On Wed, Feb 26, 2014 at 03:11:21PM +0800, Jason Wang wrote: > On 02/26/2014 02:32 PM, Qin Chuanyu wrote: > >On 2014/2/26 13:53, Jason Wang wrote: > >>On 02/25/2014 09:57 PM, Michael S. Tsirkin wrote: > >>>On Tue, Feb 25, 2014 at 02:53:58PM +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 the VHOST_MAX_PEND. This is safe because: > >>>> > >>>>- The number of pending DMAs were still limited by VHOST_MAX_PEND > >>>>- 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 > >>>> > >>>>After this patch: > >>>>VM1 to VM2 throughput is 9.3Mbit/s > >>>>Vm1 to External throughput is 93Mbit/s > >>>Would like to see CPU utilization #s as well. > >>> > >> > >>Will measure this. > >>>>Simple performance test on 40gbe shows no obvious changes in > >>>>throughput after this patch. > >>>> > >>>>The patch only solve this issue when unlimited sndbuf. We still need a > >>>>solution for limited sndbuf. > >>>> > >>>>Cc: Michael S. Tsirkin<mst@xxxxxxxxxx> > >>>>Cc: Qin Chuanyu<qinchuanyu@xxxxxxxxxx> > >>>>Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx> > >>>I think this needs some thought. > >>> > >>>In particular I think this works because VHOST_MAX_PEND > >>>is much smaller than the ring size. > >>>Shouldn't max_pend then be tied to the ring size if it's small? > >>> > >> > >>Yes it should. I just reuse the VHOST_MAX_PEND since it was there for a > >>long time. > >>>Another question is about stopping vhost: > >>>ATM it's waiting for skbs to complete. > >>>Should we maybe hunt down skbs queued and destroy them > >>>instead? > >>>I think this happens when a device is removed. > >>> > >>>Thoughts? > >>> > >> > >>Agree, vhost net removal should not be blocked by a skb. But since the > >>skbs could be queued may places, just destroy them may need extra locks. > >> > >>Haven't thought this deeply, but another possible sloution is to rcuify > >>destructor_arg and assign it to NULL during vhost_net removing. > > > >Xen treat it by a timer, for those skbs which has been delivered for a > >while, netback would exchange page of zero_copy's skb with dom0's page. > > > >but there is still a race between host's another process handle the skb > >and netback exchange its page. (This problem has been proved by testing) > > > >and Xen hasn't solved this problem yet, because if anyone want to solve > >this problem completely, a page lock is necessary, but it would be > >complex and expensive. > > > >rcuify destructor arg and assign it to NULL couldn't solve the problem > >of page release that has been reserved by host's another process. > > > > There're two issues: > > 1) if a zerocopy skb won't be freed or frags orphaned in time, > vhost_net removal will be blocked since it was waiting for the > refcnt of ubuf to zero. > 2) whether or not we should free all pending skbs during vhost_net removing. all pending *zero copy* skbs. > My proposal is for issue 1. Another idea is not wait for the refcnt > to be zero and then we can defer the freeing of vhost_net during the > release method of kref_put(). It's not freeing that's at issue. We must not access guest memory after vhost stop, too. > For issue 2, I'm still not sure we should do this or not. Looks like > there's a similar issue for the packets sent by tcp_sendpage() was > blocked or delayed. What's the issue exactly? How would you trigger it? > >The key problem is how to release the memory of zero_copy's skb while > >been reserved. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
