/proc/sys/fs/file-max

[Bill Warner]

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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