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TUNING(7) FreeBSD Miscellaneous Information Manual TUNING(7) NAME tuning -- performance tuning under FreeBSD SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP When using disklabel(8) to lay out your filesystems 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 filesystems and swap closer to the outer tracks, follow with the larger filesystems, and end with the largest filesystems. It is also important to size system standard filesystems 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 mem- ory. If you do not have a lot of RAM, though, you will generally 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 expan- sion when sizing the swap partition. The kernel's VM paging algorithms are tuned to perform best when there is at least 2x swap versus main mem- ory. 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 mem- ory 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 (up to four drives). 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. Don't worry about overdoing it a little, swap space is the sav- ing grace of UNIX and even if you don't 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 isn't 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, but it is usually a good idea to make /tmp a softlink to /var/tmp after the fact. Dedicating a partition for tempo- rary file storage is important for two reasons: first, it reduces the possibility of filesystem 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. 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 don't have to worry about undersizing things! Well, there are several reasons this isn't a good idea. First, each partition has different operational characteristics and separating them allows the filesystem 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/inode, and cylinders/group. FreeBSD performs best when using 8K or 16K filesystem block sizes. The default filesystem 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 applications 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 filesystem that requires a very large number of inodes or is intended to hold a large number of very small files. Such a filesystem 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) options 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 a database files, you can increase the bytes/inode ratio which reduces the number of inodes (maximum number of files and directories that can be created) for that partition. Decreasing the number of inodes in a filesystem 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 filesys- tem that has lots of free space remaining but cannot accommodate any more files. Using 32768, 65536, or 262144 bytes/inode 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 filesystem. This command can be run in single-user mode without having to reformat the filesystem. How- ever, this is possibly the most abused program in the system. Many peo- ple attempt to increase available filesystem space by setting the min- free percentage to 0. This can lead to severe filesystem fragmentation and we do not recommend that you do this. Really the only tunefs(8) option 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)). Softupdates drastically improves meta-data performance, mainly file creation and deletion. We recommend enabling softupdates on all of your filesystems. There are two downsides to softupdates that you should be aware of. First, softupdates guarantees filesystem consistency in the case of a crash but could very easily be several seconds (even a minute!) behind updating the physical disk. If you crash you may lose more work than otherwise. Secondly, softupdates delays the freeing of filesystem blocks. If you have a filesystem (such as the root filesystem) 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. 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. Don't 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. 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 exam- ple, the /var filesystem customarily holds mailboxes, and atime (in com- bination 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 filesystem by splitting I/O operations across two or more disks. The vinum(8) and ccdconfig(8) utilities may be used to cre- ate simple striped filesystems. Generally speaking, striping smaller partitions such as the root and /var/tmp, or essentially read-only parti- tions 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. Filesystems 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 aren't. 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 filesystem 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-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. There are various 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. This controls 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 connec- tions 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 intro- duce 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 then 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 out of certain types of network links; specif- ically, gigabit WAN links and high-latency satellite links. RFC1323 sup- port is enabled by default. We recommend that you turn on (set to 1) and leave on the net.inet.tcp.always_keepalive control. The default is usually off. This introduces a small amount of additional network bandwidth but guarantees that dead TCP connections will eventually be recognized and cleared. Dead TCP connections are a particular problem on systems accessed by users operating over dialups, because users often disconnect their modems without properly closing active connections. 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 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 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 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 then 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. 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 net- work memory resouces, etc. As of FreeBSD 4.5, kern.maxusers is automati- cally 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. 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 approximate 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 filesystem 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. 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 don't use. Removing things like INET6 and drivers you don't have will reduce the size of your ker- nel, sometimes by a megabyte or more, leaving more memory available for applications. SCSI_DELAY and IDE_DELAY may be used to reduce system boot times. The defaults are fairly high and can be responsible for 15+ seconds of delay in the boot process. Reducing SCSI_DELAY to 5 seconds usually works (especially with modern drives). Reducing IDE_DELAY also works but you have to be a little more careful. 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 I386_CPU and 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 recog- nized 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 bet- ter-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, which increases its efficiency under heavy syscall loads. IDE WRITE CACHING 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 when under heavy disk loads. A crash or power failure can result in serious filesystem 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 ata(4.) There is a new experimental feature for IDE hard drives called hw.ata.tags (you also set this in the boot loader) which allows write caching to be safely turned on. This brings SCSI tagging features to IDE drives. As of this writing only IBM DPTA and DTLA drives support the feature. Warning! These drives apparently have quality control problems and I do not recommend purchasing them at this time. If you need perfor- mance, 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 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. 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 backoff 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 then 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 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), ata(4), dummynet(4), login.conf(5), rc.conf(5), sysctl.conf(5), firewall(7), hier(7), ports(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), vinum(8) HISTORY The tuning manual page was originally written by Matthew Dillon and first appeared in FreeBSD 4.3, May 2001. FreeBSD 11.1 May 25, 2001 FreeBSD 11.1
NAME | SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP | STRIPING DISKS | SYSCTL TUNING | LOADER TUNABLES | KERNEL CONFIG TUNING | IDE WRITE CACHING | CPU, MEMORY, DISK, NETWORK | SEE ALSO | HISTORY
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