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Message-Id: <7ABD5D73-6C1B-423D-9B55-56B351933A86@redhat.com> Date: Mon, 17 Mar 2014 19:21:18 +0200 From: Yan Vugenfirer <yvugenfi@...hat.com> To: Ronen Hod <rhod@...hat.com> Cc: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, Dmitry Fleytman <dfleytma@...hat.com>, kvm@...r.kernel.org, virtio-dev@...ts.oasis-open.org, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Qin Chuanyu <qinchuanyu@...wei.com> Subject: Re: [PATCH net V2] vhost: net: switch to use data copy if pending DMAs exceed the limit On Mar 17, 2014, at 8:43 AM, Ronen Hod <rhod@...hat.com> wrote: > On 03/13/2014 09:28 AM, Jason Wang wrote: >> On 03/10/2014 04:03 PM, Michael S. Tsirkin wrote: >>> On Fri, Mar 07, 2014 at 01:28:27PM +0800, Jason Wang wrote: >>>>> We used to stop the handling of tx when the number of pending DMAs >>>>> exceeds VHOST_MAX_PEND. This is used to reduce the memory occupation >>>>> of both host and guest. But it was too aggressive in some cases, since >>>>> any delay or blocking of a single packet may delay or block the guest >>>>> transmission. Consider the following setup: >>>>> >>>>> +-----+ +-----+ >>>>> | VM1 | | VM2 | >>>>> +--+--+ +--+--+ >>>>> | | >>>>> +--+--+ +--+--+ >>>>> | tap0| | tap1| >>>>> +--+--+ +--+--+ >>>>> | | >>>>> pfifo_fast htb(10Mbit/s) >>>>> | | >>>>> +--+--------------+---+ >>>>> | bridge | >>>>> +--+------------------+ >>>>> | >>>>> pfifo_fast >>>>> | >>>>> +-----+ >>>>> | eth0|(100Mbit/s) >>>>> +-----+ >>>>> >>>>> - start two VMs and connect them to a bridge >>>>> - add an physical card (100Mbit/s) to that bridge >>>>> - setup htb on tap1 and limit its throughput to 10Mbit/s >>>>> - run two netperfs in the same time, one is from VM1 to VM2. Another is >>>>> from VM1 to an external host through eth0. >>>>> - result shows that not only the VM1 to VM2 traffic were throttled but >>>>> also the VM1 to external host through eth0 is also throttled somehow. >>>>> >>>>> This is because the delay added by htb may lead the delay the finish >>>>> of DMAs and cause the pending DMAs for tap0 exceeds the limit >>>>> (VHOST_MAX_PEND). In this case vhost stop handling tx request until >>>>> htb send some packets. The problem here is all of the packets >>>>> transmission were blocked even if it does not go to VM2. >>>>> >>>>> We can solve this issue by relaxing it a little bit: switching to use >>>>> data copy instead of stopping tx when the number of pending DMAs >>>>> exceed half of the vq size. This is safe because: >>>>> >>>>> - The number of pending DMAs were still limited (half of the vq size) >>>>> - The out of order completion during mode switch can make sure that >>>>> most of the tx buffers were freed in time in guest. >>>>> >>>>> So even if about 50% packets were delayed in zero-copy case, vhost >>>>> could continue to do the transmission through data copy in this case. >>>>> >>>>> Test result: >>>>> >>>>> Before this patch: >>>>> VM1 to VM2 throughput is 9.3Mbit/s >>>>> VM1 to External throughput is 40Mbit/s >>>>> CPU utilization is 7% >>>>> >>>>> After this patch: >>>>> VM1 to VM2 throughput is 9.3Mbit/s >>>>> Vm1 to External throughput is 93Mbit/s >>>>> CPU utilization is 16% >>>>> >>>>> Completed performance test on 40gbe shows no obvious changes in both >>>>> throughput and cpu utilization with this patch. >>>>> >>>>> The patch only solve this issue when unlimited sndbuf. We still need a >>>>> solution for limited sndbuf. >>>>> >>>>> Cc: Michael S. Tsirkin <mst@...hat.com> >>>>> Cc: Qin Chuanyu <qinchuanyu@...wei.com> >>>>> Signed-off-by: Jason Wang <jasowang@...hat.com> >>> I thought hard about this. >>> Here's what worries me: if there are still head of line >>> blocking issues lurking in the stack, they will still >>> hurt guests such as windows which rely on timely >>> completion of buffers, but it makes it >>> that much harder to reproduce the problems with >>> linux guests which don't. >>> And this will make even it harder to figure out >>> whether zero copy is actually active to diagnose >>> high cpu utilization cases. >> Yes. >>> >>> So I think this is a good trick, but let's make >>> this path conditional on a new debugging module parameter: >>> how about head_of_line_blocking with default off? >> Sure. But the head of line blocking was only partially solved in the >> patch since we only support in-order completion of zerocopy packets. >> Maybe we need consider switching to out of order completion even for >> zerocopy skbs? > > Yan, Dima, > > I remember that there is an issue with out-of-order packets and WHQL. The test considers out of order packets reception as a failure. Yan. > > Ronen. > >>> This way if we suspect packets are delayed forever >>> somewhere, we can enable that and see guest networking block. >>> >>> Additionally, I think we should add a way to count zero copy >>> and non zero copy packets. >>> I see two ways to implement this: add tracepoints in vhost-net >>> or add counters in tun accessible with ethtool. >>> This can be a patch on top and does not have to block >>> this one though. >>> >> Yes, I post a RFC about 2 years ago, see >> https://lkml.org/lkml/2012/4/9/478 which only traces generic vhost >> behaviours. I can refresh this and add some -net specific tracepoints. >> -- >> To unsubscribe from this list: send the line "unsubscribe netdev" in >> the body of a message to majordomo@...r.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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