Descriptors and Apache
This document has not been fully updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.
A descriptor, also commonly called a file
handle is an object that a program uses to read or write
an open file, or open network socket, or a variety of other
devices. It is represented by an integer, and you may be
stderr which are descriptors 0, 1, and 2
respectively. Apache needs a descriptor for each log file, plus
one for each network socket that it listens on, plus a handful
of others. Libraries that Apache uses may also require
descriptors. Normal programs don't open up many descriptors at
all, and so there are some latent problems that you may
experience should you start running Apache with many
descriptors (i.e., with many virtual hosts).
The operating system enforces a limit on the number of
descriptors that a program can have open at a time. There are
typically three limits involved here. One is a kernel
limitation, depending on your operating system you will either
be able to tune the number of descriptors available to higher
numbers (this is frequently called FD_SETSIZE). Or you
may be stuck with a (relatively) low amount. The second limit
is called the hard resource limit, and it is sometimes
set by root in an obscure operating system file, but frequently
is the same as the kernel limit. The third limit is called the
soft resource limit. The soft limit is always less
than or equal to the hard limit. For example, the hard limit
may be 1024, but the soft limit only 64. Any user can raise
their soft limit up to the hard limit. Root can raise the hard
limit up to the system maximum limit. The soft limit is the
actual limit that is used when enforcing the maximum number of
files a process can have open.
#open files <= soft limit <= hard limit <= kernel limit
You control the hard and soft limits using the
limit (csh) or
directives. See the respective man pages for more information.
For example you can probably use
unlimited to raise your soft limit up to the hard limit.
You should include this command in a shell script which starts
Unfortunately, it's not always this simple. As mentioned
above, you will probably run into some system limitations that
will need to be worked around somehow. Work was done in version
1.2.1 to improve the situation somewhat. Here is a partial list
of systems and workarounds (assuming you are using 1.2.1 or
Under BSDI 2.0 you can build Apache to support more
descriptors by adding
EXTRA_CFLAGS (where nnn is the number of
descriptors you wish to support, keep it less than the hard
limit). But it will run into trouble if more than
approximately 240 Listen directives are used. This may be
cured by rebuilding your kernel with a higher
Similar to the BSDI 2.0 case, you should define
FD_SETSIZE and rebuild. But the extra Listen
limitation doesn't exist.
By default Linux has a kernel maximum of 256 open
descriptors per process. There are several patches available
for the 2.0.x series which raise this to 1024 and beyond, and
you can find them in the "unofficial patches" section of the Linux Information HQ.
None of these patches are perfect, and an entirely different
approach is likely to be taken during the 2.1.x development.
Applying these patches will raise the FD_SETSIZE used to
compile all programs, and unless you rebuild all your
libraries you should avoid running any other program with a
soft descriptor limit above 256. As of this writing the
patches available for increasing the number of descriptors do
not take this into account. On a dedicated webserver you
probably won't run into trouble.
Solaris has a kernel hard limit of 1024 (may be lower in
earlier versions). But it has a limitation that files using
the stdio library cannot have a descriptor above 255. Apache
uses the stdio library for the ErrorLog directive. When you
have more than approximately 110 virtual hosts (with an error
log and an access log each) you will need to build Apache
-DHIGH_SLACK_LINE=256 added to
EXTRA_CFLAGS. You will be limited to
approximately 240 error logs if you do this.
AIX version 3.2?? appears to have a hard limit of 128
descriptors. End of story. Version 4.1.5 has a hard limit of
/etc/conf/cf.d/stune file or use
/etc/conf/cf.d/configure choice 7 (User and
Group configuration) and modify the
kernel parameter to a suitably higher value. SCO recommends a
number between 60 and 11000, the default is 110. Relink and
reboot, and the new number of descriptors will be
open_max_hard to 4096 in the proc subsystem.
Do a man on sysconfig, sysconfigdb, and
max-vnodes to a large number which
is greater than the number of apache processes * 4096
(Setting it to 250,000 should be good for most people).
Do a man on sysconfig, sysconfigdb, and
- If you are using Tru64 5.0, 5.0A, or 5.1, define
NO_SLACK to work around a bug in the OS.
If you have details on another operating system, please
submit it through our Bug Report
In addition to the problems described above there are
problems with many libraries that Apache uses. The most common
example is the bind DNS resolver library that is used by pretty
much every unix, which fails if it ends up with a descriptor
above 256. We suspect there are other libraries that similar
limitations. So the code as of 1.2.1 takes a defensive stance
and tries to save descriptors less than 16 for use while
processing each request. This is called the low slack
Note that this shouldn't waste descriptors. If you really
are pushing the limits and Apache can't get a descriptor above
16 when it wants it, it will settle for one below 16.
In extreme situations you may want to lower the low slack
line, but you shouldn't ever need to. For example, lowering it
can increase the limits 240 described above under Solaris and
BSDI 2.0. But you'll play a delicate balancing game with the
descriptors needed to serve a request. Should you want to play
this game, the compile time parameter is
LOW_SLACK_LINE and there's a tiny bit of
documentation in the header file
Finally, if you suspect that all this slack stuff is causing
you problems, you can disable it. Add
EXTRA_CFLAGS and rebuild. But please report it
to our Bug
Report Page so that we can investigate.