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Subject: Re: Intel 8265 unstable/asymmetric ping response time (only under
 Linux)
To:     =?UTF-8?Q?Micha=c5=82_Kazior?= <kazikcz@gmail.com>
Cc:     linux-wireless <linux-wireless@vger.kernel.org>
References: <edaca500-8ae2-aef4-1c9d-b754af7e899f@gmx.com>
 <CABvG-CUJNAq=4XWf3bVYw_EF4W8zmmL7fvc=nt+Zae82Wn7yug@mail.gmail.com>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
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Date:   Fri, 12 Oct 2018 09:34:29 +0800
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From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: =?UTF-8?Q?Micha=c5=82_Kazior?= <kazikcz@gmail.com>
Cc: linux-wireless <linux-wireless@vger.kernel.org>
Message-ID: <ca5a768b-3d81-6bc3-dc28-b0b737e1a7cb@gmx.com>
Subject: Re: Intel 8265 unstable/asymmetric ping response time (only under
 Linux)
References: <edaca500-8ae2-aef4-1c9d-b754af7e899f@gmx.com>
 <CABvG-CUJNAq=4XWf3bVYw_EF4W8zmmL7fvc=nt+Zae82Wn7yug@mail.gmail.com>
In-Reply-To: <CABvG-CUJNAq=4XWf3bVYw_EF4W8zmmL7fvc=nt+Zae82Wn7yug@mail.gmail.com>

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On 2018/10/12 =E4=B8=8A=E5=8D=883:30, Micha=C5=82 Kazior wrote:
> On Thu, 11 Oct 2018 at 15:56, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:=

>>
>> Hi,
>>
>> I found one pretty strange behavior for the Intel 8265/8275 wireless
>> (from ThinkPad X1 carbon)
>>
>> If I ping *FROM* my router/desktop to the wireless laptop, the respons=
e
>> time is very unstable and slow, like:
>> ------
>> $ ping -4 thinkpad
>> PING thinkpad.lan (172.16.0.100) 56(84) bytes of data.
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D1 ttl=3D64 time=3D=
143 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D2 ttl=3D64 time=3D=
166 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D3 ttl=3D64 time=3D=
188 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D4 ttl=3D64 time=3D=
211 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D5 ttl=3D64 time=3D=
29.8 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D6 ttl=3D64 time=3D=
51.7 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D7 ttl=3D64 time=3D=
72.8 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D8 ttl=3D64 time=3D=
94.6 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D9 ttl=3D64 time=3D=
117 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D10 ttl=3D64 time=
=3D140 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D11 ttl=3D64 time=
=3D163 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D12 ttl=3D64 time=
=3D186 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D13 ttl=3D64 time=
=3D207 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D14 ttl=3D64 time=
=3D25.1 ms
>> 64 bytes from thinkpad.lan (172.16.0.100): icmp_seq=3D15 ttl=3D64 time=
=3D47.6 ms
>> -----
>>
>> It looks like the response time get increase by 20ms and flipped over
>> around 200ms.
> [...]
>> So is this something wrong with the wireless card or something wrong
>> with the firmware/powersaving setting of ThinkPad X1 Carbon 6th gen?
>=20
> What you're seeing here is powersave.
>=20
> When you ping your laptop its wifi card may be in powersave so AP will
> store those icmp requests in a queue. The AP will start advertising
> that fact in the next beacon frame (which it broadcasts regularily).
> The wifi client can then see that there's traffic queued up for it,
> wake up, receive it and possibly reply (with icmp response in your
> case). The interval between each ping is long enough for the wifi
> client to re-enter powersave due to "nothing more to do" every time
> causing the same sequence (sleep, queue, advertise, wake, txrx).

Thanks for your detailed explanation, this really helps me to understand
the problem.

>=20
> The time drift comes from the fact beacon interval tends to be
> slightly off from your ping interval. The fact it wraps around at
> ~200msec for you means your AP's beacon interval is roughly that long
> as well which you can probably confirm with your AP/wifi router
> settings. Most APs tend to run 100 or 200msec (or TU, actually, which
> is 1024/1000 of msec) by default.
>=20
> If you run more traffic the lag will go away. I find this sort of
> behavior behavior annoying for mostly idle systems where I want to do
> remoting with ssh (or any other bursty and interactive work).

Yeah, completely the same case.

>=20
> You can try disabling powersave with:
>   iw wlan0 set power_save off
>=20
> You might need to replace wlan0 with wlpXsY depending on your system.
> You'll need root privileges to do it, e.g. via sudo, or su.

Works like a charm.

>=20
> Keep in mind it'll cause more power to be outputted and e.g. leave you
> with slightly shorter battery runtime. On intel compute sticks
> disabling wifi powersave can cause its fan to kick in more often due
> to thermal impact and unfortunate fan trip points.

Not a problem for Thinkpad with better thermal paste and extra tweaked
CPU parameters.

>=20
> Wifi powersave might be disabled in Windows for some reason. Maybe it
> does that disable it when AC is plugged in, and if you were to plug it
> out and run off of battery only, it'd enable it and show similar
> (laggy) behavior?

On battery, windows behaves exactly the same as Linux.

I only need to add some hook for AC/battery switch for Linux.

Thanks for your help!
Qu

>=20
>=20
> Micha=C5=82
>=20


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