7.6 Tuning the FreeBSD VM system

A concerted effort has been made to make the FreeBSD kernel dynamically tune itself. Typically you do not need to mess with anything beyond the maxusers and NMBCLUSTERS kernel config options. That is, kernel compilation options specified in (typically) /usr/src/sys/i386/conf/CONFIG_FILE. A description of all available kernel configuration options can be found in /usr/src/sys/i386/conf/LINT.

In a large system configuration you may wish to increase maxusers. Values typically range from 10 to 128. Note that raising maxusers too high can cause the system to overflow available KVM resulting in unpredictable operation. It is better to leave maxusers at some reasonable number and add other options, such as NMBCLUSTERS, to increase specific resources.

If your system is going to use the network heavily, you may want to increase NMBCLUSTERS. Typical values range from 1024 to 4096.

The NBUF parameter is also traditionally used to scale the system. This parameter determines the amount of KVA the system can use to map filesystem buffers for I/O. Note that this parameter has nothing whatsoever to do with the unified buffer cache! This parameter is dynamically tuned in 3.0-CURRENT and later kernels and should generally not be adjusted manually. We recommend that you not try to specify an NBUF parameter. Let the system pick it. Too small a value can result in extremely inefficient filesystem operation while too large a value can starve the page queues by causing too many pages to become wired down.

By default, FreeBSD kernels are not optimized. You can set debugging and optimization flags with the makeoptions directive in the kernel configuration. Note that you should not use -g unless you can accommodate the large (typically 7 MB+) kernels that result.

makeoptions      DEBUG="-g"
makeoptions      COPTFLAGS="-O -pipe"

Sysctl provides a way to tune kernel parameters at run-time. You typically do not need to mess with any of the sysctl variables, especially the VM related ones.

Run time VM and system tuning is relatively straightforward. First, use Soft Updates on your UFS/FFS filesystems whenever possible. /usr/src/sys/ufs/ffs/README.softupdates contains instructions (and restrictions) on how to configure it.

Second, configure sufficient swap. You should have a swap partition configured on each physical disk, up to four, even on your “work” disks. You should have at least 2x the swap space as you have main memory, and possibly even more if you do not have a lot of memory. You should also size your swap partition based on the maximum memory configuration you ever intend to put on the machine so you do not have to repartition your disks later on. If you want to be able to accommodate a crash dump, your first swap partition must be at least as large as main memory and /var/crash must have sufficient free space to hold the dump.

NFS-based swap is perfectly acceptable on 4.X or later systems, but you must be aware that the NFS server will take the brunt of the paging load.