Re: [PATCH v2 0/2] Change sk_pacing_shift in ieee80211_hw for best tx throughput

[Arend van Spriel <arend.vanspriel@broadcom.com>]

+ Eric

On 8/10/2018 9:52 PM, Ben Greear wrote:
> On 08/10/2018 12:28 PM, Arend van Spriel wrote:
>> On 8/10/2018 3:20 PM, Toke Høiland-Jørgensen wrote:
>>> Arend van Spriel <arend.vanspriel@broadcom.com> writes:
>>>
>>>> On 8/8/2018 9:00 PM, Peter Oh wrote:
>>>>>
>>>>>
>>>>> On 08/08/2018 03:40 AM, Wen Gong wrote:
>>>>>> Add a field for ath10k to adjust the sk_pacing_shift, mac80211 set
>>>>>> the default value to 8, and ath10k will change it to 6. Then mac80211
>>>>>> will use the changed value 6 as sk_pacing_shift since 6 is the best
>>>>>> value for tx throughput by test result.
>>>>> I don't think you can convince people with the numbers unless you
>>>>> provide latency along with the numbers and also measurement result on
>>>>> different chipsets as Michal addressed (QCA4019, QCA9984, etc.) From
>>>>> users view point, I also agree on Toke that we cannot scarify latency
>>>>> for the small throughput improvement.
>>>>
>>>> Yeah. The wireless industry (admittedly that is me too :-p ) has been
>>>> focused on just throughput long enough.
>>>
>>> Tell me about it ;)
>>>
>>>> All the preaching about bufferbloat from Dave and others is (just)
>>>> starting to sink in here and there.
>>>
>>> Yeah, I've noticed; this is good!
>>>
>>>> Now as for the value of the sk_pacing_shift I think we agree it
>>>> depends on the specific device so in that sense the api makes sense,
>>>> but I think there are a lot of variables so I was wondering if we
>>>> could introduce a sysctl parameter for it. Does that make sense?
>>>
>>> I'm not sure a sysctl parameter would make sense; for one thing, it
>>> would be global for the host, while different network interfaces will
>>> probably need different values. And for another, I don't think it's
>>> something a user can reasonably be expected to set correctly, and I
>>> think it *is* actually possible to pick a value that works well at the
>>> driver level.
>>
>> I not sure either. Do you think a user could come up with something
>> like this (found here [1]):
>>
>> sysctl -w net.core.rmem_max=8388608
>> sysctl -w net.core.wmem_max=8388608
>> sysctl -w net.core.rmem_default=65536
>> sysctl -w net.core.wmem_default=65536
>> sysctl -w net.ipv4.tcp_rmem='4096 87380 8388608'
>> sysctl -w net.ipv4.tcp_wmem='4096 65536 8388608'
>> sysctl -w net.ipv4.tcp_mem='8388608 8388608 8388608'
>> sysctl -w net.ipv4.route.flush=1
>>
>> Now the page listing this config claims this is for use "on Linux 2.4+
>> for high-bandwidth applications". Beats me if it still is correct in
>> 4.17.
>>
>> Anyway, sysctl is nice for parameterizing code that is built-in the
>> kernel so you don't need to rebuild it. mac80211 tends to be a module
>> in most distros so
>> maybe sysctl is not a good fit. So lets agree on that.
>>
>> Picking a value at driver level may be possible, but a driver tends to
>> support a number of different devices. So how do you see the picking
>> work. Some static
>> table with entries for the different devices?
>
> Some users are not going to care about latency, and for others, latency may
> be absolutely important and they don't care about bandwidth.
>
> So, it should be tunable.  sysctl can support per network-device settings,
> right?  Or, probably could use ethtool API to set a per-netdev value as
> well.
> That might be nice for other network devices as well, not just wifi.

I was under the impression that the parameters are all global, but your 
statement made me look. I came across some references here [2] so I 
checked the kernel sources under net/ and found net/ipv4/devinet.c [3]. 
So that confirms it supports per-netdev settings.

The sk_pacing_shift is actually a socket setting although I did not find 
a user-space api to change it. For instance it is not a socket option. 
There might be a good reason for not exposing it to user-space (added Eric).

Regards,
Arend

[2] 
https://askubuntu.com/questions/316492/disabling-ipv6-on-a-single-interface
[3] https://elixir.bootlin.com/linux/latest/source/net/ipv4/devinet.c#L2309


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