RE: [PATCH net-next v3 08/12] net: ethernet: oa_tc6: implement transmit path to transfer tx ethernet frames

[Selvamani Rajagopal]

> -----Original Message-----
> From: Andrew Lunn <andrew@xxxxxxx>
> Sent: Thursday, March 21, 2024 12:42 PM
> To: Selvamani Rajagopal <Selvamani.Rajagopal@xxxxxxxxxx>
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> Subject: Re: [PATCH net-next v3 08/12] net: ethernet: oa_tc6:
> implement transmit path to transfer tx ethernet frames
> 
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> > > > This second part is clearly an optimisation. If you have lots of
> > > > full MTU packets, 1514 bytes, they take around 24 chunks. Having
> > > > the last chunk only 1/2 full does not waste too much bandwidth.
> > > > But if you are carrying lots of small packets, say voice, 130
> > > > bytes, the wasted bandwidth starts to add up. But is there a use
> > > > case for 10Mbps of small packets? I doubt it.
> > > Yes, for sure there is a possibility to get into this scenario and
> > > the protocol also supports that. But as proposed by you below, let's
> > > implement it as part of optimization later.
> > > >
> > > > So if you don't have the ability to combine two packets into one
> > > > chunk, i would do that later. Lets get the basics merged first, it
> > > > can be optimised later.
> > > Yes, I agree with this proposal to get the basic version merged first.
> >
> > While latency is important, so is using the available bandwidth
> > efficiently. Here is a suggestion.  We know that the tx credit
> > available basically tells us, how many chunks could be transmitted
> without overflow. Instead of stopping the netif queue based on number
> of skbs queued, why not stop the queue based on number of bytes
> accumulated? Basically, at any given point of time, we enqueue the
> tx_skb_q until we are have enough bytes to cross the threshold of (tc6-
> >tc_credit * OA_TC6_CHUNK_PAYLOAD_SIZE).
> > This way, during the next transmit, we could utilize the whole available
> credits. Bandwidth utilization between bigger frames and smaller
> frames would be not be vastly different.
> 
> Please configure your email client to wrap emails at around 70
> characters.
> 
> tc_credit is 5 bits. So it is a maximum of 32.
> 
> A 1514 frame takes around 24 chunks. So you only need two full size
> frames to consume all your possible credit.
> 
> If you happen to have smaller voice packets, say 130 bytes, you need
> three chunks to send it. So you might want to have 10 such packets on
> hand in order to make use of all your credit. But if you have 10 voice
> packets to send in a burst, your voice quality is going to be bad, they
> should be 10ms to 20ms apart, not in a burst...
> 
> I don't like the original idea of having lots of packets in a transmit queue.
> But having 1/2 dozen should not be an issue.
> 
> In general, we prefer things to be simple. We can then optimise later,
> and use benchmarks to show the optimisations really do bring a benefit
> to justify the added complexity.

True. I should get some performance numbers to see where we are
with the current code. That would be time to look at the improvement. 

> 
>    Andrew