Yibo Zhao <yiboz@xxxxxxxxxxxxxx> 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
