TUNING(7) FreeBSD Miscellaneous Information Manual TUNING(7)
NAME
tuning – performance tuning under FreeBSD
SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP
The swap partition should typically be approximately 2x the size of main
memory for systems with less than 4GB of RAM, or approximately equal to
the size of main memory if you have more. Keep in mind future memory
expansion when sizing the swap partition. Configuring too little swap
can lead to inefficiencies in the VM page scanning code as well as create
issues later on if you add more memory to your machine. On larger
systems with multiple SCSI disks (or multiple IDE disks operating on
different controllers), configure swap on each drive. The swap
partitions on the drives should be approximately the same size. The
kernel can handle arbitrary sizes but internal data structures scale to 4
times the largest swap partition. Keeping the swap partitions near the
same size will allow the kernel to optimally stripe swap space across the
N disks. Do not worry about overdoing it a little, swap space is the
saving grace of UNIX and even if you do not normally use much swap, it
can give you more time to recover from a runaway program before being
forced to reboot.
It is not a good idea to make one large partition. First, each partition
has different operational characteristics and separating them allows the
file system to tune itself to those characteristics. For example, the
root and /usr partitions are read-mostly, with very little writing, while
a lot of reading and writing could occur in /var/tmp. By properly
partitioning your system fragmentation introduced in the smaller more
heavily write-loaded partitions will not bleed over into the mostly-read
partitions.
Properly partitioning your system also allows you to tune newfs(8), and
tunefs(8) parameters. The only tunefs(8) option worthwhile turning on is
softupdates with “tunefs -n enable /filesystem”. Softupdates drastically
improves meta-data performance, mainly file creation and deletion. We
recommend enabling softupdates on most file systems; however, there are
two limitations to softupdates that you should be aware of when
determining whether to use it on a file system. First, softupdates
guarantees file system consistency in the case of a crash but could very
easily be several seconds (even a minute!) behind on pending write to the
physical disk. If you crash you may lose more work than otherwise.
Secondly, softupdates delays the freeing of file system blocks. If you
have a file system (such as the root file system) which is close to full,
doing a major update of it, e.g., “make installworld”, can run it out of
space and cause the update to fail. For this reason, softupdates will
not be enabled on the root file system during a typical install. There
is no loss of performance since the root file system is rarely written
to.
A number of run-time mount(8) options exist that can help you tune the
system. The most obvious and most dangerous one is async. Only use this
option in conjunction with gjournal(8), as it is far too dangerous on a
normal file system. A less dangerous and more useful mount(8) option is
called noatime. UNIX file systems normally update the last-accessed time
of a file or directory whenever it is accessed. This operation is
handled in FreeBSD with a delayed write and normally does not create a
burden on the system. However, if your system is accessing a huge number
of files on a continuing basis the buffer cache can wind up getting
polluted with atime updates, creating a burden on the system. For
example, if you are running a heavily loaded web site, or a news server
with lots of readers, you might want to consider turning off atime
updates on your larger partitions with this mount(8) option. However,
you should not gratuitously turn off atime updates everywhere. For
example, the /var file system customarily holds mailboxes, and atime (in
combination with mtime) is used to determine whether a mailbox has new
mail. You might as well leave atime turned on for mostly read-only
partitions such as / and /usr as well. This is especially useful for /
since some system utilities use the atime field for reporting.
STRIPING DISKS
In larger systems you can stripe partitions from several drives together
to create a much larger overall partition. Striping can also improve the
performance of a file system by splitting I/O operations across two or
more disks. The gstripe(8), gvinum(8), and ccdconfig(8) utilities may be
used to create simple striped file systems. Generally speaking, striping
smaller partitions such as the root and /var/tmp, or essentially read-
only partitions such as /usr is a complete waste of time. You should
only stripe partitions that require serious I/O performance, typically
/var, /home, or custom partitions used to hold databases and web pages.
Choosing the proper stripe size is also important. File systems tend to
store meta-data on power-of-2 boundaries and you usually want to reduce
seeking rather than increase seeking. This means you want to use a large
off-center stripe size such as 1152 sectors so sequential I/O does not
seek both disks and so meta-data is distributed across both disks rather
than concentrated on a single disk. If you really need to get
sophisticated, we recommend using a real hardware RAID controller from
the list of FreeBSD supported controllers.
