Re: [PATCH V3 3/4] mac80211: fix low throughput in multi-clients situation

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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	weight	Tx_time_round  wt_sum	s_vt	g_vt  valid_for_next_Tx
> 1	256	2048		12800	2048	2000	N
> 2	256	2048			2048		N
> .	.	.			.		.
> .	.	.			.		.
> .	.	.			.		.
> 50	256	2048			2048		N
>
> After this round, all the stations are not valid for next transmission due to
> accumulative deviation.
>
> And if we add a new #51,
> STA	weight	Tx_time_round  wt_sum	s_vt	g_vt  valid_for_next_Tx
> 1	256	2048		13056	2048	2020	N
> 2	256	2048			2048		N
> .	.	.			.
> .	.	.			.
> .	.	.			.
> 50	256	2048			2048		N
> 51	256	1024			2524		N

That's better :)

> Sync is done by:
> max(g_vt of last round - grace period, s_vt)
> and s_vt of #51 = max(2000 - 500, 0) + 1024 = 2524, and it is more than 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