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

Andrew Lunn
2024-03-19T13:19:40Z

On Tue, Mar 19, 2024 at 12:54:30PM +0000, Parthiban.Veerasooran@microchip.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