How to determine what's causing: "Error: connect EMFILE" (Node.js)

I came across a bunch of posts on stackoverflow suggesting I use something like graceful-js to solve my problem, or to increase ulimit. Essentially, they’re workarounds. I wanted to know what the root problem was. Here’s the process I came up with. If anyone knows a better way, please let me know in the comments.

What This Error Means

There is a number of file handles a process can have open. Note: that sockets also create a file handle. Once you reach the limit you cannot open anymore and a cryptic error message such as “Error connect EMFILE” will end up in your log file (hopefully). The default limit (at least on my ubuntu system) is 1024.

How To Isolate

This command will output the number of open handles for nodejs processes: lsof -i -n -P | grep nodejs

COMMAND     PID    USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
...
nodejs    12211    root 1012u  IPv4 151317015      0t0  TCP 10.101.42.209:40371->54.236.3.170:80 (ESTABLISHED)
nodejs    12211    root 1013u  IPv4 151279902      0t0  TCP 10.101.42.209:43656->54.236.3.172:80 (ESTABLISHED)
nodejs    12211    root 1014u  IPv4 151317016      0t0  TCP 10.101.42.209:34450->54.236.3.168:80 (ESTABLISHED)
nodejs    12211    root 1015u  IPv4 151289728      0t0  TCP 10.101.42.209:52691->54.236.3.173:80 (ESTABLISHED)
nodejs    12211    root 1016u  IPv4 151305607      0t0  TCP 10.101.42.209:47707->54.236.3.172:80 (ESTABLISHED)
nodejs    12211    root 1017u  IPv4 151289730      0t0  TCP 10.101.42.209:45423->54.236.3.171:80 (ESTABLISHED)
nodejs    12211    root 1018u  IPv4 151289731      0t0  TCP 10.101.42.209:36090->54.236.3.170:80 (ESTABLISHED)
nodejs    12211    root 1019u  IPv4 151314874      0t0  TCP 10.101.42.209:49176->54.236.3.172:80 (ESTABLISHED)
nodejs    12211    root 1020u  IPv4 151289768      0t0  TCP 10.101.42.209:45427->54.236.3.171:80 (ESTABLISHED)
nodejs    12211    root 1021u  IPv4 151289769      0t0  TCP 10.101.42.209:36094->54.236.3.170:80 (ESTABLISHED)
nodejs    12211    root 1022u  IPv4 151279903      0t0  TCP 10.101.42.209:43836->54.236.3.171:80 (ESTABLISHED)
nodejs    12211    root 1023u  IPv4 151281403      0t0  TCP 10.101.42.209:43930->54.236.3.172:80 (ESTABLISHED)
....

Notice the: 1023u (last line) —- that’s the 1024th file handle which is the default maximum.

Now, Look at the last column. That indicates which resource is open. You’ll probably see a number of lines all with the same resource name. Hopefully, that now tells you where to look in your code for the leak.

If you have multiple node processes, you can isolate it by using PID in 2nd column.

In my case above, I noticed that there were a bunch of very similar IP Addresses. They were all 54.236.3.###

So, I started doing iplookups on all the different 3rd party services I used… loggly, newrelic, pubnub… until I ultimately determined it was pubnub. Turned out we were creating a new socket each time we published an event instead of reusing.

Command Reference

Use this syntax to determine how many open handles a process has open…

To get a count of open files for a certain pid

I used this command to test the number of files that were opened after doing various events in my app.

lsof -i -n -P | grep "8465" | wc -l

root@ip-10-101-42-209:/var/www# lsof -i -n -P | grep "nodejs.*8465" | wc -l
28
root@ip-10-101-42-209:/var/www# lsof -i -n -P | grep "nodejs.*8465" | wc -l
31
root@ip-10-101-42-209:/var/www# lsof -i -n -P | grep "nodejs.*8465" | wc -l
34

What is your process limit?

ulimit -a

The line you want will look like this: open files (-n) 1024

Comments

[Eric]

It helps me a lot, thanks!

[Sai Teja]

Good Post, Thanks for the info :)

[Eddie]

This command works for me
lsof -i -n -P | grep node