From:   "dust.li" <dust.li@...ux.alibaba.com>
To:     Guangguan Wang <guangguan.wang@...ux.alibaba.com>,
        kgraul@...ux.ibm.com, davem@...emloft.net, kuba@...nel.org
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next] net/smc: Introduce receive queue flow
 control support

On Thu, Jan 20, 2022 at 02:51:40PM +0800, Guangguan Wang wrote:
>This implement rq flow control in smc-r link layer. QPs
>communicating without rq flow control, in the previous
>version, may result in RNR (reveive not ready) error, which
>means when sq sends a message to the remote qp, but the
>remote qp's rq has no valid rq entities to receive the message.
>In RNR condition, the rdma transport layer may retransmit
>the messages again and again until the rq has any entities,
>which may lower the performance, especially in heavy traffic.
>Using credits to do rq flow control can avoid the occurrence
>of RNR.

I'm wondering if SRQ can be used to solve this problem ?

One of my concern on credit-base flow control is if the RTT is
a bit longer, we may have to wait RTT/2 for peer to grant us credit
before we can really send more data. That may decrease the maximium
bandwidth we can achive in this case.