DragonFly On-Line Manual Pages
TUNING(7) DragonFly Miscellaneous Information Manual TUNING(7)
tuning -- performance tuning under DragonFly
Modern DragonFly systems typically have just three partitions on the main
drive. In order, a UFS /boot, swap, and a HAMMER root. The installer
used to create separate PFSs for half a dozen directories, but now it
just puts (almost) everything in the root. It will separate stuff that
doesn't need to be backed up into a /build subdirectory and create null-
mounts for things like /usr/obj, but it no longer creates separate PFSs
for these. If desired, you can make /build its own mount to separate-out
the components of the filesystem which do not need to be persistent.
Generally speaking the /boot partition should be 1GB in size. This is
the minimum recommended size, giving you room for backup kernels and
alternative boot schemes. DragonFly always installs debug-enabled ker-
nels and modules and these can take up quite a bit of disk space (but
will not take up any extra ram).
In the old days we recommended that swap be sized to at least 2x main
memory. These days swap is often used for other activities, including
tmpfs(5) and swapcache(8). We recommend that swap be sized to the larger
of 2x main memory or 1GB if you have a fairly small disk and 16GB or more
if you have a modestly endowed system. If you have a modest SSD + large
HDD combination, we recommend a large dedicated swap partition on the
SSD. For example, if you have a 128GB SSD and 2TB or more of HDD stor-
age, dedicating upwards of 64GB of the SSD to swap and using swapcache(8)
and tmpfs(5) will significantly improve your HDD's performance.
In an all-SSD or mostly-SSD system, swapcache(8) is not normally used but
you may still want to have a large swap partition to support tmpfs(5)
use. Our synth/poudriere build machines run with a 200GB swap partition
and use tmpfs for all the builder jails. 50-100 GB is swapped out at the
peak of the build. As a result, actual system storage bandwidth is mini-
mized and performance increased.
If you are on a minimally configured machine you may, of course, config-
ure far less swap or no swap at all but we recommend at least some swap.
The kernel's VM paging algorithms are tuned to perform best when there is
swap space configured. Configuring too little swap can lead to ineffi-
ciencies in the VM page scanning code as well as create issues later on
if you add more memory to your machine, so don't be shy about it. Swap
is a good idea even if you don't think you will ever need it as it allows
the machine to page out completely unused data and idle programs (like
getty), maximizing the ram available for your activities.
If you intend to use the swapcache(8) facility with a SSD + HDD combina-
tion we recommend configuring as much swap space as you can on the SSD.
However, keep in mind that each 1GByte of swapcache requires around
1MByte of ram, so don't scale your swap beyond the equivalent ram that
you reasonably want to eat to support it.
Finally, on larger systems with multiple drives, if the use of SSD swap
is not in the cards or if it is and you need higher-than-normal swapcache
bandwidth, you can configure swap on up to four drives and the kernel
will interleave the storage. 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 over-
doing 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. However, keep in mind
that any sort of swap space failure can lock the system up. Most
machines are setup with only one or two swap partitions.
Most DragonFly systems have a single HAMMER root. PFSs can be used to
administratively separate domains for backup purposes but tend to be a
hassle otherwise so if you don't need the administrative separation you
don't really need to use multiple HAMMER PFSs. All the PFSs share the
same allocation layer so there is no longer a need to size each individ-
ual mount. Instead you should review the hammer(8) manual page and use
the 'hammer viconfig' facility to adjust snapshot retention and other
parameters. By default HAMMER keeps 60 days worth of snapshots. Usually
snapshots are not desired on PFSs such as /usr/obj or /tmp since data on
these partitions cycles a lot.
If a very large work area is desired it is often beneficial to configure
it as a separate HAMMER mount. If it is integrated into the root mount
it should at least be its own HAMMER PFS. We recommend naming the large
work area /build. Similarly if a machine is going to have a large number
of users you might want to separate your /home out as well.
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. Do not ever
use it; it is far too dangerous. A less dangerous and more useful
mount(8) option is called noatime. UNIX filesystems normally update the
last-accessed time of a file or directory whenever it is accessed. How-
ever, this creates a massive burden on copy-on-write filesystems like
HAMMER, particularly when scanning the filesystem. DragonFly currently
defaults to disabling atime updates on HAMMER mounts. It can be enabled
by setting the vfs.hammer.noatime tunable to 0 in loader.conf(5) but we
recommend leaving it disabled. The lack of atime updates can create
issues with certain programs such as when detecting whether unread mail
is present, but applications for the most part no longer depend on it.
