Date:	Mon, 25 May 2015 15:21:25 -0400
From:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: TCP window auto-tuning sub-optimal in GRE tunnel

On Mon, 2015-05-25 at 12:05 -0700, Eric Dumazet wrote:
> On Mon, 2015-05-25 at 14:49 -0400, John A. Sullivan III wrote:
> > On Mon, 2015-05-25 at 09:58 -0700, Eric Dumazet wrote:
> > > On Mon, 2015-05-25 at 11:42 -0400, John A. Sullivan III wrote:
> > > > Hello, all.  I hope this is the correct list for this question.  We are
> > > > having serious problems on high BDP networks using GRE tunnels.  Our
> > > > traces show it to be a TCP Window problem.  When we test without GRE,
> > > > throughput is wire speed and traces show the window size to be 16MB
> > > > which is what we configured for r/wmem_max and tcp_r/wmem.  When we
> > > > switch to GRE, we see over a 90% drop in throughput and the TCP window
> > > > size seems to peak at around 500K.
> > > > 
> > > > What causes this and how can we get the GRE tunnels to use the max
> > > > window size? Thanks - John
> > > 
> > > Hi John
> > > 
> > > Is it for a single flow or multiple ones ? Which kernel versions on
> > > sender and receiver ? What is the nominal speed of non GRE traffic ?
> > > 
> > > What is the brand/model of receiving NIC  ? Is GRO enabled ?
> > > 
> > > It is possible receiver window is impacted because of GRE encapsulation
> > > making skb->len/skb->truesize ratio a bit smaller, but not by 90%.
> > > 
> > > I suspect some more trivial issues, like receiver overwhelmed by the
> > > extra load of GRE encapsulation.
> > > 
> > > 1) Non GRE session
> > > 
> > > lpaa23:~# DUMP_TCP_INFO=1 ./netperf -H lpaa24 -Cc -t OMNI
> > > OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpaa24.prod.google.com () port 0 AF_INET
> > > tcpi_rto 201000 tcpi_ato 0 tcpi_pmtu 1500 tcpi_rcv_ssthresh 29200
> > > tcpi_rtt 70 tcpi_rttvar 7 tcpi_snd_ssthresh 221 tpci_snd_cwnd 258
> > > tcpi_reordering 3 tcpi_total_retrans 711
> > > Local       Remote      Local  Elapsed Throughput Throughput  Local Local  Remote Remote Local   Remote  Service  
> > > Send Socket Recv Socket Send   Time               Units       CPU   CPU    CPU    CPU    Service Service Demand   
> > > Size        Size        Size   (sec)                          Util  Util   Util   Util   Demand  Demand  Units    
> > > Final       Final                                             %     Method %      Method                          
> > > 1912320     6291456     16384  10.00   22386.89   10^6bits/s  1.20  S      2.60   S      0.211   0.456   usec/KB  
> > > 
> > > 2) GRE session
> > > 
> > > lpaa23:~# DUMP_TCP_INFO=1 ./netperf -H 7.7.7.24 -Cc -t OMNI
> > > OMNI Send TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.7.24 () port 0 AF_INET
> > > tcpi_rto 201000 tcpi_ato 0 tcpi_pmtu 1500 tcpi_rcv_ssthresh 29200
> > > tcpi_rtt 76 tcpi_rttvar 7 tcpi_snd_ssthresh 176 tpci_snd_cwnd 249
> > > tcpi_reordering 3 tcpi_total_retrans 819
> > > Local       Remote      Local  Elapsed Throughput Throughput  Local Local  Remote Remote Local   Remote  Service  
> > > Send Socket Recv Socket Send   Time               Units       CPU   CPU    CPU    CPU    Service Service Demand   
> > > Size        Size        Size   (sec)                          Util  Util   Util   Util   Demand  Demand  Units    
> > > Final       Final                                             %     Method %      Method                          
> > > 1815552     6291456     16384  10.00   22420.88   10^6bits/s  1.01  S      3.44   S      0.177   0.603   usec/KB  
> > > 
> > > 
> > 
> > Thanks, Eric. It really looks like a windowing issue but here is the
> > relevant information:
> > We are measuring single flows.  One side is an Intel GbE NIC connected
> > to a 1 Gbps CIR Internet connection. The other side is an Intel 10 GbE
> > NIC connected to a 40 Gbps Internet connection.  RTT is ~=80ms
> > 
> > The numbers I will post below are from a duplicated setup in our test
> > lab where the systems are connected by GbE links with a netem router in
> > the middle to introduce the latency.  We are not varying the latency to
> > ensure we eliminate packet re-ordering from the mix.
> > 
> > We are measuring a single flow.  Here are the non-GRE numbers:
> > root@...q-1:~# nuttcp -T 60 -i 10 192.168.224.2
> >   666.3125 MB /  10.00 sec =  558.9370 Mbps     0 retrans
> >  1122.2500 MB /  10.00 sec =  941.4151 Mbps     0 retrans
