Re: [Xen-devel] Poor network performance between DomU with multiqueue support

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Dec 03, 2014 at 02:43:37PM +0000, Zhangleiqiang (Trump) wrote:
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: Tuesday, December 02, 2014 11:59 PM
> > To: Zhangleiqiang (Trump)
> > Cc: Wei Liu; zhangleiqiang; xen-devel@xxxxxxxxxxxxx; Luohao (brian); 
> > Xiaoding
> > (B); Yuzhou (C); Zhuangyuxin
> > Subject: Re: [Xen-devel] Poor network performance between DomU with
> > multiqueue support
> > 
> > On Tue, Dec 02, 2014 at 02:46:36PM +0000, Zhangleiqiang (Trump) wrote:
> > > Thanks for your reply, Wei.
> > >
> > > I do the following testing just now and found the results as follows:
> > >
> > > There are three DomUs (4U4G) are running on Host A (6U6G) and one DomU
> > (4U4G) is running on Host B (6U6G), I send packets from three DomUs to the
> > DomU on Host B simultaneously.
> > >
> > > 1. The "top" output of Host B as follows:
> > >
> > > top - 09:42:11 up  1:07,  2 users,  load average: 2.46, 1.90, 1.47
> > > Tasks: 173 total,   4 running, 169 sleeping,   0 stopped,   0 zombie
> > > %Cpu0  :  0.0 us,  0.0 sy,  0.0 ni, 97.3 id,  0.0 wa,  0.0 hi,  0.8
> > > si,  1.9 st
> > > %Cpu1  :  0.0 us, 27.0 sy,  0.0 ni, 63.1 id,  0.0 wa,  0.0 hi,  9.5
> > > si,  0.4 st
> > > %Cpu2  :  0.0 us, 90.0 sy,  0.0 ni,  8.3 id,  0.0 wa,  0.0 hi,  1.7
> > > si,  0.0 st
> > > %Cpu3  :  0.4 us,  1.4 sy,  0.0 ni, 95.4 id,  0.0 wa,  0.0 hi,  1.4
> > > si,  1.4 st
> > > %Cpu4  :  0.0 us, 60.2 sy,  0.0 ni, 39.5 id,  0.0 wa,  0.0 hi,  0.3
> > > si,  0.0 st
> > > %Cpu5  :  0.0 us,  2.8 sy,  0.0 ni, 89.4 id,  0.0 wa,  0.0 hi,  6.9 si,  
> > > 0.9
> > st
> > > KiB Mem:   4517144 total,  3116480 used,  1400664 free,      876
> > buffers
> > > KiB Swap:  2103292 total,        0 used,  2103292 free.  2374656
> > cached Mem
> > >
> > >   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM
> > TIME+ COMMAND
> > >  7440 root      20   0       0      0      0 R 71.10 0.000
> > 8:15.38 vif4.0-q3-guest
> > >  7434 root      20   0       0      0      0 R 59.14 0.000
> > 9:00.58 vif4.0-q0-guest
> > >    18 root      20   0       0      0      0 R 33.89 0.000
> > 2:35.06 ksoftirqd/2
> > >    28 root      20   0       0      0      0 S 20.93 0.000
> > 3:01.81 ksoftirqd/4
> > >
> > >
> > > As shown above, only two netback related processes (vif4.0-*) are running
> > with high cpu usage, and the other 2 netback processes are idle. The "ps"
> > result of vif4.0-* processes as follows:
> > >
> > > root      7434 50.5  0.0      0     0 ?        R    09:23  11:29
> > [vif4.0-q0-guest]
> > > root      7435  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q0-deall]
> > > root      7436  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q1-guest]
> > > root      7437  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q1-deall]
> > > root      7438  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q2-guest]
> > > root      7439  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q2-deall]
> > > root      7440 48.1  0.0      0     0 ?        R    09:23  10:55
> > [vif4.0-q3-guest]
> > > root      7441  0.0  0.0      0     0 ?        S    09:23   0:00
> > [vif4.0-q3-deall]
> > > root      9724  0.0  0.0   9244  1520 pts/0    S+   09:46   0:00
> > grep --color=auto
> > >
> > >
> > > 2. The "rx" related content in /proc/interupts in receiver DomU (on Host 
> > > B):
> > >
> > > 73:       2               0               2925405         0               
> > >         xen-dyn-event
> >     eth0-q0-rx
> > > 75:       43              93              0                       118     
> > >                 xen-dyn-event
> >     eth0-q1-rx
> > > 77:       2               3376    14                      1983            
> > > xen-dyn-event
> >     eth0-q2-rx
> > > 79:       2414666 0               9                       0               
> > >         xen-dyn-event
> >     eth0-q3-rx
> > >
> > > As shown above, it seems like that only q0 and q3 handles the interrupt
> > triggered by packet receving.
> > >
> > > Any advise? Thanks.
> > 
> > Netback selects queue based on the return value of skb_get_queue_mapping.
> > The queue mapping is set by core driver or ndo_select_queue (if specified by
> > individual driver). In this case netback doesn't have its implementation of
> > ndo_select_queue, so it's up to core driver to decide which queue to 
> > dispatch
> > the packet to.  I think you need to inspect why Dom0 only steers traffic to
> > these two queues but not all of them.
> > 
> > Don't know which utility is handy for this job. Probably tc(8) is useful?
> 
> Thanks Wei.
> 

> I think the reason for the above results that only two
> netback/netfront processes works hard is the queue select method. I
> have tried to send packets from multiple host/vm to a vm, and all of
> the netback/netfront processes are running with high cpu usage a few
> times.
> 

A few times? You might want to check some patches to rework RX stall
detection by David Vrabel that went in after 3.16.

