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Commit 7c142bfe authored by Baoquan He's avatar Baoquan He Committed by Jonathan Corbet
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docs: kernel-parameter: Improve the description of nr_cpus and maxcpus 



From the old description people still can't get what's the exact
difference between nr_cpus and maxcpus. Especially in kdump kernel
nr_cpus is always suggested if it's implemented in the ARCH. The
reason is nr_cpus is used to limit the max number of possible cpu
in system, the sum of already plugged cpus and hot plug cpus can't
exceed its value. However maxcpus is used to limit how many cpus
are allowed to be brought up during bootup.

Signed-off-by: default avatarBaoquan He <bhe@redhat.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 6d232c80

Documentation/kernel-parameters.txt

+13 −7
Original line number Diff line number Diff line
@@ -2161,10 +2161,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 
			than or equal to this physical address is ignored.



	maxcpus=	[SMP] Maximum number of processors that	an SMP kernel

			should make use of.  maxcpus=n : n >= 0 limits the

			kernel to using 'n' processors.  n=0 is a special case,

			it is equivalent to "nosmp", which also disables

			the IO APIC.

			will bring up during bootup.  maxcpus=n : n >= 0 limits

			the kernel to bring up 'n' processors. Surely after

			bootup you can bring up the other plugged cpu by executing

			"echo 1 > /sys/devices/system/cpu/cpuX/online". So maxcpus

			only takes effect during system bootup.

			While n=0 is a special case, it is equivalent to "nosmp",

			which also disables the IO APIC.



	max_loop=	[LOOP] The number of loop block devices that get

	(loop.max_loop)	unconditionally pre-created at init time. The default
@@ -2773,9 +2776,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 


	nr_cpus=	[SMP] Maximum number of processors that	an SMP kernel

			could support.  nr_cpus=n : n >= 1 limits the kernel to

			supporting 'n' processors. Later in runtime you can not

			use hotplug cpu feature to put more cpu back to online.

			just like you compile the kernel NR_CPUS=n

			support 'n' processors. It could be larger than the

			number of already plugged CPU during bootup, later in

			runtime you can physically add extra cpu until it reaches

			n. So during boot up some boot time memory for per-cpu

			variables need be pre-allocated for later physical cpu

			hot plugging.



	nr_uarts=	[SERIAL] maximum number of UARTs to be registered.