/proc/sys/fs/file-max

[Mike Cantrell]

One thing you should be aware of when increasing max files is increasing
your inodes as well ( /proc/sys/fs/inode-max ). A good rule is 3 times as
many inodes as files (at least that's what I've read).


Regards,
Mike Cantrell

On 2 Sep 2000, Bill Warner wrote:

> Kevin,
> 
> This is one of the most informative things I have read in a long
> time.  I have never really worked in the kernel other than well 
> documented howto bug fixes type stuff but I was able to understand 
> most of what you were talking about and I think I could almost do 
> the same now.  I don't know if I would have the confidence to go 
> changing things but this is a very interesting read for problem 
> trouble shooting.  Please post more like this if you have a chance.
> 
> Thanks
> Bill Warner
> 
> > I learned about /proc/sys/fs/file-nr and /proc/sys/fs/file-max
> > earlier tonight.
> > 
> > I was sending some mail, cc'd myself and was running fetchmail to
> > watch for replies.  (Sure enough, the person I wanted to correspond
> > with was working late too...)  I noticed that even though fetchmail
> > said it was fetching some mail (my cc), it wasn't being delivered via
> > sendmail (which fetchmail invokes).
> > 
> > Diagnosing this problem simply consisted of reading /var/log/maillog.
> > (Apparently I had had a number of such incidents throughout the day.)
> > The mail log told me that the system had run out of open files.  The
> > exact message was:
> > 
> >     Too many open files in system
> > 
> > This led me to wonder about how one determines how many open files
> > you are allowed to have and how you find out how many your system
> > is using...  (remember, that we're *not* talking about per-process
> > limits here, but rather system limits.)
> > 
> > I chose to use the kernel sources as my documentation.  ;-)
> > 
> > In fs/file_table.c, I noticed the following declaration early
> > in the file:
> > 
> >     int max_files = NR_FILE;/* tunable */
> > 
> > NR_FILES is defined to be 4096 in include/linux/fs.h.  If you
> > get close to the limit, a user process won't be allowed to open
> > a new file if it gets within NR_RESERVED_FILES (10) of the limit.
> > 
> > In addition to declaring/defining `max_files', file_table.c also
> > declares `nr_files' and `nr_free_files'.
> > 
> > `nr_files' is the total number of file structs that the kernel
> > has allocated thus far.  This value is not allowed to exceed
> > `max_files'.  `nr_free_files' is self descriptive, it simply
> > tells the kernel how many (of the `nr_files' that are allocated)
> > are free.
> > 
> > Searching for `max_files' in the kernel sources turned out an
> > occurence in a data structure in sysctl.c which controls how part of
> > the structure of the /proc file system gets set up.
> > 
> > It turns out that /proc/sys/fs/file-max contains the value of
> > `max_files'.  You may set this value via, e.g,
> > 
> >       echo 5200 >/proc/sys/fs/file-max
> > 
> > The file /proc/sys/fs/file-nr is also of interest.  It is read-only,
> > and contains three values (on one line).  These are as follows:
> > 
> >     - the total number of file structs allocated by the system,
> >       i.e, the value given by `nr_files' in the kernel sources.
> >     - the number of free file structs, i.e, the value given by
> >       `nr_free_files'.
> >     - the maximum number of file structs which may be allocated
> >       by the system.  This will be the value of `max_files' in
> >       the kernel sources and is also available by examining
> >       /proc/sys/fs/file-max.
> > 
> > So, for example, after my fiasco with my mail not getting delivered
> > (which also entailed me frantically closing down a lot of applications
> > that I wasn't using), I had the following situation:
> > 
> >     ocotillo:linux$ cat /proc/sys/fs/file-nr
> >     4096    1323    4096
> >     ocotillo:linux$ cat /proc/sys/fs/file-max
> >     4096
> > 
> > Note that the first and third number being equal indicates that I
> > actually hit the limit.  (Actually, if these numbers even get close,
> > you're probably in trouble - remember that non-root processes will
> > fail when the system gets within 10 files of the maximum.)
> > 
> > Then, as root, I did the following:
> > 
> >     [root@ocotillo kev]# echo 4608 >/proc/sys/fs/file-max 
> > 
> > Running the same two `cat' commands now yields the following
> > results:
> > 
> >     ocotillo:linux$ cat /proc/sys/fs/file-nr
> >     4096    1323    4608
> >     ocotillo:linux$ cat /proc/sys/fs/file-max
> >     4608
> > 
> > Hopefully, this will give me a bit of a cushion for the days to
> > come.
> > 
> > I was curious to see how many file structs the kernel was using on
> > a different (less heavily used) system.  Here's what I saw:
> > 
> >     saguaro:kev$ cat /proc/sys/fs/file-nr
> >     1969    461     4096
> >     saguaro:kev$ cat /proc/sys/fs/file-max
> >     4096
> > 
> > Comparing the first and third numbers indicates that this other system
> > had come no where close to hitting the limit of 4096.
> > 
> > I have a hunch that those of you who run big web servers and other
> > sorts of things which may need lots of open files already knew how to
> > increase the number of maximum number of open files on your system. 
> > But even if you did, I hope that my description of the methodology
> > that I used to ferret the information out of the kernel was of some
> > interest.
> > 
> > Kevin
> > 
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> 
> 
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