SYSCTL TUNING
sysctl(8) variables permit system behavior to be monitored and controlled
at run-time. Some sysctls simply report on the behavior of the system;
others allow the system behavior to be modified; some may be set at boot
time using rc.conf(5), but most will be set via sysctl.conf(5). There
are several hundred sysctls in the system, including many that appear to
be candidates for tuning but actually are not. In this document we will
only cover the ones that have the greatest effect on the system.
The vm.overcommit sysctl defines the overcommit behaviour of the vm
subsystem. The virtual memory system always does accounting of the swap
space reservation, both total for system and per-user. Corresponding
values are available through sysctl vm.swap_total, that gives the total
bytes available for swapping, and vm.swap_reserved, that gives number of
bytes that may be needed to back all currently allocated anonymous
memory.
Setting bit 0 of the vm.overcommit sysctl causes the virtual memory
system to return failure to the process when allocation of memory causes
vm.swap_reserved to exceed vm.swap_total. Bit 1 of the sysctl enforces
RLIMIT_SWAP limit (see getrlimit(2)). Root is exempt from this limit.
Bit 2 allows to count most of the physical memory as allocatable, except
wired and free reserved pages (accounted by vm.stats.vm.v_free_target and
vm.stats.vm.v_wire_count sysctls, respectively).
The kern.ipc.maxpipekva loader tunable is used to set a hard limit on the
amount of kernel address space allocated to mapping of pipe buffers. Use
of the mapping allows the kernel to eliminate a copy of the data from
writer address space into the kernel, directly copying the content of
mapped buffer to the reader. Increasing this value to a higher setting,
such as `25165824' might improve performance on systems where space for
mapping pipe buffers is quickly exhausted. This exhaustion is not fatal;
however, and it will only cause pipes to fall back to using double-copy.
The kern.ipc.shm_use_phys sysctl defaults to 0 (off) and may be set to 0
(off) or 1 (on). Setting this parameter to 1 will cause all System V
shared memory segments to be mapped to unpageable physical RAM. This
feature only has an effect if you are either (A) mapping small amounts of
shared memory across many (hundreds) of processes, or (B) mapping large
amounts of shared memory across any number of processes. This feature
allows the kernel to remove a great deal of internal memory management
page-tracking overhead at the cost of wiring the shared memory into core,
making it unswappable.
The vfs.vmiodirenable sysctl defaults to 1 (on). This parameter controls
how directories are cached by the system. Most directories are small and
use but a single fragment (typically 2K) in the file system and even less
(typically 512 bytes) in the buffer cache. However, when operating in
the default mode the buffer cache will only cache a fixed number of
directories even if you have a huge amount of memory. Turning on this
sysctl allows the buffer cache to use the VM Page Cache to cache the
directories. The advantage is that all of memory is now available for
caching directories. The disadvantage is that the minimum in-core memory
used to cache a directory is the physical page size (typically 4K) rather
than 512 bytes. We recommend turning this option off in memory-
constrained environments; however, when on, it will substantially improve
the performance of services that manipulate a large number of files.
Such services can include web caches, large mail systems, and news
systems. Turning on this option will generally not reduce performance
even with the wasted memory but you should experiment to find out.
The vfs.write_behind sysctl defaults to 1 (on). This tells the file
system to issue media writes as full clusters are collected, which
typically occurs when writing large sequential files. The idea is to
avoid saturating the buffer cache with dirty buffers when it would not
benefit I/O performance. However, this may stall processes and under
certain circumstances you may wish to turn it off.
The vfs.hirunningspace sysctl determines how much outstanding write I/O
may be queued to disk controllers system-wide at any given time. It is
used by the UFS file system. The default is self-tuned and usually
sufficient but on machines with advanced controllers and lots of disks
this may be tuned up to match what the controllers buffer. Configuring
this setting to match tagged queuing capabilities of controllers or
drives with average IO size used in production works best (for example:
16 MiB will use 128 tags with IO requests of 128 KiB). Note that setting
too high a value (exceeding the buffer cache's write threshold) can lead
to extremely bad clustering performance. Do not set this value
arbitrarily high! Higher write queuing values may also add latency to
reads occurring at the same time.
