Re: [PATCH] bus: mhi: pci-generic: configurable network interface MRU

On Mon, 19 Jul 2021 at 23:44, Richard Laing
[off-list ref] wrote:

Hi Loic,

On 7/19/21 10:11 PM, Loic Poulain wrote:

quoted

For my interest do you have some numbers here highlighting improvement?

These are some of the numbers we found from initial testing using an
external packet generator:

packet size    packets sent  throughput (%pps)
64             1000000        6.21%
128            1000000        7.42%
256            1000000        10.79%
512            1000000        16.40%
1024           1000000        34.34%
1262           1000000        43.82%
1263           1000000        22.45%    <--
1280           1000000        23.15%
1500           1000000        46.32%
1518           1000000        46.84%

You can see the sudden drop of almost 50% between 1262 and 1263 byte
packets. This is what caused us to investigate further. Following the
change to 32KB buffers the drop in throughput is no longer seen.

packet size    packets sent  throughput (%pps)
64             1000000       4.41%
128            1000000       7.70%
256            1000000       14.26%
512            1000000       27.06%
1024           1000000       49.39%
1280           1000000       58.82%
1428           1000000       62.63%

In all cases we were testing with the modem itself in internal loopback
mode.

We have noted that our modem defaults to 32KB buffers (and a maximum of
32 packets per buffer) and also that these values can be changed. We are
considering adding the ability to tune the buffer size, perhaps adding a
sysfs entry or netlink message to change the buffer size instead of the
hard coded value. Any comments would be appreciated.

Thanks for the info, that's interesting.

Note that the default MRU you define is not MHI controller specific
but MHI channel specific (IP/MBIM channel), so it should not be a
property of the MHI controller. AFAIK, The MHI specification already
defines MRU for the transfered buffers which is 65535. I would
recommend to move this prop to the channel config.

Regards,
Loic

Regards,
Richard