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sched_setaffinity.2: Rework text about dealing with systems with more than 1024 CPUs 

After comments from Florian Weimer, who pointed out various
confusions in the earlier text.

Reported-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-07-21 17:00:40 +02:00
parent abbbace718
commit dde6d764a3
1 changed files with 10 additions and 21 deletions
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31
man2/sched_setaffinity.2
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@ -242,7 +242,10 @@ system call returns the size (in bytes) of the
data type that is used internally by the kernel to
represent the CPU set bit mask.
.SS Handling systems with more than 1024 CPUs
The
The underlying system calls (which represent CPU masks as bit masks of type
.IR "unsigned long\ *" )
impose no restriction on the size of the CPU mask.
However, the
.I cpu_set_t
data type used by glibc has a fixed size of 128 bytes,
meaning that the maximum CPU number that can be represented is 1023.
@ -260,29 +263,15 @@ size specified in
.I cpusetsize
is smaller than the size of the affinity mask used by the kernel.
.PP
The underlying system calls (which represent CPU masks as bit masks of type
.IR "unsigned long\ *" )
impose no restriction on the size of the mask.
To handle systems with more than 1024 CPUs, one must dynamically allocate the
When working on systems with more than 1024 CPUs,
one must dynamically allocate the
.I mask
argument using
.BR CPU_ALLOC (3)
and manipulate the mask using the "_S" macros described in
.BR CPU_ALLOC (3).
Using an allocation based on the number of online CPUs:
cpu_set_t *mask = CPU_ALLOC(CPU_ALLOC_SIZE(
sysconf(_SC_NPROCESSORS_CONF)));
is probably sufficient, although the value returned by the
.BR sysconf ()
call can in theory change during the lifetime of the process.
Alternatively, one can obtain a value that is guaranteed to be
stable for the lifetime of the process by probing for the size
argument.
Currently, the only way to do this is by probing for the size
of the required mask using
.BR sched_getaffinity ()
calls with increasing mask sizes until the call does not fail with the error
.BR EINVAL .
calls with increasing mask sizes (until the call does not fail with the error
.BR EINVAL ).
.SH EXAMPLE
The program below creates a child process.
The parent and child then each assign themselves to a specified CPU