open files (was Re: Virtual Hosting)

[George Toft]

Hi Kevin,

Kevin Buettner wrote:
> 
> On Jan 8,  6:24am, George Toft wrote:
> 
> > So if I issue the cat command as described, and compare that against
> > a ulimit command, I get the following discrepancy:
> > [georgetoft@biff georgetoft]$ ulimit -a | grep files
> > open files                  1024
> > [georgetoft@biff georgetoft]$ cat /proc/sys/fs/file-max
> > 4096
> > [georgetoft@biff georgetoft]$
> >
> > The number given by ulimit is the one that takes effect (243 virtual
> > hosts * 4 log files per host = 979, plus a few other open files for
> > mail, logs, login shells, etc is pretty close to 1024), so I'm
> > wondering what effect /proc/sys/fs/file-max has.  The write up you
> > pointed me to seems to conflict with what I observed.
> >
> > Any clarification would be appreciated.
> 
> The ulimit command specifies a per-process limit.  The value contained
> in /proc/sys/fs/file-max is the overall limit for the entire system.
> 
> With the values that you cite above, it would only take four processes
> to max out the limit contained in /proc/sys/fs/file-max.

Thanks for the clarification.


> For your application, it sounds to me like you need to increase your
> per-process open file limit.  (You may need to increase the overall
> system limit too.)  I notice that as root, I'm able to do
> ``ulimit -n 2048'' and then ``ulimit -a'' actually shows that the limit
> has been increased.

But if you exit that shell and open another, you will find your change
is gone - ulimit only affects the current and child shells.

> I've done some tests (using calls to pipe()) and have determined that
> I'm able to create more than 1024 open files when I do this.  However,
> it seems to me that not all functions implemented by glibc will
> support more than 1024 files.  In particular, the select() function
> relies on the fd_set data structure to know which file descriptors to
> wait for.  It appears to me that you can't use this function for file
> descriptor numbers past 1023.  In /usr/include/bits/types.h (which is
> included by /usr/include/sys/types.h), I see the following:
> 
>     /* Number of descriptors that can fit in an `fd_set'.  */
>     #define __FD_SETSIZE        1024
> 
> I think you're okay if your application uses poll() instead of
> select() though.

I'm not into rewriting Apache :)

George


> Kevin
> 
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