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TUNING(7)	     BSD Miscellaneous Information Manual	     TUNING(7)

     tuning -- performance tuning under	FreeBSD

     When using	bsdlabel(8) or sysinstall(8) to	lay out	your file systems on a
     hard disk it is important to remember that	hard drives can	transfer data
     much more quickly from outer tracks than they can from inner tracks.  To
     take advantage of this you	should try to pack your	smaller	file systems
     and swap closer to	the outer tracks, follow with the larger file systems,
     and end with the largest file systems.  It	is also	important to size sys-
     tem standard file systems such that you will not be forced	to resize them
     later as you scale	the machine up.	 I usually create, in order, a 128M
     root, 1G swap, 128M /var, 128M /var/tmp, 3G /usr, and use any remaining
     space for /home.

     You should	typically size your swap space to approximately	2x main	memory
     for systems with less than	2GB of RAM, or approximately 1x	main memory if
     you have more.  If	you do not have	a lot of RAM, though, you will gener-
     ally want a lot more swap.	 It is not recommended that you	configure any
     less than 256M of swap on a system	and you	should keep in mind future
     memory expansion when sizing the swap partition.  The kernel's VM paging
     algorithms	are tuned to perform best when there is	at least 2x swap ver-
     sus main memory.  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.  Finally, on larger systems with multiple
     SCSI disks	(or multiple IDE disks operating on different controllers), we
     strongly recommend	that you configure swap	on each	drive.	The swap par-
     titions 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 sav-
     ing 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.

     How you size your /var partition depends heavily on what you intend to
     use the machine for.  This	partition is primarily used to hold mailboxes,
     the print spool, and log files.  Some people even make /var/log its own
     partition (but except for extreme cases it	is not worth the waste of a
     partition ID).  If	your machine is	intended to act	as a mail or print
     server, or	you are	running	a heavily visited web server, you should con-
     sider creating a much larger partition - perhaps a	gig or more.  It is
     very easy to underestimate	log file storage requirements.

     Sizing /var/tmp depends on	the kind of temporary file usage you think you
     will need.	 128M is the minimum we	recommend.  Also note that sysinstall
     will create a /tmp	directory.  Dedicating a partition for temporary file
     storage is	important for two reasons: first, it reduces the possibility
     of	file system corruption in a crash, and second it reduces the chance of
     a runaway process that fills up [/var]/tmp	from blowing up	more critical
     subsystems	(mail, logging,	etc).  Filling up [/var]/tmp is	a very common
     problem to	have.

     In	the old	days there were	differences between /tmp and /var/tmp, but the
     introduction of /var (and /var/tmp) led to	massive	confusion by program
     writers so	today programs haphazardly use one or the other	and thus no
     real distinction can be made between the two.  So it makes	sense to have
     just one temporary	directory and softlink to it from the other tmp	direc-
     tory locations.  However you handle /tmp, the one thing you do not	want
     to	do is leave it sitting on the root partition where it might cause root
     to	fill up	or possibly corrupt root in a crash/reboot situation.

     The /usr partition	holds the bulk of the files required to	support	the
     system and	a subdirectory within it called	/usr/local holds the bulk of
     the files installed from the ports(7) hierarchy.  If you do not use ports
     all that much and do not intend to	keep system source (/usr/src) on the
     machine, you can get away with a 1	gigabyte /usr partition.  However, if
     you install a lot of ports	(especially window managers and	Linux-emulated
     binaries),	we recommend at	least a	2 gigabyte /usr	and if you also	intend
     to	keep system source on the machine, we recommend	a 3 gigabyte /usr.  Do
     not underestimate the amount of space you will need in this partition, it
     can creep up and surprise you!

     The /home partition is typically used to hold user-specific data.	I usu-
     ally size it to the remainder of the disk.

     Why partition at all?  Why	not create one big / partition and be done
     with it?  Then I do not have to worry about undersizing things!  Well,
     there are several reasons this is not a good idea.	 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 and /var/tmp.  By	prop-
     erly partitioning your system fragmentation introduced in the smaller
     more heavily write-loaded partitions will not bleed over into the mostly-
     read partitions.  Additionally, keeping the write-loaded partitions
     closer to the edge	of the disk (i.e., before the really big partitions
     instead of	after in the partition table) will increase I/O	performance in
     the partitions where you need it the most.	 Now it	is true	that you might
     also need I/O performance in the larger partitions, but they are so large
     that shifting them	more towards the edge of the disk will not lead	to a
     significant performance improvement whereas moving	/var to	the edge can
     have a huge impact.  Finally, there are safety concerns.  Having a	small
     neat root partition that is essentially read-only gives it	a greater
     chance of surviving a bad crash intact.

