netdev - Re: [kvm-devel] QEMU PIC indirection patch for in-kernel APIC work

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Date:	Mon, 16 Apr 2007 10:22:46 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Ingo Molnar <mingo@...e.hu>, kvm-devel@...ts.sourceforge.net,
	netdev <netdev@...r.kernel.org>
Subject: Re: [kvm-devel] QEMU PIC indirection patch for in-kernel APIC work

On Thu, 2007-04-12 at 06:32 +0300, Avi Kivity wrote:
> I hadn't considered an always-blocking (or unbuffered) networking API. 
> It's very counter to current APIs, but does make sense with things like
> syslets.  Without syslets, I don't think it's very useful as you need
> some artificial threads to keep things humming along.
> 
> (How would userspace specify it? O_DIRECT when opening the tap?)

TBH, I hadn't thought that far.  Tap already has those IFF_NO_PI etc
flags, but it might make sense to just be the default.  From userspace's
POV it's not a semantic change.

OK, just tested: I can get 230,000 packets (28 byte UDP) through the tun
device in a second (130,000 actually out the 100-base-T NIC, 100,000
dropped).  If the tun driver's write() blocks until the skb is
destroyed, it's 4,000 packets.

So your intuition was right: skb_free latency on xmit (at least for this
e1000) is far too large for anything but an async solution.

Will ponder further.

Thanks!
Rusty.

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