Re: [Xen-devel] large packet support in netfront driver and guest network throughput

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Sep 13, 2013 at 05:09:48PM +0000, Anirban Chakraborty wrote:
> On Sep 13, 2013, at 4:44 AM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> 
> > On Thu, Sep 12, 2013 at 05:53:02PM +0000, Anirban Chakraborty wrote:
> >> Hi All,
> >> 
> >> I am sure this has been answered somewhere in the list in the past, but I 
> >> can't find it. I was wondering if the linux guest netfront driver has GRO 
> >> support in it. tcpdump shows packets coming in with 1500 bytes, although 
> >> the eth0 in dom0 and the vif corresponding to the linux guest in dom0 is 
> >> showing that they receive large packet:
> >> 
> >> In dom0:
> >> eth0      Link encap:Ethernet  HWaddr 90:E2:BA:3A:B1:A4  
> >>          UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
> >> tcpdump -i eth0 -nnvv -s 1500 src 10.84.20.214
> >> 17:38:25.155373 IP (tos 0x0, ttl 64, id 54607, offset 0, flags [DF], proto 
> >> TCP (6), length 29012)
> >>    10.84.20.214.51041 > 10.84.20.213.5001: Flags [.], seq 276592:305552, 
> >> ack 1, win 229, options [nop,nop,TS val 65594025 ecr 65569225], length 
> >> 28960
> >> 
> >> vif4.0    Link encap:Ethernet  HWaddr FE:FF:FF:FF:FF:FF  
> >>          UP BROADCAST RUNNING NOARP PROMISC  MTU:1500  Metric:1
> >> tcpdump -i vif4.0 -nnvv -s 1500 src 10.84.20.214
> >> 17:38:25.156364 IP (tos 0x0, ttl 64, id 54607, offset 0, flags [DF], proto 
> >> TCP (6), length 29012)
> >>    10.84.20.214.51041 > 10.84.20.213.5001: Flags [.], seq 276592:305552, 
> >> ack 1, win 229, options [nop,nop,TS val 65594025 ecr 65569225], length 
> >> 28960
> >> 
> >> 
> >> In the guest:
> >> eth0      Link encap:Ethernet  HWaddr CA:FD:DE:AB:E1:E4  
> >>          inet addr:10.84.20.213  Bcast:10.84.20.255  Mask:255.255.255.0
> >>          inet6 addr: fe80::c8fd:deff:feab:e1e4/64 Scope:Link
> >>          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> >> tcpdump -i eth0 -nnvv -s 1500 src 10.84.20.214
> >> 10:38:25.071418 IP (tos 0x0, ttl 64, id 15074, offset 0, flags [DF], proto 
> >> TCP (6), length 1500)
> >>    10.84.20.214.51040 > 10.84.20.213.5001: Flags [.], seq 17400:18848, ack 
> >> 1, win 229, options [nop,nop,TS val 65594013 ecr 65569213], length 1448
> >> 
> >> Is the packet on transfer from netback to net front is segmented into MTU 
> >> size? Is GRO not supported in the guest?
> > 
> > Here is what I see in the guest, iperf server running in guest and iperf
> > client running in Dom0. Tcpdump runs with the rune you provided.
> > 
> > 10.80.238.213.38895 > 10.80.239.197.5001: Flags [.], seq
> > 5806480:5818064, ack 1, win 229, options [nop,nop,TS val 21968973 ecr
> > 21832969], length 11584
> > 
> > This is a upstream kernel. The throughput from Dom0 to DomU is ~7.2Gb/s.
> 
> Thanks for your reply. The tcpdump was captured on dom0 of the guest [at both 
> vif and the physical interfaces] , i.e. on the receive path of the server. 
> iperf server was running on the guest (10.84.20.213) and the client was at 
> another guest (on a different server) with IP 10.84.20.214. The traffic was 
> between two guests, not between dom0 and the guest.
> 
> > 
> >> 
> >> I am seeing extremely low throughput on a 10Gb/s link. Two linux guests 
> >> (Centos 6.4 64bit, 4 VCPU and 4GB of memory) are running on two different 
> >> XenServer 6.1s and iperf session between them shows at most 3.2 Gbps. 
> > 
> > XenServer might use different Dom0 kernel with their own tuning. You can
> > also try to contact XenServer support for better idea?
> > 
> 
> XenServer 6.1 is running 2.6.32.43 kernel. Since the issue is in netfront 
> driver, as it appears from the tcpdump, thats why I thought I post it here. 
> Note that checksum offloads of the interfaces (virtual and physical) were not 
> even touched, the default setting (which was set to on) was used.
> 
> > In general, off-host communication can be affected by various things. It
> > would be quite useful to identify the bottleneck first.
> > 
> > Try to run:
> > 1. Dom0 to Dom0 iperf (or you workload)
> > 2. Dom0 to DomU iperf
> > 3. DomU to Dom0 iperf
> 
> I tried dom0 to dom0 and I got 9.4 Gbps, which is what I expected (with GRO 
> turned on in the physical interface). However, when I run guest to guest, 
> things fall off. Is large packet not supported in netfront? I thought 
> otherwise. I looked at the code and I do not see any call to 
> napi_gro_receive(), rather it is using netif_receive_skb(). netback seems to 
> be sending GSO packets to the netfront, but it is being segmented to 1500 
> byte (as it appears from the tcpdump).
> 

OK, I get your problem.

Indeed netfront doesn't make use of GRO API at the moment. I've added
this to my list to work on. I will keep you posted when I get to that.

Thanks!

Wei.

> > 
> > In order to get line rate, you need to at least get line rate from Dom0
> > to Dom0 IMHO. 10G/s line rate from guest to guest has not yet been
> > achieved at the momentâ
> 
> What is the current number, without VCPU pinning etc. for 1500 byte MTU? I am 
> getting 2.2-3.2 Gbps for 4VCPU guest with 4GB of memory. It is the only vm 
> running on that server without any other traffic.
> 
> -Anirban
> 

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