From owner-freebsd-hackers Fri May 25 15:25:47 2001 Delivered-To: freebsd-hackers@freebsd.org Received: from earth.backplane.com (earth-nat-cw.backplane.com [208.161.114.67]) by hub.freebsd.org (Postfix) with ESMTP id 9EE0A37B423 for ; Fri, 25 May 2001 15:25:33 -0700 (PDT) (envelope-from dillon@earth.backplane.com) Received: (from dillon@localhost) by earth.backplane.com (8.11.3/8.11.2) id f4PMPXI44229; Fri, 25 May 2001 15:25:33 -0700 (PDT) (envelope-from dillon) Date: Fri, 25 May 2001 15:25:33 -0700 (PDT) From: Matt Dillon Message-Id: <200105252225.f4PMPXI44229@earth.backplane.com> To: Dave Hayes , hackers@FreeBSD.ORG Subject: Preliminary Tuning man page (was Re: Benchmarking FreeBSD (was ...)) References: <200105250638.XAA06408@hokkshideh.jetcafe.org> <200105251951.f4PJp1b42293@earth.backplane.com> Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG Ok, here is my first shot at a 'tuning' manual page. If anyone wants to review it, I am open to all suggestions, grammatical and formatting, fixes etc... just email me with the changes (do not email the entire document back to me, just a diff). It's not 100% complete (well duh!), but it's a good start. While this topic is broken open, if anyone has adjustments or updates for my 'security' man page (which is already in the system), email diffs for that to me as well. Now is the time. Sometime this weekend I'll commit it. I think I'll do a 'firewall' manual page as well. I'm tired of people not configuring firewalls properly and then whining about it. -Matt .\" Copyright (c) 2001, Matthew Dillon. Terms and conditions are those of .\" the BSD Copyright as specified in the file "/usr/src/COPYRIGHT" in .\" the source tree. .\" .\" $FreeBSD$ .\" .Dd May 25, 2001 .Dt TUNING 7 .Os FreeBSD .Sh NAME .Nm tuning .Nd performance tuning under FreeBSD .Sh SYSTEM SETUP - DISKLABEL, NEWFS, TUNEFS, SWAP .Pp When using .Xr 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 then 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 memory. 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 then 256M of swap on a system. The kernel's VM paging algorithms are tuned to perform best when there is at least 2x swap verses 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. .Pp How you size your .Em /var partition depends heavily on what you intend to use the machine for. This partition is primarily used to hold mailboxes and the print spool. If your machine is intended to act as a mail or print server you should consider creating a much larger partition - perhaps a gig or more. Sizing .Em /var/tmp depends on the kind of temp file usage you think you will need. 128M is the minimum we recommend. .Pp The .Em /usr partition holds the bulk of the files required to support the system and a subdirectory within it called .Em /usr/local holds the bulk of the files installed from the .Xr 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! .Pp The .Em /home partition is typically used to hold user-specific data. I usually size it to the remainder of the disk. .Pp Why partition at all? Why not create one big .Em /dev/kmem 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 properly 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 significnat performance improvement whereas moving /var to the edge can have a huge impact. .Pp Properly partitioning your system also allows you to tune .Xr newfs 8 , and .Xr tunefs 8 parameters. Tuning .Fn newfs requires more experience but can lead to significant improvements in performance. There are three parameters that are relatively safe to tune: .Em blocksize , .Em bytes/inode , and .Em cylinders/group . .Pp .Fx performs best when using 8K or 16K filesystem block sizes. The default filesystem block size is 8K. For larger partitions it is usually a good idea to use a 16K block size. This also requires you to specify a larger 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 .Fn newfs options for this would be .Li Em newfs -f 2048 -b 16384 ... Using a larger block size can cause fragmentation of the buffer cache and lead to lower performance. .Pp If a large partition is intended to be used to hold fewer, larger files, such as a database files, you can increase the .Em bytes/inode ratio which reduces the number if 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 .Xr 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 filesystem that has lots of free space remaining but cannot accomodate 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 recovery times. For example, .Li Em newfs -i 32768 ... .Pp Finally, increasing the .Em cyliners/group ratio has the effect of packing the inodes closer together. This can increase directory performance and also decrease fsck times. If you use this option at all, we recommend maxing it out. Use .Li Em newfs -c 999 and newfs will error out and tell you what the maximum is, then use that. .Pp .Xr 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. However, this is possibly the most abused program in the system. Many people 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 option worthwhile here is turning on .Em softupdates with .Li Em tunefs -n enable /filesystem. Softupdates drastically improves meta-data performance, mainly file creation and deletion. We recommend turning softupdates on on all of your filesystems. There are two downsides to softupdates that you should be aware of: First, softupdates guarentees 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 loose more work then 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. .Em make installworld, can run it out of space and cause the update to fail. .Sh SYSCTL TUNING .Pp There are several hundred .Xr sysctl 8 variables 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. .Pp The .Em 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 SysV 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. .Pp The .Em vfs.vmiodirenable sysctl defaults to 0 (off) (though soon it will default to 1) and may be set to 0 (off) or 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 then 512 bytes. We recommend turning this option on if you are running any services which manipulate large numbers 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. .Pp There are various buffer-cache and VM page cache related sysctls. We do not recommend messing around with these at all. As of .Fx 4.3 , the VM system does an extremely good job tuning itself. .Pp The .Em net.inet.tcp.sendspace and .Em net.inet.tcp.recvspace sysctls are of particular 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 is 16K. 