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

On 2013-9-16 22:21, Wei Liu wrote:
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
tcpdump -i eth0 -nnvv -s 1500 src
17:38:25.155373 IP (tos 0x0, ttl 64, id 54607, offset 0, flags [DF], proto TCP 
(6), length 29012) > 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
tcpdump -i vif4.0 -nnvv -s 1500 src
17:38:25.156364 IP (tos 0x0, ttl 64, id 54607, offset 0, flags [DF], proto TCP 
(6), length 29012) > 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:  Bcast:  Mask:
          inet6 addr: fe80::c8fd:deff:feab:e1e4/64 Scope:Link
tcpdump -i eth0 -nnvv -s 1500 src
10:38:25.071418 IP (tos 0x0, ttl 64, id 15074, offset 0, flags [DF], proto TCP 
(6), length 1500) > 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. > 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 ( and the client was at 
another guest (on a different server) with IP 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 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.

This is true.
But I am wondering why large packet is not segmented into mtu size with upstream kernel? I did see large packets with upsteam kernel on receive guest(test between 2 domus on same host).

  I've added
this to my list to work on. I will keep you posted when I get to that.



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.


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