The vfs.read_max sysctl governs VFS read-ahead and is expressed as the
number of blocks to pre-read if the heuristics algorithm decides that the
reads are issued sequentially. It is used by the UFS, ext2fs and msdosfs
file systems. With the default UFS block size of 32 KiB, a setting of 64
will allow speculatively reading up to 2 MiB. This setting may be
increased to get around disk I/O latencies, especially where these
latencies are large such as in virtual machine emulated environments. It
may be tuned down in specific cases where the I/O load is such that read-
ahead adversely affects performance or where system memory is really low.
The vfs.ncsizefactor sysctl defines how large VFS namecache may grow.
The number of currently allocated entries in namecache is provided by
debug.numcache sysctl and the condition debug.numcache < kern.maxvnodes *
vfs.ncsizefactor is adhered to.
The vfs.ncnegfactor sysctl defines how many negative entries VFS
namecache is allowed to create. The number of currently allocated
negative entries is provided by debug.numneg sysctl and the condition
vfs.ncnegfactor * debug.numneg < debug.numcache is adhered to.
There are various other buffer-cache and VM page cache related sysctls.
We do not recommend modifying these values. As of FreeBSD 4.3, the VM
system does an extremely good job tuning itself.
The net.inet.tcp.sendspace and net.inet.tcp.recvspace sysctls are of
particular interest if you are running network intensive applications.
They control the amount of send and receive buffer space allowed for any
given TCP connection. The default sending buffer is 32K; the default
receiving buffer is 64K. You can often improve bandwidth utilization by
increasing the default at the cost of eating up more kernel memory for
each connection. We do not recommend increasing the defaults if you are
serving hundreds or thousands of simultaneous connections because it is
possible to quickly run the system out of memory due to stalled
connections building up. But if you need high bandwidth over a fewer
number of connections, especially if you have gigabit Ethernet,
increasing these defaults can make a huge difference. You can adjust the
buffer size for incoming and outgoing data separately. For example, if
your machine is primarily doing web serving you may want to decrease the
recvspace in order to be able to increase the sendspace without eating
too much kernel memory. Note that the routing table (see route(8)) can
be used to introduce route-specific send and receive buffer size
defaults.
As an additional management tool you can use pipes in your firewall rules
(see ipfw(8)) to limit the bandwidth going to or from particular IP
blocks or ports. For example, if you have a T1 you might want to limit
your web traffic to 70% of the T1's bandwidth in order to leave the
remainder available for mail and interactive use. Normally a heavily
loaded web server will not introduce significant latencies into other
services even if the network link is maxed out, but enforcing a limit can
smooth things out and lead to longer term stability. Many people also
enforce artificial bandwidth limitations in order to ensure that they are
not charged for using too much bandwidth.
Setting the send or receive TCP buffer to values larger than 65535 will
result in a marginal performance improvement unless both hosts support
the window scaling extension of the TCP protocol, which is controlled by
the net.inet.tcp.rfc1323 sysctl. These extensions should be enabled and
the TCP buffer size should be set to a value larger than 65536 in order
to obtain good performance from certain types of network links;
specifically, gigabit WAN links and high-latency satellite links.
RFC1323 support is enabled by default.
The net.inet.tcp.always_keepalive sysctl determines whether or not the
TCP implementation should attempt to detect dead TCP connections by
intermittently delivering “keepalives” on the connection. By default,
this is enabled for all applications; by setting this sysctl to 0, only
applications that specifically request keepalives will use them. In most
environments, TCP keepalives will improve the management of system state
by expiring dead TCP connections, particularly for systems serving dialup
users who may not always terminate individual TCP connections before
disconnecting from the network. However, in some environments, temporary
network outages may be incorrectly identified as dead sessions, resulting
in unexpectedly terminated TCP connections. In such environments,
setting the sysctl to 0 may reduce the occurrence of TCP session
disconnections.
The net.inet.tcp.delayed_ack TCP feature is largely misunderstood.
