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

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Date:	Mon, 18 May 2009 19:40:13 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [RFC] virtio: orphan skbs if we're relying on timer to free
 them

From: Rusty Russell <rusty@...tcorp.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@...tcorp.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.
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