From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: Interesting observation with network event notification and batching Date: Mon, 17 Jun 2013 10:38:33 +0100 Message-ID: <1371461913.3967.68.camel@zakaz.uk.xensource.com> References: <20130612101451.GF2765@zion.uk.xensource.com> <20130614185303.GC21280@phenom.dumpdata.com> <20130616095433.GA27462@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130616095433.GA27462@zion.uk.xensource.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Wei Liu Cc: annie.li@oracle.com, xen-devel@lists.xen.org, andrew.bennieston@citrix.com, stefano.stabellini@eu.citrix.com, Konrad Rzeszutek Wilk List-Id: xen-devel@lists.xenproject.org On Sun, 2013-06-16 at 10:54 +0100, Wei Liu wrote: > > > Konrad, IIRC you once mentioned you discovered something with event > > > notification, what's that? > > > > They were bizzare. I naively expected some form of # of physical NIC > > interrupts to be around the same as the VIF or less. And I figured > > that the amount of interrupts would be constant irregardless of the > > size of the packets. In other words #packets == #interrupts. > > > > It could be that the frontend notifies the backend for every packet it > sends. This is not desirable and I don't expect the ring to behave that > way. It is probably worth checking that things are working how we think they should. i.e. that netback's calls to RING_FINAL_CHECK_FOR_.. and netfront's calls to RING_PUSH_..._AND_CHECK_NOTIFY are placed at suitable points to maximise batching. Is the RING_FINAL_CHECK_FOR_REQUESTS inside the xen_netbk_tx_build_gops loop right? This would push the req_event pointer to just after the last request, meaning the net request enqueued by the frontend would cause a notification -- even though the backend is actually still continuing to process requests and would have picked up that packet without further notification. n this case there is a fair bit of work left in the backend for this iteration i.e. plenty of opportunity for the frontend to queue more requests. The comments in ring.h say: * These macros will set the req_event/rsp_event field to trigger a * notification on the very next message that is enqueued. If you want to * create batches of work (i.e., only receive a notification after several * messages have been enqueued) then you will need to create a customised * version of the FINAL_CHECK macro in your own code, which sets the event * field appropriately. Perhaps we want to just use RING_HAS_UNCONSUMED_REQUESTS in that loop (and other similar loops) and add a FINAL check at the very end? > > But it was odd and I didn't go deeper in it to figure out what > > was happening. And also to figure out if for the VIF we could > > do something of #packets != #interrupts. And hopefully some > > mechanism to adjust so that the amount of interrupts would > > be lesser per packets (hand waving here). > > I'm trying to do this now. What scheme do you have in mind?