     Properly partitioning your	system also allows you to tune newfs(8), and
     tunefs(8) parameters.  Tuning newfs(8) requires more experience but can
     lead to significant improvements in performance.  There are three parame-
     ters that are relatively safe to tune: blocksize, bytes/i-node, and

     FreeBSD performs best when	using 8K or 16K	file system block sizes.  The
     default file system block size is 16K, which provides best	performance
     for most applications, with the exception of those	that perform random
     access on large files (such as database server software).	Such applica-
     tions tend	to perform better with a smaller block size, although modern
     disk characteristics are such that	the performance	gain from using	a
     smaller block size	may not	be worth consideration.	 Using a block size
     larger than 16K can cause fragmentation of	the buffer cache and lead to
     lower performance.

     The defaults may be unsuitable for	a file system that requires a very
     large number of i-nodes or	is intended to hold a large number of very
     small files.  Such	a file system should be	created	with an	8K or 4K block
     size.  This also requires you to specify a	smaller	fragment size.	We
     recommend always using a fragment size that is 1/8	the block size (less
     testing has been done on other fragment size factors).  The newfs(8) op-
     tions for this would be "newfs -f 1024 -b 8192 ...".

     If	a large	partition is intended to be used to hold fewer,	larger files,
     such as database files, you can increase the bytes/i-node ratio which re-
     duces the number of i-nodes (maximum number of files and directories that
     can be created) for that partition.  Decreasing the number	of i-nodes in
     a file system can greatly reduce fsck(8) recovery times after a crash.
     Do	not use	this option unless you are actually storing large files	on the
     partition,	because	if you overcompensate you can wind up with a file sys-
     tem that has lots of free space remaining but cannot accommodate any more
     files.  Using 32768, 65536, or 262144 bytes/i-node	is recommended.	 You
     can go higher but it will have only incremental effects on	fsck(8)	recov-
     ery times.	 For example, "newfs -i	32768 ...".

     tunefs(8) may be used to further tune a file system.  This	command	can be
     run in single-user	mode without having to reformat	the file system.  How-
     ever, this	is possibly the	most abused program in the system.  Many peo-
     ple attempt to increase available file system space by setting the	min-
     free percentage to	0.  This can lead to severe file system	fragmentation
     and we do not recommend that you do this.	Really the only	tunefs(8) op-
     tion worthwhile here is turning on	softupdates with "tunefs -n enable
     /filesystem".  (Note: in FreeBSD 4.5 and later, softupdates can be	turned
     on	using the -U option to newfs(8), and sysinstall(8) will	typically en-
     able softupdates automatically for	non-root file systems).	 Softupdates
     drastically improves meta-data performance, mainly	file creation and
     deletion.	We recommend enabling softupdates on most file systems;	how-
     ever, there are two limitations to	softupdates that you should be aware
     of	when determining whether to use	it on a	file system.  First, softup-
     dates 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 other-
     wise.  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	han-
     dled in FreeBSD with a delayed write and normally does not	create a bur-
     den 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 utili-
     ties use the atime	field for reporting.

     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 sophisti-
     cated, we recommend using a real hardware RAID controller from the	list
     of	FreeBSD	supported controllers.

     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 sub-
     system.  The virtual memory system	always does accounting of the swap
     space reservation,	both total for system and per-user. Corresponding val-
     ues 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	mem-

     Setting bit 0 of the vm.overcommit	sysctl causes the virtual memory sys-
     tem 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 to fall back to using double-

     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 1K) 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 di-
     rectories 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 di-
     rectories.	 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-con-
     strained 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	sys-
     tems.  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 sys-
     tem to issue media	writes as full clusters	are collected, which typically
     occurs when writing large sequential files.  The idea is to avoid satu-
     rating the	buffer cache with dirty	buffers	when it	would not benefit I/O
     performance.  However, this may stall processes and under certain circum-
     stances 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	instance.  The
     default is	usually	sufficient but on machines with	lots of	disks you may
     want to bump it up	to four	or five	megabytes.  Note that setting too high
     a value (exceeding	the buffer cache's write threshold) can	lead to	ex-
     tremely bad clustering performance.  Do not set this value	arbitrarily
     high!  Also, higher write queueing	values may add latency to reads	occur-
     ring at the same time.