The single most important thing you can do is have at least one solid-
state drive in your system, and configure your swap space on that drive.
If you are using a combination of a smaller SSD and a very larger HDD,
you can use swapcache(8) to automatically cache data from your HDD. But
even if you do not, having swap space configured on your SSD will signif-
icantly improve performance under even modest paging loads. It is par-
ticularly useful to configure a significant amount of swap on a worksta-
tion, 32GB or more is not uncommon, to handle bloated leaky applications
such as browsers.
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 kern.ipc.shm_use_phys sysctl defaults to 1 (on) 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.write_behind sysctl defaults to 1 (on). This tells the filesys-
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
extremely 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.bufcache_bw controls data cycling within
the buffer cache. I/O bandwidth less than this specification (per sec-
ond) will cycle into the much larger general VM page cache while I/O
bandwidth in excess of this specification will be recycled within the
buffer cache, reducing the load on the rest of the VM system. The
default value is 200 megabytes (209715200), which means that the system
will try harder to cache data coming off a slower hard drive and less
hard trying to cache data coming off a fast SSD. This parameter is par-
ticularly important if you have NVMe drives in your system as these stor-
age devices are capable of transferring well over 2GBytes/sec into the
There are various other buffer-cache and VM page cache related sysctls.
We do not recommend modifying their values.
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. However, DragonFly now auto-tunes these parameters using
a number of other related sysctls (run 'sysctl net.inet.tcp' to get a
list) and usually no longer need to be tuned manually. We do not recom-
mend increasing or decreasing the defaults if you are managing a very
large number of connections. Note that the routing table (see route(8))
can be used to introduce route-specific send and receive buffer size
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; specifi-
cally, gigabit WAN links and high-latency satellite links. RFC 1323 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
intermittently delivering ``keepalives'' on the connection. By default,
this is now enabled for all applications. We do not recommend turning it
off. The extra network bandwidth is minimal and this feature will clean-
up stalled and long-dead connections that might not otherwise be cleaned
up. In the past people using dialup connections often did not want to
use this feature in order to be able to retain connections across long
disconnections, but in modern day the only default that makes sense is
for the feature to be turned on.
The net.inet.tcp.delayed_ack TCP feature is largely misunderstood. His-
torically 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 pro-
tocol (e.g. SMTP, WWW, POP3) and can cut the number of tiny packets flow-
ing across the network in half. The DragonFly delayed-ack implementa-
tion 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 connec-
tion. While we aren't 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.tcp.inflight_enable sysctl turns on bandwidth delay product
limiting for all TCP connections. This feature is now turned on by
default and we recommend that it be left on. It will slightly reduce the
maximum bandwidth of a connection but the benefits of the feature in
reducing packet backlogs at router constriction points are enormous.
These benefits make it a whole lot easier for router algorithms to manage
QOS for multiple connections. 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).
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 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.
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.
Adjusting net.inet.tcp.inflight_stab is not recommended. This parameter
defaults to 50, representing +5% fudge when calculating the bwnd from the
bw. This fudge is on top of an additional fixed +2*maxseg added to bwnd.
The fudge factor is required to stabilize the algorithm at very high
speeds while the fixed 2*maxseg stabilizes the algorithm at low speeds.
If you increase this value excessive packet buffering may occur.
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 an IP_PORTRANGE
setsockopt() call. Most network programs use the default range which is
controlled by net.inet.ip.portrange.first and net.inet.ip.portrange.last,
which defaults 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 serves 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
we recommend increasing net.inet.ip.portrange.last modestly. A value of
10000 or 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 we
do not recommend that net.inet.ip.portrange.first be lowered.
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.maxvnodes specifies how many vnodes and related file structures
the kernel will cache. The kernel uses a very generous default for this
parameter based on available physical memory. You generally do not want
to mess with this parameter as it directly effects how well the kernel
can cache not only file structures but also the underlying file data.
But you can lower it if kernel memory use is higher than you would like.
