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Calculating network throughput

Author(s): @dcavalheiro @bernieongewe ,   Published: 2019-08-07

Daniel Cavalheiro and Bernie Ongewe | Google

Contributed by Google employees.

This document describes how to calculate network throughput, both within Google Cloud and to your on-premises or third-party cloud locations. This document includes information on how to analyze results, explanations of variables that can affect network performance, and troubleshooting tips.

Common use cases

Choosing and configuring your applications and machine types for optimal network throughput is essential for balancing costs and networking performance.

Here are the common use cases for a network throughput assessment:

Know your tools

When calculating network throughput, it's important to use well-tested, well-documented tools:

  • iPerf3: A commonly used network testing tool that can create TCP/UDP data streams (single-thread or multi-thread) and measure the throughput of the network that carries them.

    Note: iPerf3 may only be appropriate for single-CPU machines.

  • Netperf: Similar to iPerf3 but more appropriate for throughput testing on multi-CPU instances that are CPU-bound on a single CPU.

  • traceroute: A computer network diagnostic tool that measures and displays the routes that packets take across a network. The traceroute tool records the route's history as the round-trip times of packets received from each successive host in a route. The sum of the mean times in each hop is an approximate measure of the total time spent to establish the TCP connection.

  • tcpdump: A command-line packet analyzer that captures packet details and TCP/IP communications for more advanced troubleshooting. tcpdump is compatible with other tools, such as Wireshark.

  • TCP throughput calculator: A calculator on the SWITCH Foundation website that measures theoretical network limits based on the TCP window and RTT.

  • Perfkit Benchmarker: A tool that contains a set of benchmarks to measure and compare cloud offerings. The benchmarks use defaults to reflect what most users will see.

  • gcping: A command-line tool that provides median latencies to Google Cloud regions.

Round-trip time, latency, and distances

When analyzing network performance, one of the most common mistakes is to measure data transfer speed without considering the characteristics of the network or the actual distance between network endpoints. Network performance analysis is highly dependent on factors such as latency and distance. You need to consider these real-world characteristics, and not rely on simple assumptions.

For example, the speed of light traveling in a vacuum is around 300,000 km/s, but the speed of light through the fiber-optic cable that makes up most of the internet is approximatey 200,000 km/s.

Another important consideration is that the distance between two virtual machines (VMs) isn't normally a straight line, but is instead a complex maze of connections, increasing the total distance traveled. This is analogous to the fact that the distance between New York City and Los Angeles is almost 3,944 km in a straight line, but driving on roads increases this value to at least 4,466 km.

Because of the bandwidth-delay product, even connections with large bandwidth capabilities will perform poorly when tested via TCP, since their TCP window may not allow for full link utilization.

Choosing the closest Google Cloud location to your on-premises location is the best approach to reduce latency.

You can use the gcping command-line tool to determine median latencies from any location to various Google Cloud regions.

Egress per virtual machine limit (all network interfaces)

To choose the best egress bandwidth cap for your application, refer to the per-instance VPC resource limits for the maximum egress data rate.

Single-thread and multi-thread connection modes

Another important concept is the use of single or parallel threads when sending data over a network connection. For example, a machine's CPU can process network requests over a single or over multiple processor threads. Multi-connection, multi-thread, or parallel-thread mode gives you maximum potential speed by using multiple CPU threads in parallel to send network packets.

For example, if you sequentially download a single file from start to end, you use a single thread, but when you download many smaller files, or break a large file into small chunks and download them at the same time, each thread is responsible for each file or chunk.

It is possible to achieve values close to the maximum egress data rate using multi-thread mode. However, because the maximum egress data rate is a limit, the data rate will not go over the maximum egress data rate, even under ideal conditions.

Single-connection mode is best for testing over a VPN (non-HA) or simulating the download of a single file. Expect it to have lower transfer speeds than multi-thread mode.

Measuring throughput with iPerf3

Follow this procedure to measure throughput from the perspective of a single virtual machine.

Choose a large machine type

To perform throughput tests, we recommend that you use a large machine type, such as n1-standard-8. This machine type provides a maximum egress throughput limit of 16 Gbps, so the per-VM egress throughput will not interfere with the tests.

Install tcpdump and iPerf3

Install tcpdump and iPerf3 on a Linux VM machine.

To install both pieces of software on a Debian or Ubuntu system, run the following commands:

sudo apt-get update
sudo apt-get install tcpdump
sudo apt-get install iperf3

Run an iPerf3 server

The iPerf3 server accepts connections on both the VM's internal address and (if configured) its external IP address. By default, the server listens on port 5201 for both TCP and UDP traffic. To change the port, use the -p flag.

iperf3 -s -p [PORT_NUMBER]

The -s flag tells iPerf3 to run as a server.

Allow the server and client communication through the Google Cloud firewall

Create a firewall rule to the iPerf3 server to allow ingress TCP on the selected port, as described in Creating firewall rules.

Run an iPerf3 client

Run iPerf3 with the -c (client) flag and specify the destination IP address of the iPerf3 server. By default, iPerf3 uses TCP, unless you specify the -u (UDP) flag for the client. No such flag is needed server-side for UDP; however, a firewall rule allowing incoming UDP traffic to the server is required.

If you run the server on a custom port, you need to specify that same port using the -p (port) flag. If you omit the port flag, the client assumes that the destination port is 5201.

