Solaris file descriptor question

Question

Hi,
We have an application on Solaris 2.6 and
the shell in which the server runs has a
file descriptor limit of 1024. What does
this mean? Does this mean that every process
from the shell will have 1024 fds? What
is the maximum # of fds that a solaris 2.6
system can provide?
When I run "sysdef", I get:
ffffffff:fffffffd file descriptors
How do I interpret this line?
Is this 64K - some value?
If system limit is 64K and if each
shell has 1024, how are the fds allocated
to the shells?
What I mean is:
say I have 3 shells each with descriptor
limit of 1024, then is the distribution
something like 1024-2047 for shell 1,
2048 - 3071 for shell 2 (i.e. 3072) and
3072 - 4095 for shell 3?
Appreciate any explanation of this anyone
can offer.
thanks,
mshyam

Answer 1

Hi There,
About File Descriptors and Their Limitations:
All versions of Solaris (including Solaris 7 64-bit) have a default "soft" limit of 64 and a default "hard" limit of 1024.
Processes may need to open many files or sockets as file descriptors. Standard I/O (stdio) library functions have a defined limit of 256 file descriptors as the fopen() call, datatype char, will fail if it can not get a file descriptor between 0 and 255. The open() system call is of datatype int, removing this limitation. However, if open() has opened 0 to 255 file descriptors without closing any, fopen() will
not be able to open any file descriptors as all the low-numbered ones have been used up. Applications that need to use many file descriptors to open a large number of sockets, or other raw files, should be forced to use descriptors numbered above 256. This allows system functions such as name services, to work as they depend upon stdio routines.
(See p 368 "Performance and Tuning - Java and the Internet").
There are limitations on the number of file descriptors
available to the current shell and its descendents. (See the ulimit man page). The maximum number of file descriptors that can be safely used for the shell and Solaris processes is 1024.
This limitation has been lifted for Solaris 7 64-bit which can be 64k (65536).
Therefore the recommended maximum values to be added to /etc/system are:
set rlim_fd_cur=1024
set rlim_fd_max=1024
To use the limit command with csh:
% limit descriptors 1024
To use the ulimit command with Bourne or ksh:
$ ulimit -n 1024
However, some third-party applications need the max raised. A possible recommendation would be to increase rlim_fd_max, but not the default (rlim_fd_cur). Then rlim_fd_cur can be raised on a per-process basis if needed, but the higher setting
for rlim_fd_max doesn't affect all processes.
I hope this helps your understanding about systemwide file descriptor max limit in conjunction with shell and per process file descriptor limits.
....jagruti
Deveoper Technical Support
Sun Microsystems Inc.