> >   720.8750 MB /  10.00 sec =  604.7129 Mbps     0 retrans
> >  1122.3125 MB /  10.00 sec =  941.4622 Mbps     0 retrans
> >  1122.2500 MB /  10.00 sec =  941.4101 Mbps     0 retrans
> >  1122.3125 MB /  10.00 sec =  941.4668 Mbps     0 retrans
> > 
> >  5888.5000 MB /  60.19 sec =  820.6857 Mbps 4 %TX 13 %RX 0 retrans 80.28 msRTT
> > 
> > For some reason, nuttcp does not show retransmissions in our environment
> > even when they do exist.
> > 
> > gro is active on the send side:
> > root@...q-1:~# ethtool -k eth0
> > Features for eth0:
> > rx-checksumming: on
> > tx-checksumming: on
> >         tx-checksum-ipv4: on
> >         tx-checksum-unneeded: off [fixed]
> >         tx-checksum-ip-generic: off [fixed]
> >         tx-checksum-ipv6: on
> >         tx-checksum-fcoe-crc: off [fixed]
> >         tx-checksum-sctp: on
> > scatter-gather: on
> >         tx-scatter-gather: on
> >         tx-scatter-gather-fraglist: off [fixed]
> > tcp-segmentation-offload: on
> >         tx-tcp-segmentation: on
> >         tx-tcp-ecn-segmentation: off [fixed]
> >         tx-tcp6-segmentation: on
> > udp-fragmentation-offload: off [fixed]
> > generic-segmentation-offload: on
> > generic-receive-offload: on
> > large-receive-offload: off [fixed]
> > rx-vlan-offload: on
> > tx-vlan-offload: on
> > ntuple-filters: off [fixed]
> > receive-hashing: on
> > highdma: on [fixed]
> > rx-vlan-filter: on [fixed]
> > vlan-challenged: off [fixed]
> > tx-lockless: off [fixed]
> > netns-local: off [fixed]
> > tx-gso-robust: off [fixed]
> > tx-fcoe-segmentation: off [fixed]
> > fcoe-mtu: off [fixed]
> > tx-nocache-copy: on
> > loopback: off [fixed]
> > 
> > and on the receive side:
> > root@...tgwingest-1:~# ethtool -k eth5
> > Offload parameters for eth5:
> > rx-checksumming: on
> > tx-checksumming: on
> > scatter-gather: on
> > tcp-segmentation-offload: on
> > udp-fragmentation-offload: off
> > generic-segmentation-offload: on
> > generic-receive-offload: on
> > large-receive-offload: off
> > rx-vlan-offload: on
> > tx-vlan-offload: on
> > ntuple-filters: off
> > receive-hashing: on
> > 
> > The CPU is also lightly utilized.  These are fairly high powered
> > gateways.  We have measure 16 Gbps throughput on them with no strain at
> > all. Checking individual CPUs, we occasionally see one become about half
> > occupied with software interrupts.  
> > 
> > gro is also active on the intermediate netem Linux router.
> > lro is disabled.  I gather there is a bug in the ixgbe driver which can
> > cause this kind of problem if both gro and lro are enabled.
> > 
> > Here are the GRE numbers:
> > root@...q-1:~# nuttcp -T 60 -i 10 192.168.126.1
> >    21.4375 MB /  10.00 sec =   17.9830 Mbps     0 retrans
> >    23.2500 MB /  10.00 sec =   19.5035 Mbps     0 retrans
> >    23.3125 MB /  10.00 sec =   19.5559 Mbps     0 retrans
> >    23.3750 MB /  10.00 sec =   19.6084 Mbps     0 retrans
> >    23.2500 MB /  10.00 sec =   19.5035 Mbps     0 retrans
> >    23.3125 MB /  10.00 sec =   19.5560 Mbps     0 retrans
> > 
> >   138.0000 MB /  60.09 sec =   19.2650 Mbps 9 %TX 6 %RX 0 retrans 80.33 msRTT
> > 
> > 
> > Here is top output during GRE testing on the receive side (which is much
> > lower powered than the send side):
> > 
> > top - 14:37:29 up 200 days, 17:03,  1 user,  load average: 0.21, 0.22, 0.17
> > Tasks: 186 total,   1 running, 185 sleeping,   0 stopped,   0 zombie
> > Cpu0  :  0.0%us,  2.4%sy,  0.0%ni, 93.6%id,  0.0%wa,  0.0%hi,  4.0%si,  0.0%st
> > Cpu1  :  0.0%us,  0.1%sy,  0.0%ni, 99.9%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu2  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu3  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu4  :  0.0%us,  0.0%sy,  0.0%ni, 99.9%id,  0.1%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu5  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu6  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu7  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu8  :  0.0%us,  0.1%sy,  0.0%ni, 99.9%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu9  :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu10 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu11 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu12 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu13 :  0.1%us,  0.0%sy,  0.0%ni, 99.9%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu14 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Cpu15 :  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