The kern.maxfiles sysctl determines how many open files the system sup-
ports. The default is typically based on available physical memory but
you may need to bump it up if you are running databases or large descrip-
tor-heavy daemons. The read-only kern.openfiles sysctl may be interro-
gated 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 than 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. On DragonFly, 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.
kern.nbuf sets how many filesystem buffers the kernel should cache.
Filesystem buffers can be up to 128KB each. UFS typically uses an 8KB
blocksize while HAMMER typically uses 64KB. The defaults usually suf-
fice. The cached buffers represent wired physical memory so specifying a
value that is too large can result in excessive kernel memory use, and is
also not entirely necessary since the pages backing the buffers are also
cached by the VM page cache (which does not use wired memory). The buf-
fer cache significantly improves the hot path for cached file accesses.
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 and kern.ipc.nmbjclusters may be adjusted to
increase the number of network mbufs the system is willing to allocate.
Each normal cluster represents approximately 2K of memory, so a value of
1024 represents 2M of kernel memory reserved for network buffers. Each
'j' cluster is typically 4KB, so a value of 1024 represents 4M of kernel
memory. You can do a simple calculation to figure out how many you need
but keep in mind that tcp buffer sizing is now more dynamic than it used
The defaults usually suffice but you may want to bump it up on service-
heavy machines. Modern machines often need a large number of mbufs to
operate services efficiently, values of 65536, even upwards of 262144 or
more are common. If you are running a server, it is better to be gener-
ous than to be frugal. Remember the memory calculation though.
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.
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. 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
If your motherboard is AHCI-capable then we strongly recommend turning on
AHCI mode in the BIOS if it is not the default.
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 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
machines, and so forth.
Finally, you might run out of network suds. Optimize the network path as
much as possible. If you are operating a machine as a router you may
need to setup a pf(4) firewall (also see firewall(7). DragonFly has a
very good fair-share queueing algorithm for QOS in pf(4).
SOURCE OF KERNEL MEMORY USAGE
The primary sources of kernel memory usage are:
kern.maxvnodes The maximum number of cached vnodes in the system. These
can eat quite a bit of kernel memory, primarily due to
auxiliary structures tracked by the HAMMER filesystem.
It is relatively easy to configure a smaller value, but
we do not recommend reducing this parameter below 100000.
Smaller values directly impact the number of discrete
files the kernel can cache data for at once.
Calculate approximately 2KB per normal cluster and 4KB
per jumbo cluster. Do not make these values too low or
you risk deadlocking the network stack.
kern.nbuf The number of filesystem buffers managed by the kernel.
The kernel wires the underlying cached VM pages, typi-
cally 8KB (UFS) or 64KB (HAMMER) per buffer.
swap/swapcache Swap memory requires approximately 1MB of physical ram
for each 1GB of swap space. When swapcache is used,
additional memory may be required to keep VM objects
around longer (only really reducable by reducing the
value of kern.maxvnodes which you can do post-boot if you
tmpfs Tmpfs is very useful but keep in mind that while the file
data itself is backed by swap, the meta-data (the direc-
tory topology) requires wired kernel memory.
mmu page tables
Even though the underlying data pages themselves can be
paged to swap, the page tables are usually wired into
memory. This can create problems when a large number of
processes are mmap()ing very large files. Sometimes
turning on machdep.pmap_mmu_optimize suffices to reduce
overhead. Page table kernel memory use can be observed
by using 'vmstat -z'
It is sometimes necessary to force shared memory to use
physical memory when running a large database which uses
shared memory to implement its own data caching. The use
of sysv shared memory in this regard allows the database
to distinguish between data which it knows it can access
instantly (i.e. without even having to page-in from
swap) verses data which it might require and I/O to
If you use this feature be very careful with regards to
the database's shared memory configuration as you will be
wiring the memory.
netstat(1), systat(1), dm(4), dummynet(4), nata(4), pf(4), login.conf(5),
pf.conf(5), rc.conf(5), sysctl.conf(5), firewall(7), hier(7), boot(8),
ccdconfig(8), config(8), disklabel(8), fsck(8), ifconfig(8), ipfw(8),
loader(8), mount(8), newfs(8), route(8), sysctl(8), tunefs(8)
The tuning manual page was inherited from FreeBSD and first appeared in
FreeBSD 4.3, May 2001.
The tuning manual page was originally written by Matthew Dillon.
DragonFly 4.9 April 27, 2017 DragonFly 4.9