Re: [RFC] virtio: orphan skbs if we're relying on timer to free them

[David Miller <davem@davemloft.net>]

From: Rusty Russell <rusty@rustcorp.com.au>
Date: Mon, 18 May 2009 22:18:47 +0930

> We check for finished xmit skbs on every xmit, or on a timer (unless
> the host promises to force an interrupt when the xmit ring is empty).
> This can penalize userspace tasks which fill their sockbuf.  Not much
> difference with TSO, but measurable with large numbers of packets.
> 
> There are a finite number of packets which can be in the transmission
> queue.  We could fire the timer more than every 100ms, but that would
> just hurt performance for a corner case.  This seems neatest.
 ...
> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

If this is so great for virtio it would also be a great idea
universally, but we don't do it.

What you're doing by orphan'ing is creating a situation where a single
UDP socket can loop doing sends and monopolize the TX queue of a
device.  The only control we have over a sender for fairness in
datagram protocols is that send buffer allocation.

I'm guilty of doing this too in the NIU driver, also because there I
lack a "TX queue empty" interrupt and this can keep TCP sockets from
getting stuck.

I think we need a generic solution to this issue because it is getting
quite common to see cases where the packets in the TX queue of a
device can sit there indefinitely.