netdev - Re: [RFC PATCH net-next 0/1] ibmveth: Implement BQL

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Date:   Tue, 25 Oct 2022 11:41:48 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Nick Child <nnac123@...ux.ibm.com>
Cc:     netdev@...r.kernel.org, nick.child@....com, dave.taht@...il.com
Subject: Re: [RFC PATCH net-next 0/1] ibmveth: Implement BQL

On Mon, 24 Oct 2022 16:38:27 -0500 Nick Child wrote:
> Labeled as RFC because I am unsure if adding Byte Queue Limits (BQL) is
> positively effecting the ibmveth driver. BQL is common among network
> drivers so I would like to incorporate it into the virtual ethernet
> driver, ibmveth. But I am having trouble measuring its effects.
> 
> From my understanding (and please correct me if I am wrong), BQL will 
> use the number of packets sent to the NIC to approximate the minimum
> number of packets to enqueue to a netdev_queue without starving the NIC.
> As a result, bufferbloat in the networking queues are minimized which
> may allow for smaller latencies.
> 
> After performing various netperf tests under differing loads and
> priorities, I do not see any performance effect when comparing the
> driver with and without BQL. The ibmveth driver is a virtual driver
> which has an abstracted view of the NIC so I am comfortable without
> seeing any performance deltas. That being said, I would like to know if
> BQL is actually being enforced in some way. In other words, I would
> like to observe a change in the number of queued bytes during BQL
> implementations. Does anyone know of a mechanism to measure the length
> of a netdev_queue?
> 
> I tried creating a BPF script[1] to track the bytes in a netdev_queue
> but again am not seeing any difference with and without BQL. I do not
> believe anything is wrong with BQL (it is more likely that my tracing
> is bad) but I would like to have some evidence of BQL having a
> positive effect on the device. Any recommendations or advice would be
> greatly appreciated.

What qdisc are you using and what "netperf tests" are you running?