Nice link to increas the linux fd limit -

http://www.cs.uwaterloo.ca/~brecht/servers/openfiles.html

Tuning file descriptor limits on Linux

Linux limits the number of file descriptors that any one process may open; the default limits are 1024 per process. These limits can prevent optimum performance of both benchmarking clients (such as httperf and apachebench) and of the web servers themselves (Apache is not affected, since it uses a process per connection, but single process web servers such as Zeus use a file descriptor per connection, and so can easily fall foul of the default limit).
The open file limit is one of the limits that can be tuned with the ulimit command. The command ulimit -aS displays the current limit, and ulimit -aH displays the hard limit (above which the limit cannot be increased without tuning kernel parameters in /proc).
The following is an example of the output of ulimit -aH. You can see that the current shell (and its children) is restricted to 1024 open file descriptors.

core file size (blocks)     unlimited
data seg size (kbytes)      unlimited
file size (blocks)          unlimited
max locked memory (kbytes)  unlimited
max memory size (kbytes)    unlimited
open files                  1024
pipe size (512 bytes)       8
stack size (kbytes)         unlimited
cpu time (seconds)          unlimited
max user processes          4094
virtual memory (kbytes)     unlimited

Increasing the file descriptor limit

The file descriptor limit can be increased using the following procedure:

1. Edit /etc/security/limits.conf and add the lines:

   *       soft    nofile  1024
   *       hard    nofile  65535
   

2. Edit /etc/pam.d/login, adding the line:

   session required /lib/security/pam_limits.so
   

3. The system file descriptor limit is set in /proc/sys/fs/file-max. The following command will increase the limit to 65535:

   echo 65535 > /proc/sys/fs/file-max
   

4. You should then be able to increase the file descriptor limits using:

   ulimit -n unlimited
   

   The above command will set the limits to the hard limit specified in /etc/security/limits.conf.

Note that you may need to log out and back in again before the changes take effect.

1. [Find out where __FD_SETSIZE is defined]
   % grep "#define __FD_SETSIZE" /usr/include/*.h /usr/include/*/*.h
   /usr/include/bits/types.h:#define __FD_SETSIZE  1024
   /usr/include/linux/posix_types.h:#define __FD_SETSIZE   1024
   
   Alternately your system may use tabs in which case
   something like this should do the trick:
   % grep -E "#define\W+__FD_SETSIZE" /usr/include/*.h /usr/include/*/*.h
   [Thanks to madRat for this tip].
   
   
   [Make a local copy of these files]
   % cp /usr/include/bits/types.h include/bits/types.h
   % cp /usr/include/linux/posix_types.h include/linux/posix_types.h
   
   [Modify them so that they look something like
   #define __FD_SETSIZE  65535
   
   [Recompile httperf and/or your server so that it uses a larger file
   descriptor set size by using -I include during compliation, this
   will allow the compiler/preprocessor to use the new include files
   rather than the default versions]
   
   

2. To check and modify limits per shell.

   [Using csh: openfiles and descriptors show that the limit here is 1024]
   % limit
   cputime         unlimited
   filesize        unlimited
   datasize        unlimited
   stacksize       8192 kbytes
   coredumpsize    0 kbytes
   memoryuse       unlimited
   descriptors     1024 
   memorylocked    unlimited
   maxproc         8146 
   openfiles       1024 
   
   [To increase this to 65535 for all users (as root)]
   # vi /etc/security/limits.conf
   
   [Modify or add "nofile" (number of file) entries - note
   that a userid can be used in place of *]
   *                soft    nofile          65535
   *                hard    nofile          65535
   

   
   

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