> > Mem:  24681616k total,  1633712k used, 23047904k free,   175016k buffers
> > Swap: 25154556k total,        0k used, 25154556k free,  1084648k cached
> > 
> >   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
> > 27014 nobody    20   0  6496  912  708 S    6  0.0   0:02.26 nuttcp
> >     4 root      20   0     0    0    0 S    0  0.0 101:53.42 kworker/0:0
> >    10 root      20   0     0    0    0 S    0  0.0   1020:04 rcu_sched
> >    99 root      20   0     0    0    0 S    0  0.0  11:00.02 kworker/1:1
> >   102 root      20   0     0    0    0 S    0  0.0  26:01.67 kworker/4:1
> >   113 root      20   0     0    0    0 S    0  0.0  24:46.28 kworker/15:1
> > 18321 root      20   0  8564 4516  248 S    0  0.0  80:10.20 haveged
> > 27016 root      20   0 17440 1396  984 R    0  0.0   0:00.03 top
> >     1 root      20   0 24336 2320 1348 S    0  0.0   0:01.39 init
> >     2 root      20   0     0    0    0 S    0  0.0   0:00.20 kthreadd
> >     3 root      20   0     0    0    0 S    0  0.0 217:16.78 ksoftirqd/0
> >     5 root       0 -20     0    0    0 S    0  0.0   0:00.00 kworker/0:0H
> > 
> > A second nuttcp test shows the same but this time we took a tcpdump of
> > the traffic:
> > root@...q-1:~# nuttcp -T 60 -i 10 192.168.126.1
> >    21.2500 MB /  10.00 sec =   17.8258 Mbps     0 retrans
> >    23.2500 MB /  10.00 sec =   19.5035 Mbps     0 retrans
> >    23.3750 MB /  10.00 sec =   19.6084 Mbps     0 retrans
> >    23.2500 MB /  10.00 sec =   19.5035 Mbps     0 retrans
> >    23.3125 MB /  10.00 sec =   19.5560 Mbps     0 retrans
> >    23.3750 MB /  10.00 sec =   19.6083 Mbps     0 retrans
> > 
> >   137.8125 MB /  60.07 sec =   19.2449 Mbps 8 %TX 6 %RX 0 retrans 80.31 msRTT
> > 
> > MSS is 1436
> > Window Scale is 10
> > Window size tops out at 545 = 558080
> > Hmm . . . I would think if I could send 558080 bytes every 0.080s, that
> > would be about 56 Mbps and not 19.5.
> > ip -s -s link ls shows no errors on either side.
> > 
> > I rebooted the receiving side to reset netstat error counters and reran
> > the test with the same results.  Nothing jumped out at me in netstat -s:
> > 
> > TcpExt:
> >     1 invalid SYN cookies received
> >     1 TCP sockets finished time wait in fast timer
> >     187 delayed acks sent
> >     2 delayed acks further delayed because of locked socket
> >     47592 packets directly queued to recvmsg prequeue.
> >     48473682 bytes directly in process context from backlog
> >     90710698 bytes directly received in process context from prequeue
> >     3085 packet headers predicted
> >     88907 packets header predicted and directly queued to user
> >     21 acknowledgments not containing data payload received
> >     201 predicted acknowledgments
> >     3 times receiver scheduled too late for direct processing
> >     TCPRcvCoalesce: 677
> > 
> > Why is my window size so small?
> > Here are the receive side settings:
> > 
> > # increase TCP max buffer size setable using setsockopt()
> > net.core.rmem_default = 268800
> > net.core.wmem_default = 262144
> > net.core.rmem_max = 33564160
> > net.core.wmem_max = 33554432
> > net.ipv4.tcp_rmem = 8960 89600 33564160
> > net.ipv4.tcp_wmem = 4096 65536 33554432
> > net.ipv4.tcp_mtu_probing=1
> > 
> > and here are the transmit side settings:
> > # increase TCP max buffer size setable using setsockopt()
> >   net.core.rmem_default = 268800
> >   net.core.wmem_default = 262144
> >   net.core.rmem_max = 33564160
> >   net.core.wmem_max = 33554432
> >   net.ipv4.tcp_rmem = 8960 89600 33564160
> >   net.ipv4.tcp_wmem = 4096 65536 33554432
> >   net.ipv4.tcp_mtu_probing=1
> >   net.core.netdev_max_backlog = 3000
> > 
> > 
> > Oh, kernel versions:
> > sender: root@...q-1:~# uname -a
> > Linux gwhq-1 3.2.0-4-amd64 #1 SMP Debian 3.2.65-1+deb7u1 x86_64 GNU/Linux
> > 
> > receiver:
> > root@...tgwingest-1:/etc# uname -a
> > Linux testgwingest-1 3.8.0-38-generic #56~precise1-Ubuntu SMP Thu Mar 13 16:22:48 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
> > 
> > Thanks - John
> 
> Nothing seems giving a hint here.
> 
> Coud you post netem setup, and maybe full "tc -s qdisc" output for this
> netem host ?
> 
> 
> Also, you could use nstat at the sender this way, so that we might have
> some clue :
> 
> nstat >/dev/null
> nuttcp -T 60 -i 10 192.168.126.1
> nstat
> 
> 
> 

