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 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 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 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 caches 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 occurring at the same time.
There are various other buffer-cache and VM page cache related sysctls. We do not recommend modifying these values. As of
.Fx 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 T1s 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
.Fx 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
.Fx , it would be more beneficial to increase the slow-start flightsize via the net.inet.tcp.slowstart_flightsize sysctl rather than disable delayed acks.
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 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. 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. 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 hosts 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 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 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 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.
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
.Fx 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
.Fx 4.4 must set this value via the kernel config(8) option
.Cd maxusers instead.
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
.Fx 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.
IDE WRITE CACHING
.Fx 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. Unfortunately, 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 performance, go with SCSI.
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 CPUs), 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. 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), 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), ifconfig(8), ipfw(8), loader(8), mount(8), newfs(8), route(8), sysctl(8), sysinstall(8), tunefs(8), vinum(8)