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

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Date: Tue, 19 Mar 2024 14:19:19 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Parthiban.Veerasooran@...rochip.com
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, horms@...nel.org, saeedm@...dia.com,
	anthony.l.nguyen@...el.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, corbet@....net,
	linux-doc@...r.kernel.org, robh+dt@...nel.org,
	krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
	devicetree@...r.kernel.org, Horatiu.Vultur@...rochip.com,
	ruanjinjie@...wei.com, Steen.Hegelund@...rochip.com,
	vladimir.oltean@....com, UNGLinuxDriver@...rochip.com,
	Thorsten.Kummermehr@...rochip.com, Pier.Beruto@...emi.com,
	Selvamani.Rajagopal@...emi.com, Nicolas.Ferre@...rochip.com,
	benjamin.bigler@...nformulastudent.ch
Subject: Re: [PATCH net-next v3 08/12] net: ethernet: oa_tc6: implement
 transmit path to transfer tx ethernet frames

On Tue, Mar 19, 2024 at 12:54:30PM +0000, Parthiban.Veerasooran@...rochip.com wrote:
> Hi Andrew,
> 
> On 07/03/24 10:38 pm, Andrew Lunn wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> >> @@ -55,6 +77,14 @@
> >>                                                (OA_TC6_CTRL_MAX_REGISTERS *\
> >>                                                OA_TC6_CTRL_REG_VALUE_SIZE) +\
> >>                                                OA_TC6_CTRL_IGNORED_SIZE)
> >> +#define OA_TC6_CHUNK_PAYLOAD_SIZE            64
> >> +#define OA_TC6_DATA_HEADER_SIZE                      4
> >> +#define OA_TC6_CHUNK_SIZE                    (OA_TC6_DATA_HEADER_SIZE +\
> >> +                                             OA_TC6_CHUNK_PAYLOAD_SIZE)
> >> +#define OA_TC6_TX_SKB_QUEUE_SIZE             100
> > 
> > So you keep up to 100 packets in a queue. If use assume typical MTU
> > size packets, that is 1,238,400 bits. At 10Mbps, that is 120ms of
> > traffic. That is quite a lot of latency when a high priority packet is
> > added to the tail of the queue and needs to wait for all the other
> > packets to be sent first.
> > 
> > Chunks are 64 bytes. So in practice, you only ever need two
> > packets. You need to be able to fill a chunk with the final part of
> > one packet, and the beginning of the next. So i would try using a much
> > smaller queue size. That will allow Linux queue disciplines to give
> > you the high priority packets first which you send with low latency.
> Thanks for the detailed explanation. If I understand you correctly,
> 
> 1. The tx skb queue size (OA_TC6_TX_SKB_QUEUE_SIZE) should be 2 to avoid 
> the latency when a high priority packet added.
> 
> 2. Need to implement the handling part of the below case,
> In case if one packet ends in a chunk and that chunk still having some 
> space left to accommodate some bytes from the next packet if available 
> from network layer.

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.

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.

	Andrew