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Subject: Re: Strange performance issue when using two devices at once
To:     Marlon Smith <marlon.smith10@gmail.com>,
        linux-wireless@vger.kernel.org
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 <1580327314.26012.59.camel@gmail.com>
From:   Ben Greear <greearb@candelatech.com>
Organization: Candela Technologies
Message-ID: <c8788984-3de0-b41c-e2a1-66b67d0674a6@candelatech.com>
Date:   Wed, 29 Jan 2020 12:01:17 -0800
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On 1/29/20 11:48 AM, Marlon Smith wrote:
> On Wed, 2020-01-29 at 11:27 -0800, Ben Greear wrote:
>> On 1/29/20 11:22 AM, Marlon Smith wrote:
>>>
>>> On Wed, 2020-01-29 at 10:42 -0800, Ben Greear wrote:
>>>>
>>>> On 1/29/20 10:39 AM, Marlon Smith wrote:
>>>>>
>>>>>
>>>>> Hi everyone,
>>>>>
>>>>> I have two RT5370 devices connected to the same access point.
>>>>> Both
>>>>> devices are very slow, but the instant I disconnect one device,
>>>>> the
>>>>> other speeds up by a factor of 10.
>>>> Out of curiosity, are both of the RT5370 used on the same client
>>>> device?
>>>>>
>>>>> Did you check that they have unique MAC addresses?
>>>>> Thanks,
>>>> Ben
>>>>>
>>>>>>
>>>>>>
>>>>> The really strange part is that one device will perform slowly
>>>>> even
>>>>> if
>>>>> the other device is basically idle! I've confirmed this with a
>>>>> packet
>>>>> sniffer.
>>>>>
>>>>> I've been trying to do some debugging, and I've found that when
>>>>> both
>>>>> devices are connected to the access point, they report a large
>>>>> number
>>>>> of duplicate frames. I added some debug output
>>>>> in ieee80211_rx_h_check_dup() to confirm that this only happens
>>>>> while
>>>>> both devices are connected. The packet sniffer also shows a
>>>>> large
>>>>> number of retries while this is occurring.
>>>>>
>>>>> Using backports 5.3-rc4 for this, but also tested on 4.14-rc2.
>>>>>
>>>>> I did post about this previously on this mailing list (RT5370
>>>>> performance issues), but I thought I'd post again with this new
>>>>> information and more descriptive title. I'm a little bit stuck
>>>>> on
>>>>> this
>>>>> for a while now, so any ideas are much appreciated.
>>>>>
>>>>> Thanks!
>>>>>
>>>>> Marlon
>>>>>
>>> They are on separate devices, although the mac addresses are close.
>>> 70:F1:1C:2E:AF:B4 and
>>> 70:F1:1C:2E:AF:B6.
>>>
>>> However, I have a third device 70:F1:1C:2E:AF:BB which performs
>>> well
>>> and does not affect the performance of the other two.
>>>
>> Have you tried a different AP?
>>
>> And also tried using the exact same MAC addresses configured on a
>> different
>> radio (like ath9k)?
>>
>> Thanks,
>> Ben
>>
>>
> 
> I have tried a different access point in a different environment but no
> luck. I'll see if I can configure my laptop to use one of the
> problematic devices' mac address.

It might be tricky to determine, but if you can notice whether one of your station devices
is (block-)acking the other's frames, that would be a good clue that it is a station
side bug.  A carefully inspected sniff, especially if you can put sniffer near one station
and far from the other and so use RSSI as a sorting factor, should allow you to determine
this.

Thanks,
Ben


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com