kern.maxfiles can be raised or lowered based upon
your system requirements. This variable indicates the maximum number of file
descriptors on your system. When the file descriptor table is full,
“file: table is full” will show up
repeatedly in the system message buffer, which can be viewed with the dmesg command.
Each open file, socket, or fifo uses one file descriptor. A large-scale production server may easily require many thousands of file descriptors, depending on the kind and number of services running concurrently.
In older FreeBSD releases, the default value of
kern.maxfiles is derived from the
maxusers option in your kernel configuration file.
kern.maxfiles grows proportionally to the value of
maxusers. When compiling a custom kernel, it is a good idea
to set this kernel configuration option according to the uses of your system.
From this number, the kernel is given most of its pre-defined limits. Even though a
production machine may not actually have 256 users connected at once, the
resources needed may be similar to a high-scale web server.
kern.maxusers is automatically sized
at boot based on the amount of memory available in the system, and may be
determined at run-time by inspecting the value of the read-only
kern.maxusers sysctl. Some sites will require larger or
smaller values of
kern.maxusers and may set it as a
loader tunable; values of 64, 128, and 256 are not uncommon. We do not
recommend going above 256 unless you need a huge number of file descriptors; many
of the tunable values set to their defaults by
kern.maxusers may be individually overridden at boot-time or
run-time in /boot/loader.conf (see the loader.conf(5) manual
page or the /boot/defaults/loader.conf file for
some hints) or as described elsewhere in this document.
In older releases, the system will auto-tune maxusers for you if you explicitly set it to 0 . When setting this option, you will want to set maxusers to at least 4, especially if you are using the X Window System or compiling software. The reason is that the most important table set by maxusers is the maximum number of processes, which is set to 20 + 16 * maxusers, so if you set maxusers to 1, then you can only have 36 simultaneous processes, including the 18 or so that the system starts up at boot time and the 15 or so you will probably create when you start the X Window System. Even a simple task like reading a manual page will start up nine processes to filter, decompress, and view it. Setting maxusers to 64 will allow you to have up to 1044 simultaneous processes, which should be enough for nearly all uses. If, however, you see the dreaded proc table full error when trying to start another program, or are running a server with a large number of simultaneous users (like ftp.FreeBSD.org), you can always increase the number and rebuild.
Note: maxusers does not limit the number of users which can log into your machine. It simply sets various table sizes to reasonable values considering the maximum number of users you will likely have on your system and how many processes each of them will be running.
kern.ipc.somaxconn sysctl variable limits the
size of the listen queue for accepting new TCP connections. The default value of
128 is typically too low for robust handling of new
connections in a heavily loaded web server environment. For such environments, it
is recommended to increase this value to 1024 or higher.
The service daemon may itself limit the listen queue size (e.g., sendmail(8), or
Apache) but will often have a directive in its
configuration file to adjust the queue size. Large listen queues also do a better
job of avoiding Denial of Service (DoS) attacks.
The NMBCLUSTERS kernel configuration option dictates the
amount of network Mbufs available to the system. A heavily-trafficked server with a
low number of Mbufs will hinder FreeBSD's ability. Each cluster represents
approximately 2 K of memory, so a value of 1024 represents 2 megabytes of
kernel memory reserved for network buffers. A simple calculation can be done to
figure out how many are needed. If you have a web server which maxes out at
1000 simultaneous connections, and each connection eats a 16 K receive and
16 K send buffer, you need approximately 32 MB worth of network buffers to
cover the web server. A good rule of thumb is to multiply by 2, so
2x32 MB / 2 KB =
64 MB / 2 kB = 32768. We recommend values between 4096
and 32768 for machines with greater amounts of memory. Under no circumstances should
you specify an arbitrarily high value for this parameter as it could lead to a
boot time crash. The
-m option to netstat(1) may be used
to observe network cluster use.