Historically speaking, this feature was designed to allow the
acknowledgement to transmitted data to be returned along with the
response. For example, when you type over a remote shell, the
acknowledgement to the character you send can be returned along with the
data representing the echo of the character. With delayed acks turned
off, the acknowledgement may be sent in its own packet, before the remote
service has a chance to echo the data it just received. This same
concept also applies to any interactive protocol (e.g., SMTP, WWW, POP3),
and can cut the number of tiny packets flowing across the network in
half. The FreeBSD delayed ACK implementation also follows the TCP
protocol rule that at least every other packet be acknowledged even if
the standard 100ms timeout has not yet passed. Normally the worst a
delayed ACK can do is slightly delay the teardown of a connection, or
slightly delay the ramp-up of a slow-start TCP connection. While we are
not sure we believe that the several FAQs related to packages such as
SAMBA and SQUID which advise turning off delayed acks may be referring to
the slow-start issue.
The net.inet.ip.portrange.* sysctls 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, selectable via the
IP_PORTRANGE setsockopt(2) call. Most network programs use the default
range which is controlled by net.inet.ip.portrange.first and
net.inet.ip.portrange.last, which default to 49152 and 65535,
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 a server
which handles 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 out of ports, we recommend decreasing
net.inet.ip.portrange.first modestly. A range of 10000 to 30000 ports
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. By default net.inet.ip.portrange.last is
set at the maximum allowable port number.
The kern.ipc.somaxconn sysctl 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, we recommend increasing this value
to 1024 or higher. The service daemon may itself limit the listen queue
size (e.g., sendmail(8), apache) but will often have a directive in its
configuration file to adjust the queue size up. Larger listen queues
also do a better job of fending off denial of service attacks.
The kern.maxfiles sysctl determines how many open files the system
supports. The default is typically a few thousand but you may need to
bump this up to ten or twenty thousand if you are running databases or
large descriptor-heavy daemons. The read-only kern.openfiles sysctl may
be interrogated to determine the current number of open files on the
system.
The vm.swap_idle_enabled sysctl is useful in large multi-user systems
where you have lots of users entering and leaving the system and lots of
idle processes. Such systems tend to generate a great deal of continuous
pressure on free memory reserves. Turning this feature on and adjusting
the swapout hysteresis (in idle seconds) via vm.swap_idle_threshold1 and
vm.swap_idle_threshold2 allows you to depress the priority of pages
associated with idle processes more quickly then the normal pageout
algorithm. This gives a helping hand to the pageout daemon. Do not turn
this option on unless you need it, because the tradeoff you are making is
to essentially pre-page memory sooner rather than later, eating more swap
and disk bandwidth. In a small system this option will have a
detrimental effect but in a large system that is already doing moderate
paging this option allows the VM system to stage whole processes into and
out of memory more easily.
LOADER TUNABLES
Some aspects of the system behavior may not be tunable at runtime because
memory allocations they perform must occur early in the boot process. To
change loader tunables, you must set their values in loader.conf(5) and
reboot the system.
kern.maxusers controls the scaling of a number of static system tables,
including defaults for the maximum number of open files, sizing of
network memory resources, etc. As of FreeBSD 4.5, 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 as described elsewhere in this
document. Systems older than FreeBSD 4.4 must set this value via the
kernel config(8) option maxusers instead.
The kern.dfldsiz and kern.dflssiz tunables set the default soft limits
for process data and stack size respectively. Processes may increase
these up to the hard limits by calling setrlimit(2). The kern.maxdsiz,
kern.maxssiz, and kern.maxtsiz tunables set the hard limits for process
data, stack, and text size respectively; processes may not exceed these
limits. The kern.sgrowsiz tunable controls how much the stack segment
will grow when a process needs to allocate more stack.
kern.ipc.nmbclusters may be adjusted to increase the number of network
mbufs the system is willing to allocate. Each cluster represents
approximately 2K of memory, so a value of 1024 represents 2M of kernel
memory reserved for network buffers. You can do a simple calculation to
figure out how many you need. If you have a web server which maxes out
at 1000 simultaneous connections, and each connection eats a 16K receive
and 16K send buffer, you need approximately 32MB worth of network buffers
to deal with it. A good rule of thumb is to multiply by 2, so 32MBx2 =
64MB/2K = 32768. So for this case you would want to set
kern.ipc.nmbclusters to 32768. We recommend values between 1024 and 4096
for machines with moderates amount of memory, and between 4096 and 32768
for machines with greater amounts of memory. Under no circumstances
should you specify an arbitrarily high value for this parameter, it could
lead to a boot-time crash. The -m option to netstat(1) may be used to
observe network cluster use. Older versions of FreeBSD do not have this
tunable and require that the kernel config(8) option NMBCLUSTERS be set
instead.
