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Date: Tue, 21 Jul 2009 14:05:32 +0300
From: Or Gerlitz <ogerlitz@Voltaire.com>
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Subject: Re: [Qemu-devel] [PATCH] net: add raw backend  - some performance
	measurements
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>, Herbert Xu <herbert@gondor.apana.org.au>, netdev@vger.kernel.org, Dor Laor <dlaor@redhat.com>, qemu-devel@nongnu.org, Jan Kiszka <jan.kiszka@web.de>

Michael S. Tsirkin wrote:
> With raw sockets, send will block or fail if the TX queue for device is
> full. With tap+bridge, the buffer in tap has to fill up instead, which
> is not the same. I'm not sure this is the issue here, but could be: the
> benchmark is UDP, isn't it?

Michael, 

What/where is this tap buffer? we're talking on VM TX, so looking on tun_get_user I see a call to 
skb_copy_datagram_from_iovec() to copy from the user buffer to an skb, then a call to netif_rx_ni() and that's it... As for your question, indeed udp, the VM runs netperf/UDP_STREAM

Or.