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 simultanious 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 route table, see .Xr route 8 , can be used to introduce route-specific send and receive buffer size defaults. As an additional mangagement tool you can use pipes in your firewall rules, see .Xr 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. .Pp We recommend that you turn on (set to 1) and leave on the .Em net.inet.tcp.always_keepalive control. The default is usually off. This introduces a small amount of additional network bandwidth but guarentees that dead tcp connections will eventually be recognized and cleared. Dead tcp connections are a particular problem on systems accesed by users operating over dialups, because users often disconnect their modems without properly closing active connections. .Sh KERNEL CONFIG TUNING .Pp 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 .Xr 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 don't use. Removing things like .Em INET6 and drivers you don't have will reduce the size of your kernel, sometimes by a megabyte or more, leaving more memory available for applications. .Pp The .Em maxusers kernel option defaults to an incredibly low value. For most modern machines, you probably want to increase this value to 64, 128, or 256. We do not recommend going above 256 unless you need a huge number of file descriptors. Network buffers are also effected but can be controlled with a separate kernel option. Do not increase maxusers just to get more network mbufs. .Pp .Em NMBCLUSTERS may be adjusted to increase the number of network mbufs the system is willing to allocate. Each cluster represents approximately 16K of memory, so a value of 1024 represents 16M 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 simultanious 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/16K = 4096. So for this case you would want to se NMBCLUSTERS to 4096. We recommend values between 1024 and 4096 for machines with moderates amount of memory, and 4096, 8192, or 16384 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 machine crash. .Pp More and more programs are using the .Fn sendfile system call to transmit files over the network. The .Em NFSBUFS kernel parameter controls the number of filesystem buffers .Fn sendfile is allowed to use to perform its work. This parameter nominally scales with .Em maxusers so you should not need to mess with this parameter except under extreme circumstances. .Pp .Em SCSI_DELAY and .Em 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. .Pp There are a number of .Em XXX_CPU options that can be commented out. If you only want the kernel to run on a Pentium class cpu, you can easily remove .Em I386_CPU and .Em I486_CPU, but only remove .Em 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, which increases its efficiency under heavy syscall loads. .Sh IDE WRITE CACHING As of .Fx 4.3 , IDE write caching is turned off by default. This will reduce write bandwidth to IDE disks but is 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 is to be safe. If you are willing to risk filesystem corruption, you can return to the old behavior by setting the hw.ata.wc kernel variable back to 1. This must be done from the boot loader at boot time. Please see .Xr ata 4 , and .Xr loader 8 . .Pp There is a new experimental feature for IDE hard drives called hw.ata.tags (you also set this in the bootloader) which allows write caching to be safely turned on. This brings SCSI tagging features to IDE drives. As of this writing only IBMDPTA and DTLA drives support the feature. .Sh 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 pepetually 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 paing 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. .Xr 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. .Pp 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 are severely limited due to collision backoff and can cause other inefficiencies to build in the system (such as causing more tcp packet retries to occur due to nondeterministic delays). If you are hitting a bottleneck in your WAN link (e.g. modem, T1, DSL, whatever) and do not have the luxury of getting a bigger pipe, you may be able to use .XR ipfw 8 to partition and traffic-shape the bandwidth going over the pipe. Obviously you cannot push more bandwidth then the pipe will hold, but you can tune it so an overload with one service does not effect all services running over the pipe. .Sh SEE ALSO .Pp .Xr ata 4 , .Xr boot 8 , .Xr config 8 , .Xr disklabel 8 , .Xr fsck 8 , .Xr ifconfig 8 , .Xr ipfw 8 , .Xr loader 8 , .Xr login.conf 5 , .Xr newfs 8 , .Xr ports 7 , .Xr route 8 , .Xr sysctl 8 , .Xr systat 1 , .Xr tunefs 8 .Sh HISTORY The .Nm manual page was originally written by .An Matthew Dillon and first appeared in .Fx 4.3 , May 2001. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message