Number of kernel threads = cores?

Question

I wonder whether or not a systems max kernel threads are determined by how many cores your CPU has. Or is it decided in another way?

Answer 1

No, you can set the maximum kernel threads to very high numbers.

Note that the word "threads" is used for many different things:

• Most programmers use it to refer to independent threads of execution in the sense of POSIX threads. This is a way of organising programs and does not depend on hardware support. See Maximum number of threads per process in Linux?

• Intel use it to refer to their "Hyperthreading" technology. See Why does my Intel i7-920 display 8 cores instead of 4 cores? and What does "thread" mean as related to CPUs?

It may be that Intels use causes confusion.

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Update re kernel threads

Here are some Linux kernel threads running in CoLinux under Vista on AMD Athlon 64 X2 dual-core.

$ ps -eLf
UID        PID  PPID   LWP  C NLWP STIME TTY          TIME CMD
root         1     0     1  0    1 17:24 ?        00:00:00 init [2]
root         2     0     2  0    1 17:24 ?        00:00:00 [kthreadd]
root         3     2     3  0    1 17:24 ?        00:00:00 [ksoftirqd/0]
root         4     2     4  0    1 17:24 ?        00:00:00 [events/0]
root         5     2     5  0    1 17:24 ?        00:00:00 [khelper]
root        21     2    21  0    1 17:24 ?        00:00:00 [kblockd/0]
root        22     2    22  0    1 17:24 ?        00:00:00 [kseriod]
root        41     2    41  0    1 17:24 ?        00:00:00 [pdflush]
root        42     2    42  0    1 17:24 ?        00:00:00 [pdflush]
root        43     2    43  0    1 17:24 ?        00:00:00 [kswapd0]
root        44     2    44  0    1 17:24 ?        00:00:00 [aio/0]
root       727     2   727  0    1 17:24 ?        00:00:00 [kjournald]

LWP is the thread ID.

(See man ps: "-L Show threads, possibly with LWP and NLWP columns" … "LWP lwp (light weight process, or thread) ID of the lwp being reported. (alias spid, tid)")

kthreadd is the kernel thread daemon, I believe is is responsible for all the other kernel threads. Note I am not showing daemons like klogd which do not execute in ring 0 (as far as I know).

Number of kernel threads != number of CPU cores. (ref title of question)

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Kernel threads consist of a set of registers, a stack, and a few corresponding kernel data structures.

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The purported advantage of kernel threads over processes is faster creation and context switching compared with processes.

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Kernel threads are considered “lightweight,” and one would expect the number of threads to only be limited by address space and processor time

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In particular, operating system kernels tend to see kernel threads as a special kind of process rather than a unique entity. For example, in the Solaris kernel threads are called “light weight processes” (LWP’s). Linux actually creates kernel threads using a special variation of fork called “clone,” and until recently gave each thread a separate process ID. Because of this heritage, in practice kernel threads tend to be closer in memory and time cost to processes than user-level threads,

(Multiple Flows of Control in Migratable Parallel Programs 2006)