> However, I find another issue. Even using 6 queues and making sure
> that all of these 6 netback processes running with high cpu usage
> (indeed, any of it running with 87% cpu usage), the whole VM receive
> throughout is not very higher than results when using 4 queues. The
> results are from 4.5Gbps to 5.04 Gbps using TCP with 512 bytes length
> and 4.3Gbps to 5.78Gbps using TCP with 1460 bytes length.
> 

I would like to ask if you're still using 4U4G (4 CPU 4 G?)
configuration? If so, please make sure there are at least the same
number of vcpus as queues.

> According to the testing result from WIKI:
> http://wiki.xen.org/wiki/Xen-netback_and_xen-netfront_multi-queue_performance_testing,
> The VM receive throughput is also more lower than VM transmit. 
> 

I think that's expected, because guest RX data path still uses
grant_copy while guest TX uses grant_map to do zero-copy transmit.

> I am wondering why the VM receive throughout cannot be up to 8-10Gbps
> as VM transmit under multi-queue?  I also tried to send packets
> directly from Local Dom0 to DomU, the DomU receive throughput can
> reach about 8-12Gbps, so I am also wondering why transmitting packets
> from Dom0 to Remote DomU can only reach about 4-5Gbps throughout?

If data is from Dom0 to DomU then SKB is probably not fragmented by
network stack.  You can use tcpdump to check that.

Wei.

> 
> > Wei.
> > 
> > > ----------
> > > zhangleiqiang (Trump)
> > >
> > > Best Regards
> > >
> > >
> > > > -----Original Message-----
> > > > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > > > Sent: Tuesday, December 02, 2014 8:12 PM
> > > > To: Zhangleiqiang (Trump)
> > > > Cc: Wei Liu; zhangleiqiang; xen-devel@xxxxxxxxxxxxx; Luohao (brian);
> > > > Xiaoding (B); Yuzhou (C); Zhuangyuxin
> > > > Subject: Re: [Xen-devel] Poor network performance between DomU with
> > > > multiqueue support
> > > >
> > > > On Tue, Dec 02, 2014 at 11:50:59AM +0000, Zhangleiqiang (Trump) wrote:
> > > > > > -----Original Message-----
> > > > > > From: xen-devel-bounces@xxxxxxxxxxxxx
> > > > > > [mailto:xen-devel-bounces@xxxxxxxxxxxxx] On Behalf Of Wei Liu
> > > > > > Sent: Tuesday, December 02, 2014 7:02 PM
> > > > > > To: zhangleiqiang
> > > > > > Cc: wei.liu2@xxxxxxxxxx; xen-devel@xxxxxxxxxxxxx
> > > > > > Subject: Re: [Xen-devel] Poor network performance between DomU
> > > > > > with multiqueue support
> > > > > >
> > > > > > On Tue, Dec 02, 2014 at 04:30:49PM +0800, zhangleiqiang wrote:
> > > > > > > Hi, all
> > > > > > >     I am testing the performance of xen netfront-netback
> > > > > > > driver that with
> > > > > > multi-queues support. The throughput from domU to remote dom0 is
> > > > > > 9.2Gb/s, but the throughput from domU to remote domU is only
> > > > > > 3.6Gb/s, I think the bottleneck is the throughput from dom0 to
> > > > > > local domU. However, we have done some testing and found the
> > > > > > throughput from dom0 to local domU is 5.8Gb/s.
> > > > > > >     And if we send packets from one DomU to other 3 DomUs on
> > > > > > > different
> > > > > > host simultaneously, the sum of throughout can reach 9Gbps. It
> > > > > > seems like the bottleneck is the receiver?
> > > > > > >     After some analysis, I found that even the max_queue of
> > > > > > > netfront/back
> > > > > > is set to 4, there are some strange results as follows:
> > > > > > >     1. In domU, only one rx queue deal with softirq
> > > > > >
> > > > > > Try to bind irq to different vcpus?
> > > > >
> > > > > Do you mean we try to bind irq to different vcpus in DomU? I will try 
> > > > > it
> > now.
> > > > >
> > > >
> > > > Yes. Given the fact that you have two backend threads running while
> > > > only one DomU vcpu is busy, it smells like misconfiguration in DomU.
> > > >
> > > > If this phenomenon persists after correctly binding irqs, you might
> > > > want to check traffic is steering correctly to different queues.
> > > >
> > > > > >
> > > > > > >     2. In dom0, only two netback queues process are scheduled,
> > > > > > > other two
> > > > > > process aren't scheduled.
> > > > > >
> > > > > > How many Dom0 vcpu do you have? If it only has two then there
> > > > > > will only be two processes running at a time.
> > > > >
> > > > > Dom0 has 6 vcpus, and 6G memory. There are only one DomU running
> > > > > in
> > > > Dom0 and so four netback processes are running in Dom0 (because the
> > > > max_queue param of netback kernel module is set to 4).
> > > > > The phenomenon is that only 2 of these four netback process were
> > > > > running
> > > > with about 70% cpu usage, and another two use little CPU.
> > > > > Is there a hash algorithm to determine which netback process to
> > > > > handle the
> > > > input packet?
> > > > >
> > > >
> > > > I think that's whatever default algorithm Linux kernel is using.
> > > >
> > > > We don't currently support other algorithms.
> > > >
> > > > Wei.

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