Re: [PATCHv2 10/14] virtio_net: limit xmit polling

Krishna Kumar2
2011-05-24T12:50:35Z

"Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote on 05/24/2011 04:59:39 PM:

> > > > Maybe Rusty means it is a simpler model to free the amount
> > > > of space that this xmit needs. We will still fail anyway
> > > > at some time but it is unlikely, since earlier iteration
> > > > freed up atleast the space that it was going to use.
> > >
> > > Not sure I nderstand. We can't know space is freed in the previous
> > > iteration as buffers might not have been used by then.
> >
> > Yes, the first few iterations may not have freed up space, but
> > later ones should. The amount of free space should increase
> > from then on, especially since we try to free double of what
> > we consume.
>
> Hmm. This is only an upper limit on the # of entries in the queue.
> Assume that vq size is 4 and we transmit 4 enties without
> getting anything in the used ring. The next transmit will fail.
>
> So I don't really see why it's unlikely that we reach the packet
> drop code with your patch.

I was assuming 256 entries :) I will try to get some
numbers to see how often it is true tomorrow.

> > I am not sure of why it was changed, since returning TX_BUSY
> > seems more efficient IMHO.
> > qdisc_restart() handles requeue'd
> > packets much better than a stopped queue, as a significant
> > part of this code is skipped if gso_skb is present
>
> I think this is the argument:
> http://www.mail-archive.com/virtualization-cunTk1MwBs/ROKNJybVBZg@public.gmane.org
> foundation.org/msg06364.html

Thanks for digging up that thread! Yes, that one skb would get
sent first ahead of possibly higher priority skbs. However,
from a performance point, TX_BUSY code skips a lot of checks
and code for all subsequent packets till the device is
restarted. I can test performance with both cases and report
what I find (the requeue code has become very simple and clean
from "horribly complex", thanks to Herbert and Dave).

> > (qdisc
> > will eventually start dropping packets when tx_queue_len is
>
> tx_queue_len is a pretty large buffer so maybe no.

I remember seeing tons of drops (pfifo_fast_enqueue) when
xmit returns TX_BUSY.

> I think the packet drops from the scheduler queue can also be
> done intelligently (e.g. with CHOKe) which should
> work better than dropping a random packet?

I am not sure of that - choke_enqueue checks against a random
skb to drop current skb, and also during congestion. But for
my "sample driver xmit", returning TX_BUSY could still allow
to be used with CHOKe.

thanks,

- KK