On 2013-11-20 18:53, Karl Beldan wrote: > On Wed, Nov 20, 2013 at 06:32:57PM +0100, Felix Fietkau wrote: >> On 2013-11-20 17:19, Karl Beldan wrote: >> > On Wed, Nov 20, 2013 at 04:49:55PM +0100, Felix Fietkau wrote: >> >> On 2013-11-20 15:50, Karl Beldan wrote: >> >> > On Wed, Nov 20, 2013 at 03:04:34PM +0100, Felix Fietkau wrote: >> >> >> On 2013-11-20 14:56, Karl Beldan wrote: >> >> >> > On Wed, Nov 20, 2013 at 08:32:32AM +0100, Felix Fietkau wrote: >> >> >> >> On 2013-11-20 01:51, Karl Beldan wrote: >> >> >> >> > From: Karl Beldan <karl.beldan@xxxxxxxxxxxxxxxx> >> >> >> >> > >> >> >> >> > Commit 3e8b1eb "mac80211/minstrel_ht: improve rate selection stability" >> >> >> >> > introduced a local capped prob in minstrel_ht_calc_tp but omitted to use >> >> >> >> > it to compute the rate throughput. >> >> >> >> > >> >> >> >> > Signed-off-by: Karl Beldan <karl.beldan@xxxxxxxxxxxxxxxx> >> >> >> >> > CC: Felix Fietkau <nbd@xxxxxxxxxxx> >> >> >> >> Nice catch! >> >> >> >> Acked-by: Felix Fietkau <nbd@xxxxxxxxxxx> >> >> >> >> >> >> >> > Interestingly enough, consecutive coding rates (5/6, 3/4, 2/3) max ratio >> >> >> > is 9/10, did you do it on purpose ? (e.g. (9/10) * (5/6) == 3/4, >> >> >> > (9/10) * (3/4) == 2/3 + 11/120). >> >> >> The change has nothing to do with coding rates, it's only about >> >> >> retransmissions caused by collisions under load. >> >> >> >> >> > I understand this, my point was that along with this comes the following: >> >> > let's say my SNR is just not so good to get 5/6 as good as 3/4, and e.g. >> >> > case1 htMCS7 has 91% >> >> > htMCS6 has 100% success >> >> > case2 htMCS7 has 80% >> >> > htMCS6 has 100% success >> >> > capping at 90% will prefer htMCS7 in case1 and htMCS6 in case2 both >> >> > achieving best real throughput. >> >> > capping at 80% will prefer htMCS7 in case1 _but_ htMCS7 in case2 the >> >> > latter being the worst real throughput(90% of 5/6 == 100% of 3/4 > 80% >> >> > of 5/6). >> >> Not sure if that's a meaningful comparison at all - you're leaving out >> >> the per-packet overhead, which is important for the throughput >> >> calculation as well. >> >> >> > The overhead breaks these numbers but the more we aggregate the more >> > this math is realistic as then the rates converge to these numbers .. >> > plus, IMHO using the overhead for throughput is wasteful since >> > throughputs are ranked and used relatively to each others and overhead >> > is shared by all rates. >> The throughput metric (as displayed in debugfs) is calculated as: >> tp = 10 ms * prob / (overhead_time / ampdu_len + packet_tx_time) >> >> When you have two rates that are relatively close to each other, and the >> faster rate is less reliable than the slower one, the throughput metric >> can prefer the slower rate without aggregation, and the faster one with >> aggregation. >> >> The overhead may be shared between all rates, but that doesn't mean it >> does not affect the relative comparison between rates. >> > I did not say the overhead doesn't affect the relative comparison. > ampdu_len and overhead_time are shared by all the rates, what's the > purpose of computing overhead_time then ? since the rate selection is > only mere comparison of the said computed tps. Right, I guess we could add a mi->overhead_ampdu that gets adjusted based on the average ampdu length before recalculating all rates. - Felix -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