     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 name-
     cache is allowed to create.  The number of	currently allocated negative
     entries is	provided by debug.numneg sysctl	and the	condition vfs.ncneg-
     factor * 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 par-
     ticular 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 connec-
     tion.  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	build-
     ing up.  But if you need high bandwidth over a fewer number of connec-
     tions, 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 re-
     mainder 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; specifi-
     cally, gigabit WAN	links and high-latency satellite links.	 RFC1323 sup-
     port 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 in-
     termittently delivering "keepalives" on the connection.  By default, this
     is	enabled	for all	applications; by setting this sysctl to	0, only	appli-
     cations that specifically request keepalives will use them.  In most en-
     vironments, 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 dis-
     connecting	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,	set-
     ting the sysctl to	0 may reduce the occurrence of TCP session disconnec-

     The net.inet.tcp.delayed_ack TCP feature is largely misunderstood.	 His-
     torically speaking, this feature was designed to allow the	acknowledge-
     ment to transmitted data to be returned along with	the response.  For ex-
     ample, 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.  In	FreeBSD, it
     would be more beneficial to increase the slow-start flightsize via	the
     net.inet.tcp.slowstart_flightsize sysctl rather than disable delayed

     The net.inet.tcp.inflight.enable sysctl turns on bandwidth	delay product
     limiting for all TCP connections.	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	opti-
     mum throughput.  This feature is useful if	you are	serving	data over
     modems, GigE, or 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 de-
     bugging), and for production use setting net.inet.tcp.inflight.min	to at
     least 6144	may be beneficial.  Note however, 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
     router 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 affects data transmission (uploading / server-side).	It
     does not affect data reception (downloading).

     Adjusting net.inet.tcp.inflight.stab is not recommended.  This parameter
     defaults to 20, representing 2 maximal packets added to the bandwidth de-
     lay product window	calculation.  The additional window is required	to
     stabilize the algorithm and improve responsiveness	to changing condi-
     tions, but	it can also result in higher ping times	over slow links
     (though still much	lower than you would get without the inflight algo-
     rithm).  In such cases you	may wish to try	reducing this parameter	to 15,
     10, or 5, and you 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.

     The net.inet.ip.portrange.* sysctls control the port number ranges	auto-
     matically 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 fire-
     wall 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 ac-
     cepting 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	sup-
     ports.  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 in-
     terrogated	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 asso-
     ciated with idle processes	more quickly then the normal pageout algo-
     rithm.  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 detrimen-
     tal 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.

     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	net-
     work memory resources, etc.  As of	FreeBSD	4.5, kern.maxusers is automat-
     ically sized at boot based	on the amount of memory	available in the sys-
     tem, 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; val-
     ues 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	docu-
     ment.  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 approx-
     imately 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 mod-
     erates 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	clus-
     ter 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.

     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.	 Gen-
     erally 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 ker-
     nel, sometimes by a megabyte or more, leaving more	memory available for

     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 rec-
     ognized 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 oper-
     ations.  Additionally, higher-end CPUs support 4MB	MMU pages, which the
     kernel uses to map	the kernel itself into memory, increasing its effi-
     ciency under heavy	syscall	loads.

     FreeBSD 4.3 flirted with turning off IDE write caching.  This reduced
     write bandwidth to	IDE disks but was considered necessary due to serious
     data consistency issues introduced	by hard	drive vendors.	Basically the
     problem is	that IDE drives	lie about when a write completes.  With	IDE
     write caching turned on, IDE hard drives will not only write data to disk
     out of order, they	will sometimes delay some of the blocks	indefinitely
     under heavy disk load.  A crash or	power failure can result in serious
     file system corruption.  So our default was changed to be safe.  Unfortu-
     nately, the result	was such a huge	loss in	performance that we caved in
     and changed the default back to on	after the release.  You	should check
     the default on your system	by observing the hw.ata.wc sysctl variable.
     If	IDE write caching is turned off, you can turn it back on by setting
     the hw.ata.wc loader tunable to 1.	 More information on tuning the	ATA
     driver system may be found	in the ata(4) manual page.  If you need	per-
     formance, go with SCSI.

     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	moving
     to	an SMP motherboard (multiple CPU's), 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 ma-
     chines, 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.  The first line of defense
     for improving network performance is to make sure you are using switches
     instead of	hubs, especially these days where switches are almost as
     cheap.  Hubs have severe problems under heavy loads due to	collision
     back-off and one bad host can severely degrade the	entire LAN.  Second,
     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 100BaseT rather than 10BaseT, or 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).

     netstat(1), systat(1), sendfile(2), ata(4), dummynet(4), login.conf(5),
     rc.conf(5), sysctl.conf(5), firewall(7), hier(7), ports(7), boot(8),
     bsdlabel(8), ccdconfig(8),	config(8), fsck(8), gjournal(8), gstripe(8),
     gvinum(8),	ifconfig(8), ipfw(8), loader(8), mount(8), newfs(8), route(8),
     sysctl(8),	sysinstall(8), tunefs(8)

     The tuning	manual page was	originally written by Matthew Dillon and first
     appeared in FreeBSD 4.3, May 2001.

BSD			       October 16, 2010				   BSD


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