Is my ulimit exceeded

Under high load it happens from time to time that network connections fail although everything they work perfectly under low load. The reason be the ulimit set on a process. A ulimit restricts the process from opening up more files (and network connections) than a certain number.

To display you ulimit settings use the command ulimit:

# ulimit -a
core file size          (blocks, -c) unlimited
data seg size           (kbytes, -d) unlimited
file size               (blocks, -f) unlimited
pending signals                 (-i) 32768
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 32768
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

You can permanently set the limits in /etc/security/limits.conf. You will have to re-login afterwards. To set the number of file descriptors for all users, the syntax is:

*                hard    nofile           10000
*		 soft    nofile           10000

From my practice I can tell that the most prominent ulimit is -n, the count of open files allowed per process. I can also tell that the most interesting questions that are never answered in man pages are:

• how is my ulimit for a given process?
• how much of the ulimit is already used up?
• have there been problem with the limit beeing set too small?

How is my ulimit for a given process?

Let's take firefox as an example:

# ps -A | grep firefox
10975 ?        00:00:01 firefox
# cd /proc/10975/
# cat limits 
Limit                     Soft Limit           Hard Limit           Units     
Max cpu time              unlimited            unlimited            seconds   
Max file size             unlimited            unlimited            bytes     
Max data size             unlimited            unlimited            bytes     
Max stack size            8388608              unlimited            bytes     
Max core file size        0                    unlimited            bytes     
Max resident set          unlimited            unlimited            bytes     
Max processes             11848                11848                processes 
Max open files            1024                 4096                 files     
Max locked memory         65536                65536                bytes     
Max address space         unlimited            unlimited            bytes     
Max file locks            unlimited            unlimited            locks     
Max pending signals       11848                11848                signals   
Max msgqueue size         819200               819200               bytes     
Max nice priority         0                    0                    
Max realtime priority     0                    0                    
Max realtime timeout      unlimited            unlimited            us    

How much of the ulimit is already used up?

Let's see for firefox:

# ps -A | grep firefox
10975 ?        00:00:03 firefox
# cd /proc/10975/fd
# ls -1|wc -l
55

ok, so firefox is consuming 55 of 1024 open file descriptors, much headroom left.

Test case

I wrote a simple C program that does nothing but open files:

main.c

#include <stdio.h>

int main()
{
  FILE *handle;
  for (int i=0;i<=2048;i++)
  {
    printf("%d",(fopen("testfile", "wb")==0));
  }
  while (true){};
}

Compile this file with the command

g++ main.c 

Run this program (and send it to the background) then with the command

./a.out &

Now find out the process ID:

# ps -A|grep a.out
29232 pts/3    00:04:15 a.out

Now go into the process' file descriptor directory:

cd /proc/29232/fd

And count the number of files:

# ls -1 | wc -l
1024

strace'ing it gives me:

open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 1023
open("testfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 EMFILE (Too many open files)