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Update tuning so that it mentions maxusers, nmbclusters, and nsfbufs as

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tunables in loader.conf rather than just kernel options.

MFC after: 3 days
This commit is contained in:
Mike Silbersack 2001-10-29 22:29:01 +00:00
parent 12396bdca7
commit 2fa7b7ca77
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=85709
1 changed files with 48 additions and 37 deletions
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share/man/man7/tuning.7
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@ -369,6 +369,54 @@ rather then 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.
.Sh BOOT-TIME SYSCTL TUNING
Some sysctls may not be tunable at runtime because the memory allocations
they perform must occur early in the boot process. To change these sysctls,
you must set their value in
.Xr loader.conf 5
and reboot the system.
.Pp
The
.Em kern.maxusers
sysctl 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 affected but can be controlled with a separate
kernel option. Do not increase maxusers just to get more network mbufs.
Systems older than FreeBSD 4.4 do not have this sysctl and require that
the kernel config option maxusers be set instead.
.Pp
.Em 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
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 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
.Xr netstat 1
may be used to observe network cluster use.
Older versions of FreeBSD do not have this sysctl and require that the
kernel config option NMBCLUSTERS be set instead.
.Pp
More and more programs are using the
.Fn sendfile
system call to transmit files over the network. The
.Em kern.ipc.nsfbufs
sysctl 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
.Sh 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
@ -382,43 +430,6 @@ things like
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 affected 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 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 se 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
.Xr netstat 1
may be used to observe network cluster use.
.Pp
More and more programs are using the
.Fn sendfile
system call to transmit files over the network. The
.Em NSFBUFS
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