We already do have intelligent qdiscs - via TC. Anyone using htb for rate pacing, will achieve better control (less jitter) if the DMA queue latency is kept down.
One very important issue for 802.11 is the software rate control. At a certain point in time the rate control algorithm may determine it's time to start transmitting frames at a much lower rate - now all the frames already in the DMA queue will be transmitted at the old higher rate - and hence will suffer a lower probability of being received correctly. If the number of frames in the DMA queue can be kept down then the rate change happens faster. Obviously the limiting factor is keeping the DMA queue full enough that the hardware is not starved in the worst case of softirq latency. This is a time based limit. Simon -----Original Message----- From: Michael Buesch [mailto:[EMAIL PROTECTED] Sent: Wednesday, August 23, 2006 1:57 PM To: Simon Barber Cc: Jiri Benc; [EMAIL PROTECTED]; netdev@vger.kernel.org Subject: Re: [PATCH] d80211: add ieee80211_stop_queues() On Wednesday 23 August 2006 22:32, Simon Barber wrote: > Right - and what I'm proposing is that we don't just count the number > of frames in the ring - but also count the amount of frame time the > ring takes too. This way if there are a number of large, slow frames > we can stop adding more frames into the ring well before we reach the > limit on the number of frames. Ah, now I understand what you are trying to explain. :) But why do we actually _want_ to stop a DMA ring, if it's not full (has free descriptors)? I can't see any benefit on it. Adding frames to the ring is done on the fly without stopping or otherwise interferring with any running transmissions queued earlier. I would say we don't care about the time it takes for the ring to go into idle on the d80211 level. I think the only thing we care at d80211 level is: Can we queue another frame? We have that logic. If a queue is not stopped, we can queue another frame. Or do you want to make the qdisc intelligent? Say, it drops a few beacons, if there are already packets queued for the next 300ms, for example. Do you want to optimize latency of payload data by dropping low-priority packets while the queue is heavily loaded? I can't see another usage for time based DMA ring accounting in d80211, yet. -- Greetings Michael. - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
