[erlang-questions] why is gen_tcp:send slow?

David Mercer <>
Tue Jun 24 22:02:11 CEST 2008

I tried some alternative block sizes (using the blksize option).  I found
that from 1 to somewhere around––maybe a bit short of––1000 bytes, the test
was able to send about 300,000 blocks in 10 seconds regardless of size.
(That means, 0.03 MB/sec for block size of 1, 0.3 MB/sec for block size of
10, 3 MB/sec  for block size of 100, etc.)  I suspect the system was CPU
bound at those levels.


Above 1000, the number of blocks sent seemed to decrease, though this was
more than offset by the increased size of the blocks.  Above about 10,000
byte blocks (may have been less, I didn’t check any value between 4,000 and
10,000), however, performance peaked and block size no longer mattered: it
always sent between 70 and 80 MB/sec.  My machine is clearly slower than





[mailto:] On Behalf Of Rapsey
Sent: Tuesday, June 24, 2008 14:01
Subject: Re: [erlang-questions] why is gen_tcp:send slow?


You're using very large packets. I think the results would be much more
telling if the packets would be a few kB at most. That is closer to most
real life situations. 


On Tue, Jun 24, 2008 at 8:43 PM, Edwin Fine <>

I wrote a small benchmark in Erlang to see how fast I could get socket
communications to go. All the benchmark does is pump the same buffer to a
socket for (by default) 10 seconds. It uses {active, once} each time, just
like you do. 

Server TCP options:
     {active, once},
        {reuseaddr, true},
        {packet, 0},
        {packet_size, 65536},
        {recbuf, 1000000}

Client TCP options:
        {packet, raw},
        {packet_size, 65536},
        {sndbuf, 1024 * 1024},
        {send_timeout, 3000}

Here are some results using Erlang R12B-3 (erl +K true in the Linux

Linux (Ubuntu 8.10 x86_64, Intel Core 2 Q6600, 8 GB):
- Using localhost ( 7474.14 MB in 10.01 secs (746.66 MB/sec)
- Using 192.168.x.x IP address: 8064.94 MB in 10.00 secs (806.22 MB/sec)
[Don't ask me why it's faster than using loopback, I repeated the tests and
got the same result]

Windows XP SP3 (32 bits), Intel Core 2 Duo E6600:
- Using loopback: 2166.97 MB in 10.02 secs (216.35 MB/sec)
- Using 192.168.x.x IP address: 2140.72 MB in 10.02 secs (213.75 MB/sec)
- On Gigabit Ethernet to the Q6600 Linux box: 1063.61 MB in 10.02 secs
(106.17 MB/sec) using non-jumbo frames. I don't think my router supports
jumbo frames.

There's undoubtedly a huge discrepancy between the two systems, whether
because of kernel poll in Linux, or that it's 64 bits, or unoptimized
Windows TCP/IP flags, I don't know. I don't believe it's the number of CPUs
(there's only 1 process sending and one receiving), or the CPU speed (they
are both 2.4 GHz Core 2s).

Maybe some Erlang TCP/IP gurus could comment.

I've attached the code for interest. It's not supposed to be production
quality, so please don't beat me up :) although I am always open to
suggestions for improvement. If you do improve it, I'd like to see what
you've done. Maybe there is another simple Erlang tcp benchmark program out
there (i.e. not Tsung), but I couldn't find one in a cursory Google search.

To run:


tb_server:start(Port, Opts).
tb_server:stop() to stop.

Port = integer()
Opts = []|[opt()]
opt() = {atom(), term()} (Accepts inet setopts options, too)

The server prints out the transfer rate (for simplicity).

tb_client(Host, Port, Opts).

Host = atom()|string() hostname or IP address
Port, Opts as in tb_server

Runs for 10 seconds, sending a 64K buffer as fast as possible to Host/Port.
You can change this to 20 seconds (e.g.) by adding the tupls {time_limit,
20000} to Opts.
You can change buffer size by adding the tuple {blksize, Bytes} to Opts.

2008/6/20 Rapsey <>:

All data goes through nginx which acts as a proxy. Its CPU consumption is
never over 1%.



On Thu, Jun 19, 2008 at 9:35 PM, Javier París Fernández <>

El 19/06/2008, a las 20:06, Rapsey escribió:


It loops from another module, that way I can update the code at any time
without disrupting anything.
The packets are generally a few hundred bytes big, except keyframes which
tend to be in the kB range. I haven't tried looking with wireshark.  Still
it seems a bit odd that a large CPU consumption would be the symptom. The
traffic is strictly one way. Either someone is sending the stream or
receiving it.
The transmit could of course be written with a passive receive, but the code
would be significantly uglier. I'm sure someone here knows if setting
{active, once} every packet is CPU intensive or not.
It seems the workings of gen_tcp is quite platform dependent. If I run the
code in windows, sending more than 128 bytes per gen_tcp call significantly
decreases network output.
Oh and I forgot to mention I use R12B-3.



Without being an expert.

200-300 mb/s  in small (hundreds of bytes) packets means a *lot* of system
calls if you are doing a gen_tcp:send for each one. If you buffer 3 packets,
you are reducing that by a factor of 3 :). I'd try to do an small test doing
the same thing in C and compare the results. I think it will also eat a lot
of CPU.

About the proxy CPU... I'm a bit lost about it, but speculating wildly it is
possible that the time spent doing the system calls that gen_tcp is doing is
added to the proxy CPU process.




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