How fast is 127.0.0.1?

Question

I wonder if it is tied to my NIC at all or if the OS or driver intercepts and immediately returns data sent to the loopback address?

Does the signal actually travel to my NIC then the NIC returns it?

Answer 1

All 127.xx.xx.xx traffic never hits the physical network, it gets processed by a loop back adapter in the kernel.

Answer 2

You don't mention a particular OS but for most all that happens is that the data travels down the stack until it gets to IP at which point it's pretty much sent back. That's a massive oversimplification but means that the entire process is usually CPU bound so its performance is therefore directly linked to CPU speed plus stack efficiency. In practical terms modern CPUs and OSs should be able to 'bounce' loopback traffic considerably faster than 40Gbps - which is the fastest NIC I think I'm capable of buying today. Hope this helps.

Answer 3

You may be interested in the "Loopback Fast Path" feature Microsoft added in Windows 8 and Windows Server 2012. See:

http://blogs.technet.com/b/wincat/archive/2012/12/05/fast-tcp-loopback-performance-and-low-latency-with-windows-server-2012-tcp-loopback-fast-path.aspx

It also provides an illustration of the path taken by the loopback with and without the optimization.

Answer 4

127.0.0.1 is no faster any other local ip. The local ip fast, because it doesn't use physical device and 2-layers.( It never hits your NIC)

iperf -c 109.191.109.xxx 1111 -t 5
iperf: ignoring extra argument -- 1111
------------------------------------------------------------
Client connecting to 109.191.109.241, TCP port 5001
TCP window size: 49.7 KByte (default)
------------------------------------------------------------
[  3] local 109.191.109.xxx port 32840 connected with 109.191.109.xxx port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 5.0 sec  4.72 GBytes  8.12 Gbits/sec

$ iperf -c 127.0.0.1 1111 -t 5
iperf: ignoring extra argument -- 1111
------------------------------------------------------------
Client connecting to 127.0.0.1, TCP port 5001
TCP window size: 49.7 KByte (default)
------------------------------------------------------------
[  3] local 127.0.0.1 port 56482 connected with 127.0.0.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 5.0 sec  4.62 GBytes  7.94 Gbits/sec

Answer 5

It depends on what OS you're running, but Windows 2000 had a bug in it where requests on the loopback adaptor would be slow. There's some useless trivia for you!

Please see this kb article for more information.

Answer 6

As stated above, on Linux machine at least, both localhost and any other local address bypasses the NIC drivers - it is directly served by the kernel IP stack (sidenote: many ethernet hardware MAC are simply not capable of reading back an outgoing packet).

What speed can be achieved over a loopback interface? I run some test via iperf3 -s & iperf3 -c localhost on the following machines:

• an old Intel i7-860@2.8 GHz achieves 23 Gb/s
• a more recent Intel Pentium G3260@3.3 GHz achieves 26 Gb/s
• a Ryzen 7 1700@3.0 GHz achieves 29.4 Gb/s

All three machines above had a CPU utilization of about 150% (ie: 1.5 cores were used). A faster processor (with higher single-thread speed) will obviously give greater performance. Anyway, at these speed memory copy operations are a significant bottleneck: re-running the same test on the Ryzen machine using a "zero-copy transfer" (iperf3 -Z -c localhost) resulted in >38 Gb/s

Answer 7

I'd rather say that you find your answer in it's name itself. The name says: "Local Loopback Address", which in itself means that the network never intervenes in the process and the request is in itself looped back locally.

Answer 8

If you are on Linux...

type in "ifconfig -a" as root...

Notice the "interrupt" line on all NICs...notice there is no "interrupt" line on "lo"...that means it does not even have an interrupt assigned to it, so pretty darn fast ;-)

Answer 9

Localhost "local loopback" runs as fast as your CPU, RAM, and chipset (SATA, IDE, etc.), and other local physical limits of the computer. Localhost off a USB 2.0 Hard drive will yield maybe 480 Mbps or less because of the bus of the local hard drive that you are using to run the local webserver via localhost. On the other hand, if you are using SSD or something yielding 100 Gigabytes per second per direction data rate and system bus capable of handling that with other activities on the computer, then it will be fast. There is no actual hard set limit. There is a little overhead on the CPU to process the TCP packet stuff but even an i9-10900K cpu maybe limited to data running at some speed less than 45.8 Gigabytes per second. SATA gives you limits of like 6 Gbps. PCIe x16 based SSD could go to speeds of over 60 GBytes per second per direction!! PCIe 6.0 specs takes the cake to over 120 Gigabytes per direction !!!!

No one can specifically state a singular number as to how fast localhost will be. Several factors will impact the actual measured speed and that is dependent on your computer and the impact of OS and other software and hardware on the system. These numbers given above are theoretical limits not necessarily actual performance measured. The bottleneck is the weakest link in your computer. Usually the speed of data coming to and from your hard drive on the computer and sometimes, you can cache in RAM for some speed improvement but that isn't always an easy thing or an easily/available option for every OS.

You will likely yield less than 10GBase-T performance if you are using a SATA based drive. If you are using an M.2 drive, you might beat that performance. SSDs running on a PCIe x16 bus speed will yield performance above 40gigabits per second but your CPU and memory bus will impose its own limits as well as the overall system bus. The slowest bus that is involved with the operation of localhost loopback process is your most limiting factor other than the CPU itself which maybe loaded down with stuff which can effect data rate. An i5 will be more limiting than the an i9-10900K cpu.