More and more programs are using the sendfile(2) system call to transmit
files over the network. The kern.ipc.nsfbufs sysctl controls the number
of file system buffers sendfile(2) is allowed to use to perform its work.
This parameter nominally scales with kern.maxusers so you should not need
to modify this parameter except under extreme circumstances. See the
TUNING section in the sendfile(2) manual page for details.
KERNEL CONFIG TUNING
There are a number of kernel options that you may have to fiddle with in
a large-scale system. In order to change these options you need to be
able to compile a new kernel from source. The config(8) manual page and
the handbook are good starting points for learning how to do this.
Generally the first thing you do when creating your own custom kernel is
to strip out all the drivers and services you do not use. Removing
things like INET6 and drivers you do not have will reduce the size of
your kernel, sometimes by a megabyte or more, leaving more memory
available for applications.
SCSI_DELAY may be used to reduce system boot times. The defaults are
fairly high and can be responsible for 5+ seconds of delay in the boot
process. Reducing SCSI_DELAY to something below 5 seconds could work
(especially with modern drives).
There are a number of *_CPU options that can be commented out. If you
only want the kernel to run on a Pentium class CPU, you can easily remove
I486_CPU, but only remove I586_CPU if you are sure your CPU is being
recognized as a Pentium II or better. Some clones may be recognized as a
Pentium or even a 486 and not be able to boot without those options. If
it works, great! The operating system will be able to better use higher-
end CPU features for MMU, task switching, timebase, and even device
operations. Additionally, higher-end CPUs support 4MB MMU pages, which
the kernel uses to map the kernel itself into memory, increasing its
efficiency under heavy syscall loads.
CPU, MEMORY, DISK, NETWORK
The type of tuning you do depends heavily on where your system begins to
bottleneck as load increases. If your system runs out of CPU (idle times
are perpetually 0%) then you need to consider upgrading the CPU or
perhaps you need to revisit the programs that are causing the load and
try to optimize them. If your system is paging to swap a lot you need to
consider adding more memory. If your system is saturating the disk you
typically see high CPU idle times and total disk saturation. systat(1)
can be used to monitor this. There are many solutions to saturated
disks: increasing memory for caching, mirroring disks, distributing
operations across several machines, and so forth. If disk performance is
an issue and you are using IDE drives, switching to SCSI can help a great
deal. While modern IDE drives compare with SCSI in raw sequential
bandwidth, the moment you start seeking around the disk SCSI drives
usually win.
Finally, you might run out of network suds. Optimize the network path as
much as possible. For example, in firewall(7) we describe a firewall
protecting internal hosts with a topology where the externally visible
hosts are not routed through it. Use 1000BaseT rather than 100BaseT,
depending on your needs. Most bottlenecks occur at the WAN link (e.g.,
modem, T1, DSL, whatever). If expanding the link is not an option it may
be possible to use the dummynet(4) feature to implement peak shaving or
other forms of traffic shaping to prevent the overloaded service (such as
web services) from affecting other services (such as email), or vice
versa. In home installations this could be used to give interactive
traffic (your browser, ssh(1) logins) priority over services you export
from your box (web services, email).
SEE ALSO
netstat(1), systat(1), sendfile(2), ata(4), dummynet(4), eventtimers(4),
login.conf(5), rc.conf(5), sysctl.conf(5), firewall(7), hier(7),
ports(7), boot(8), bsdinstall(8), ccdconfig(8), config(8), fsck(8),
gjournal(8), gpart(8), gstripe(8), gvinum(8), ifconfig(8), ipfw(8),
loader(8), mount(8), newfs(8), route(8), sysctl(8), tunefs(8)
HISTORY
The tuning manual page was originally written by Matthew Dillon and first
appeared in FreeBSD 4.3, May 2001. The manual page was greatly modified
by Eitan Adler <
[email protected]>.
FreeBSD 11.1-RELEASE-p4 March 22, 2017 FreeBSD 11.1-RELEASE-p4
