On Thu, May 05, 2011 at 01:33:14PM +0530, Krishna Kumar2 wrote: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote on 05/05/2011 02:53:59 AM: > > > > Not "hope" exactly. If the device is not ready, then > > > the packet is requeued. The main idea is to avoid > > > drops/stop/starts, etc. > > > > Yes, I see that, definitely. I guess it's a win if the > > interrupt takes at least a jiffy to arrive anyway, > > and a loss if not. Is there some reason interrupts > > might be delayed until the next jiffy? > > I can explain this a bit as I have three debug counters > in start_xmit() just for this: > > 1. Whether the current xmit call was good, i.e. we had > returned BUSY last time and this xmit was successful. > 2. Whether the current xmit call was bad, i.e. we had > returned BUSY last time and this xmit still failed. > 3. The free capacity when we *resumed* xmits. This is > after calling free_old_xmit_skbs where this function > is not throttled, in effect it processes *all* the > completed skbs. This counter is a sum: > > if (If_I_had_returned_EBUSY_last_iteration) > free_slots += virtqueue_get_capacity(); > > The counters after a 30 min run of 1K,2K,16K netperf > sessions are: > > Good: 1059172 > Bad: 31226 > Sum of slots: 47551557 > > (Total of Good+Bad tallies with the total number of requeues > as shown by tc: > > qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 > 1 1 1 1 > Sent 1560854473453 bytes 1075873684 pkt (dropped 718379, overlimits 0 > requeues 1090398) > backlog 0b 0p requeues 1090398 > ) > > It shows that 2.9% of the time, the 1 jiffy was not enough > to free up space in the txq. How common is it to free up space in *less than* 1 jiffy? > That could also mean that we > had set xmit_restart just before jiffies changed. But the > average free capacity when we *resumed* xmits is: > Sum of slots / (Good + Bad) = 43. > > So the delay of 1 jiffy helped the host clean up, on average, > just 43 entries, which is 16% of total entries. This is > intended to show that the guest is not sitting idle waiting > for the jiffy to expire. OK, nice, this is exactly what my patchset is trying to do, without playing with timers: tell the host to interrupt us after 3/4 of the ring is free. Why 3/4 and not all of the ring? My hope is we can get some parallelism with the host this way. Why 3/4 and not 7/8? No idea :) > > > > I can post it, mind testing this? > > > > > > Sure. > > > > Just posted. Would appreciate feedback. > > Do I need to apply all the patches and simply test? > > Thanks, > > - KK Exactly. You can also try to tune the threshold for interrupts as well. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
