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Why is Linux called a monolithic kernel?

I read that Linux is a monolithic kernel. Does monolithic kernel mean compiling and linking the complete kernel code into an executable?

If Linux is able to support modules, why not break all the subsystems into modules and load them when necessary? In that case, the kernel doesn't have to load all modules initially and could maintain an index of the functions in the module and load them when necessary.

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>   ;tl-dr - No, Linux is always monolithic.

Linux *modules* may mean *modular* in some sense.  As others have noted monolithic is usually representing a *microkernel* versus *monolithic* kernel.  A traditional *microkernel* only has these features,

 1. Scheduling
 2. Memory management
 3. Inter-process communications

There are no *hardware drivers*, *protocol stacks*, *filesystems*, *suspend/resume*, *clock management*, etc in the main kernel.  These things are *identical* to any user task (although they may have different privileges via the MMU/scheduler).

----------
[Tanenbaum's predictions][1]

   1. Microkernels are the future
   2. x86 will die out and RISC architectures will dominate the market
   3. (5 years from then) everyone will be running a free GNU OS

PC and server programmers may laugh, but two and three are certainly true for the majority of cell phones in existence.  Tanenbaum would be right on all accounts if BlackBerry QNX was a success.

Also, many [L1-hypervisors][2] have a [micro-kernel][3] underneath.  This is because a hyper-visor usually doesn't do much beside *context* switch.

Apparently three predicts the success of Linux. ;-)

----------

An argument for *microkernels* is that all of the monolithic sub-systems need to synchronize multiple values at one time.  In order to do this, they must use locks and will suffer from [Amdahl's law][4] when extended to parallel architectures.  The counter is that *microkernels* result in lots of IPC messages.

A major development is the use of [lock-free][5] programming to avoid contention in a monolithic kernel.  This avoids the locking in a monolithic kernel while also reducing IPC overhead.  Recently all CPUs have been extending their ISA to include better primitives for *lock-free* algorithms.  So Linux will probably remain a monolithic kernel for some time.

  [1]: https://softwareengineering.stackexchange.com/questions/140925/why-was-tanenbaum-wrong-in-the-tanenbaum-torvalds-debates
  [2]: http://en.wikipedia.org/wiki/Hypervisor
  [3]: http://en.wikipedia.org/wiki/L4_microkernel_family
  [4]: http://en.wikipedia.org/wiki/Amdahl%27s_law
  [5]: https://stackoverflow.com/questions/tagged/lock-free

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1

Yes, I know Tanenbaum meant Hurd. But GNU switched to Linux so the wording is funny.
– artless-noise-bye-due2AI
Commented Oct 27, 2014 at 21:06
Why is there a lack of use of x86 for mobile devices? What makes the other architectures advantageous?
– Honinbo Shusaku
Commented Aug 18, 2015 at 13:04
1

Abdul: See this question.
– artless-noise-bye-due2AI
Commented Aug 18, 2015 at 15:24
Yes, those are important. It is difficult to make an x86 SOC (system-on-chip). There is not standard HDL code to make an x86 that a SOC vendor (Broadcom, Freescale, etc) can buy.
– artless-noise-bye-due2AI
Commented Aug 18, 2015 at 15:42

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Why is Linux called a monolithic kernel?

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