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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Yibo Zhao <yiboz@codeaurora.org>, linux-wireless@vger.kernel.org
Cc:     ath10k@lists.infradead.org, Yibo Zhao <yiboz@codeaurora.org>
Subject: Re: [PATCH V3 3/4] mac80211: fix low throughput in multi-clients situation
In-Reply-To: <1569223201-1490-4-git-send-email-yiboz@codeaurora.org>
References: <1569223201-1490-1-git-send-email-yiboz@codeaurora.org> <1569223201-1490-4-git-send-email-yiboz@codeaurora.org>
Date:   Mon, 23 Sep 2019 12:55:05 +0200
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Yibo Zhao <yiboz@codeaurora.org> writes:

> Not long after the start of multi-clients test, not a single station is
> an eligible candidate for transmission since global virtual time(g_vt) is
> smaller than the virtual airtime(s_vt) of all the stations. As a result,
> the Tx has been blocked and throughput is quite low.
>
> This may mainly due to sync mechanism and accumulative deviation from the
> devision calculation of g_vt.
>
> For example:
> Suppose we have 50 clients in first round.
> Round 1:
> STA=09weight=09Tx_time_round  wt_sum=09s_vt=09g_vt  valid_for_next_Tx
> 1=09256=092048=09=0912800=092048=092000=09N
> 2=09256=092048=09=09=092048=09=09N
> .=09.=09.=09=09=09.=09=09.
> .=09.=09.=09=09=09.=09=09.
> .=09.=09.=09=09=09.=09=09.
> 50=09256=092048=09=09=092048=09=09N
>
> After this round, all the stations are not valid for next transmission du=
e to
> accumulative deviation.
>
> And if we add a new #51,
> STA=09weight=09Tx_time_round  wt_sum=09s_vt=09g_vt  valid_for_next_Tx
> 1=09256=092048=09=0913056=092048=092020=09N
> 2=09256=092048=09=09=092048=09=09N
> .=09.=09.=09=09=09.
> .=09.=09.=09=09=09.
> .=09.=09.=09=09=09.
> 50=09256=092048=09=09=092048=09=09N
> 51=09256=091024=09=09=092524=09=09N

That's better :)

> Sync is done by:
> max(g_vt of last round - grace period, s_vt)
> and s_vt of #51 =3D max(2000 - 500, 0) + 1024 =3D 2524, and it is more th=
an the final
> g_vt of this round.
>
> After this round, no more station is valid for transmission.
>
> The real situation can be more complicate, above is one of the extremely =
case.
>
> To avoid this situation to occur, the new proposal is:
>
> - Increase the airtime grace period a little more to reduce the
>   unexpected sync
>
> - If global virtual time is less than the virtual airtime of any station,
>   sync it to the airtime of first station in the red-black tree
>
> - Round the division result

I can see why we need the second part (basically, this happens because I
forgot to add a check for "no eligible stations" in may_transmit(), like
the one in next_txq()). And rounding up the division result doesn't
hurt, I guess. But why does it help to change the grace period if we're
doing all the other stuff?

-Toke