Use the -P (parallel threads) flag to specify a number of simultaneous threads, and use the -t (time) flag to specify the duration of the test, in seconds.

iperf3 -c [VM_IP] -P [THREADS] -p [PORT_NUMBER] -t [DURATION] -R

The command above uses the autotuning path. To make explicit setsockopt() calls, use this command instead:

iperf3 -c [VM_IP] -P [THREADS] -p [PORT_NUMBER] -t [DURATION] -R -w

Capturing packets

It's sometimes helpful to get packet captures on the iPerf3 client or server. The following example command shows you how to capture full packets for a given destination port range from an eth0 interface, saving a file in the working directory called mycap.pcap.

sudo /usr/sbin/tcpdump -s 100 -i eth0 dst port [PORT_NUMBER] -w mycap.pcap

Important: If you use the -s (snapshot length) flag with a value of 0, then tcpdump will capture entire packets. Be aware that the packet captures can contain sensitive information. Most tcpdump implementations interpret -s 0 to be the same as -s 262144. See the tcpdump man page for details. To reduce the chances of capturing sensitive information, you can capture just packet headers by providing a lower snapshot length value, such as -s 100.

Modify this command to suit your use case; for example, the interface name isn't always eth0 on all distributions, and you'll need to specifically choose the right interface for a multiple network interface VM.

Limitations of using iPerf3

CPU throttling is likely to be a bottleneck when testing with iperf3. The -P flag launches multiple client streams, but these all run in a single thread.

From the iperf3 man pages:

> -P, --parallel n \
> number of parallel client streams to run. Note that iperf3 is single threaded,
> so if you are CPU bound, this will not yield higher throughput.

Also, any attempt to launch multiple clients returns the following complaint on the second attempt:

$ iperf3 -c 10.150.0.12 -t 3600 \
iperf3: error - the server is busy running a test. try again later

Allowing only a single process to run is also by design choice.

To exercise multiple CPUs, use Netperf.

Measuring throughput with multiple CPUs with Netperf

Install Netperf:

sudo apt-get install git build-essential autoconf texinfo -y
git clone https://github.com/HewlettPackard/netperf.git
cd netperf
./autogen.sh
./configure --enable-histogram --enable-demo=yes
make

Start the server:

src/netserver

Start the client:

src/netperf -D $reporting_interval -H $server_ip -f $reporting_units

The following is an example command to run the Netperf client:

src/netperf -D 2 -H 10.150.0.12 -f G

The following is an example of output from Netperf:

MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 10.150.0.12
\
() port 0 AF_INET : histogram : demo Interim result: 1.66 GBytes/s over 2.444
\
seconds ending at 1582948175.660 Interim result: 1.81 GBytes/s over 2.008 \
seconds ending at 1582948177.669 Interim result: 1.81 GBytes/s over 2.000 \
seconds ending at 1582948179.669 Interim result: 1.81 GBytes/s over 2.001 \
seconds ending at 1582948181.670 Interim result: 1.80 GBytes/s over 1.546 \
seconds ending at 1582948183.216 Recv Send Send Socket Socket Message Elapsed
\
Size Size Size Time Throughput bytes bytes bytes secs. GBytes/sec \
\
87380 16384 16384 10.00 1.77

You can then start multiple clients, assigning them to different CPUs. The example below starts 10 processes and assigns them to the respective CPUs:

for i in {1..10}
do
taskset -c $i src/netperf -D 2 -H [server_IP_address] -f G &
done

Measuring VPC network throughput with PerfKit Benchmarker

To measure network throughput performance from a given machine type in a specific zone, use PerfKit Benchmarker. This tool is valuable because it creates an instance and measures its performance, without the need to install the tools on an existing VM.

Replace the placeholders in the commands below with the following:

  • [MACHINE_TYPE]: The machine type that you want to test (for example, n1-standard-32).
  • [ZONE]: The zone to create the instance in.
  • [NUMBER_OF_VCPUS]: The number of vCPUs of the instance (for example, 32 for the n1-standard-32 machine type).

Measure single-thread performance:

./pkb.py --cloud=GCP --machine_type=[MACHINE_TYPE]
--benchmarks=iperf --ip_addresses=INTERNAL --zones=[ZONE]

Measure multi-thread performance:

./pkb.py --cloud=GCP --machine_type=,[MACHINE_TYPE]
--benchmarks=iperf --ip_addresses=INTERNAL --zones=[ZONE]
--iperf_sending_thread_count=[NUMBER_OF_VCPUS]

Speed test

To measure network data transfer speed from a VM to the global internet, you can choose from many available tools, including the following:

Troubleshooting

This section includes tips for investigating and troubleshooting some common issues that may occur when measuring network throughput.

iPerf3 server and client are not able to establish a connection

  1. Verify that the firewall rules allow ingress and egress traffic to and from the VM on your VPC.
  2. If using multiple VPCs, the firewall must allow traffic on the selected port for all VPCs.
  3. Selecting the nic0 of the instance will open a console page with the breakdown of the firewall rules and route that affect this specific instance, which is a very valuable source of information.

TCP window size and RTT not optimized

If you are not able to realize your full connection bandwidth over a TCP connection, the cause may be TCP send and receive windows. For an explanation of this behavior, see this video on the bandwidth delay problem. You can use the TCP throughput calculator to understand the impact of window size on your connection bandwidth.

There are two different sets of sysctls in Linux that affect TCP window sizes:

The first set is the net.core set, which come into play when applications such as iperf3 make explicit setsockopt() calls to set SO_SNDBUF or SO_RCVBUF. Making explicit setsockopt() calls for SO_SNDBUF or SO_RCVBUF disables Linux's autotuning of socket buffers and thus TCP window size. The following are the tuning options for this case:

  • SO_RCVBUF is influenced by increasing net.core.rmem_default and net.core.rmem_max.
  • SO_SNDBUF is influenced by increasing net.core.wmem_default and net.core.wmem_max.

The second set is the net.ipv4 set, which come into play when the application does not make explicit setsockopt() calls, and so when Linux's autotuning of socket buffer and thus TCP window size is still active:

  • net.ipv4.tcp_rmem
  • net.ipv4.tcp_wmem

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