kern.ipc.nmbclusters loader tunable should be used
to tune this at boot time. Only older versions of FreeBSD will require you to use
the NMBCLUSTERS kernel config(8)
For busy servers that make extensive use of the sendfile(2) system
call, it may be necessary to increase the number of sendfile(2) buffers
via the NSFBUFS kernel configuration option or by
setting its value in /boot/loader.conf (see loader(8) for
details). A common indicator that this parameter needs to be adjusted is when
processes are seen in the sfbufa state. The sysctl variable
kern.ipc.nsfbufs is a read-only glimpse at the
kernel configured variable. This parameter nominally scales with
kern.maxusers, however it may be necessary to tune
Important: Even though a socket has been marked as non-blocking, calling sendfile(2) on the non-blocking socket may result in the sendfile(2) call blocking until enough struct sf_buf's are made available.
net.inet.ip.portrange.* sysctl variables
control the port number ranges automatically bound to TCP and UDP sockets. There
are three ranges: a low range, a default range, and a high range. Most network
programs use the default range which is controlled by the
net.inet.ip.portrange.last, which default to 1024 and 5000,
respectively. Bound port ranges are used for outgoing connections, and it is
possible to run the system out of ports under certain circumstances. This most
commonly occurs when you are running a heavily loaded web proxy. The port
range is not an issue when running servers which handle mainly incoming
connections, such as a normal web server, or has a limited number of outgoing
connections, such as a mail relay. For situations where you may run yourself
out of ports, it is recommended to increase
net.inet.ip.portrange.last modestly. A value of 10000, 20000 or 30000 may be reasonable. You should also consider firewall
effects when changing the port range. Some firewalls may block large ranges of
ports (usually low-numbered ports) and expect systems to use higher ranges of
ports for outgoing connections -- for this reason it is not recommended that
net.inet.ip.portrange.first be lowered.
The TCP Bandwidth Delay Product Limiting is similar to TCP/Vegas in NetBSD. It
can be enabled by setting
sysctl variable to 1. The system will attempt to
calculate the bandwidth delay product for each connection and limit the amount of
data queued to the network to just the amount required to maintain optimum
This feature is useful if you are serving data over modems, Gigabit Ethernet, or
even high speed WAN links (or any other link with a high bandwidth delay product),
especially if you are also using window scaling or have configured a large
send window. If you enable this option, you should also be sure to set
net.inet.tcp.inflight.debug to 0
(disable debugging), and for production use setting
net.inet.tcp.inflight.min to at least 6144 may be beneficial. However, note that setting high
minimums may effectively disable bandwidth limiting depending on the link. The
limiting feature reduces the amount of data built up in intermediate route
and switch packet queues as well as reduces the amount of data built up in the
local host's interface queue. With fewer packets queued up, interactive
connections, especially over slow modems, will also be able to operate with lower
Round Trip Times. However,
note that this feature only effects data transmission (uploading / server side). It
has no effect on data reception (downloading).
net.inet.tcp.inflight.stab is not recommended. This parameter
defaults to 20, representing 2 maximal packets added to the bandwidth delay product
window calculation. The additional window is required to stabilize the algorithm
and improve responsiveness to changing conditions, but it can also result in
higher ping times over slow links (though still much lower than you would get
without the inflight algorithm). In such cases, you may wish to try reducing
this parameter to 15, 10, or 5; and may also have to reduce
net.inet.tcp.inflight.min (for example, to 3500) to get the
desired effect. Reducing these parameters should be done as a last resort only.
A vnode is the internal representation of a file or directory. So increasing the number of vnodes available to the operating system cuts down on disk I/O. Normally this is handled by the operating system and does not need to be changed. In some cases where disk I/O is a bottleneck and the system is running out of vnodes, this setting will need to be increased. The amount of inactive and free RAM will need to be taken into account.
To see the current number of vnodes in use:
# sysctl vfs.numvnodes vfs.numvnodes: 91349
To see the maximum vnodes:
# sysctl kern.maxvnodes kern.maxvnodes: 100000
If the current vnode usage is near the maximum, increasing
kern.maxvnodes by a value of 1,000 is probably a good idea.
Keep an eye on the number of
vfs.numvnodes. If it
climbs up to the maximum again,
need to be increased further. A shift in your memory usage as reported by top(1) should be
visible. More memory should be active.
The auto-tuning algorithm sets maxusers equal to the amount of memory in the system, with a minimum of 32, and a maximum of 384.