From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark McLoughlin Subject: Re: [PATCH 3/4] virtio_net: don't free buffers in xmit ring Date: Tue, 02 Jun 2009 09:13:42 +0100 Message-ID: <1243930422.9146.60.camel@blaa> References: <200905292346.24141.rusty@rustcorp.com.au> Reply-To: Mark McLoughlin Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, virtualization@lists.linux-foundation.org To: Rusty Russell Return-path: Received: from mx2.redhat.com ([66.187.237.31]:54271 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754401AbZFBINq (ORCPT ); Tue, 2 Jun 2009 04:13:46 -0400 In-Reply-To: <200905292346.24141.rusty@rustcorp.com.au> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, 2009-05-29 at 23:46 +0930, Rusty Russell wrote: > The virtio_net driver is complicated by the two methods of freeing old > xmit buffers (in addition to freeing old ones at the start of the xmit > path). > > The original code used a 1/10 second timer attached to xmit_free(), > reset on every xmit. Before we orphaned skbs on xmit, the > transmitting userspace could block with a full socket until the timer > fired, the skb destructor was called, and they were re-woken. The timer was actually added to solve a hang when trying to unload nf_conntrack AFAIR - nf_conntrack was blocking on the skb being freed and we never actually freed it. I think skb_orphan() is enough to prevent this, is it? > So we added the VIRTIO_F_NOTIFY_ON_EMPTY feature: supporting devices > send an interrupt (even if normally suppressed) on an empty xmit ring > which makes us schedule xmit_tasklet(). This was a benchmark win. > > Unfortunately, VIRTIO_F_NOTIFY_ON_EMPTY makes quite a lot of work: a > host which is faster than the guest will fire the interrupt every xmit > packet (slowing the guest down further). Ouch. So, does simply disabling host support for VIRTIO_F_NOTIFY_ON_EMPTY speed up current guests? Cheers, Mark.