Thanks, Eric. I really appreciate the help. This is a problem holding up
a very high profile, major project and, for the life of me, I can't
figure out why my TCP window size is reduced inside the GRE tunnel.

Here is the netem setup although we are using this merely to reproduce
what we are seeing in production.  We see the same results bare metal to
bare metal across the Internet.

qdisc prio 10: root refcnt 17 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 32578077286 bytes 56349187 pkt (dropped 15361, overlimits 0 requeues 61323)
 backlog 0b 1p requeues 61323
qdisc netem 101: parent 10:1 limit 1000 delay 40.0ms
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
qdisc netem 102: parent 10:2 limit 1000 delay 40.0ms
 Sent 32434562015 bytes 54180984 pkt (dropped 15361, overlimits 0 requeues 0)
 backlog 0b 1p requeues 0
qdisc netem 103: parent 10:3 limit 1000 delay 40.0ms
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0


root@...ter-001:~# tc -s qdisc show dev eth2
qdisc prio 2: root refcnt 17 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 296515482689 bytes 217794609 pkt (dropped 11719, overlimits 0 requeues 5307)
 backlog 0b 2p requeues 5307
qdisc netem 21: parent 2:1 limit 1000 delay 40.0ms
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
qdisc netem 22: parent 2:2 limit 1000 delay 40.0ms
 Sent 289364020190 bytes 212892539 pkt (dropped 11719, overlimits 0 requeues 0)
 backlog 0b 2p requeues 0
qdisc netem 23: parent 2:3 limit 1000 delay 40.0ms
 Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0

I'm not sure how helpful these stats are as we did set this router up
for packet loss at one point.  We did suspect netem at some point and
did things like change the limit but that had no effect.

I had never used nstat - thank you for pointing it out.  Here is the
output from the sender (which happens to be a production gateway so
there is much more than just the test traffic running on it:

root@...q-1:~# nstat
#kernel
IpInReceives                    318054             0.0
IpForwDatagrams                 161654             0.0
IpInDelivers                    245859             0.0
IpOutRequests                   437620             0.0
IpOutDiscards                   17101577           0.0
IcmpOutErrors                   9                  0.0
IcmpOutTimeExcds                9                  0.0
IcmpMsgOutType3                 9                  0.0
TcpActiveOpens                  2                  0.0
TcpInSegs                       51300              0.0
TcpOutSegs                      105238             0.0
UdpInDatagrams                  14359              0.0
UdpNoPorts                      3                  0.0
UdpOutDatagrams                 34028              0.0
Ip6InReceives                   158                0.0
Ip6InMcastPkts                  158                0.0
Ip6InOctets                     23042              0.0
Ip6InMcastOctets                23042              0.0
TcpExtDelayedACKs               1                  0.0
TcpExtTCPPrequeued              5                  0.0
TcpExtTCPDirectCopyFromPrequeue 310                0.0
TcpExtTCPHPHits                 12                 0.0
TcpExtTCPHPHitsToUser           2                  0.0
TcpExtTCPPureAcks               178                0.0
TcpExtTCPHPAcks                 51083              0.0
IpExtInMcastPkts                313                0.0
IpExtOutMcastPkts               253                0.0
IpExtInBcastPkts                466                0.0
IpExtInOctets                   116579794          0.0
IpExtOutOctets                  281148038          0.0
IpExtInMcastOctets              19922              0.0
IpExtOutMcastOctets             17136              0.0
IpExtInBcastOctets              50192              0.0




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