SMP Kernel On Old CPUs?

Woodsman · 06-29-2008, 01:54 AM

Pat V. recommends using the SMP kernel even on single core CPUs. Is that a good idea on really old CPUs?

I want to update my old box to Slackware 12.1. Currently on that box is Slackware 12.0 with a recompiled 2.6.21.5 non-SMP kernel.

I don't want to (and can't) use my kernel configuration file for my new box. That box is a dual core AM2 BE-2400 (K8) with 4 GB of RAM. I also have more hardware on the new box that is not available on the old box. I don't even run udev on the old box because the hardware is static and I don't use USB there.

My old box runs a 400 MHz K6-III+ with 256 MB of RAM. So even if the recommendation remains to use the SMP kernel, I need to modify the configuration down to that level.

Typically I would update the config file by running make oldconfig. I'm still inclined to continue that habit unless somebody explains in a compelling manner why I should switch to the SMP kernel for this old box.

Thanks.

salasi · 06-29-2008, 04:51 AM

If you want to use a recent kernel, its hard to do anything else.

The argument from the kernel developers is that, these days, the performance hit of using an smp capable kernel on a non-smp cpu is so low, why should they support anything else? So, where you once had to get a kernel with 'smp' in the name to get smp support, nowadays the default kernel is smp (but doesn't have smp in the name) unless someone downstream has hacked it to take out the smp capabilities.

I've been trying to investigate when exactly this happened, and the closest I can come up with is this comment:

http://kerneltrap.org/node/13911

that it was already 'recent history' by July 21, 2007.

Woodsman · 06-29-2008, 01:05 PM

Hmm. Perhaps then I have been using an SMP kernel on my old box and never noticed the difference. With that presumption I'm going to proceed with make oldconfig.

the3dfxdude · 06-29-2008, 04:42 PM

It seems like it would make the kernel larger to build with SMP.

hitest · 06-29-2008, 05:04 PM

Quote:

Originally Posted by Woodsman

My old box runs a 400 MHz K6-III+ with 256 MB of RAM. So even if the recommendation remains to use the SMP kernel, I need to modify the configuration down to that level.

I'm running the stock smp kernel on Slackware 12.1 on my Plll 667 IBM with 256 MB RAM, it runs just fine. I think you should be fine on your K-6.

T3slider · 06-29-2008, 05:12 PM

Quote:

Originally Posted by salasi

So, where you once had to get a kernel with 'smp' in the name to get smp support, nowadays the default kernel is smp (but doesn't have smp in the name) unless someone downstream has hacked it to take out the smp capabilities.

Um...I don't think any hacking is needed, unless the kernel is really really screwy. There is a CONFIG_SMP option -- the help states this:

Code:

  │ CONFIG_SMP:                                                             │
  │                                                                         │
  │ This enables support for systems with more than one CPU. If you have    │
  │ a system with only one CPU, like most personal computers, say N. If     │
  │ you have a system with more than one CPU, say Y.                        │
  │                                                                         │
  │ If you say N here, the kernel will run on single and multiprocessor     │
  │ machines, but will use only one CPU of a multiprocessor machine. If     │
  │ you say Y here, the kernel will run on many, but not all,               │
  │ singleprocessor machines. On a singleprocessor machine, the kernel      │
  │ will run faster if you say N here.                                      │
  │                                                                         │
  │ Note that if you say Y here and choose architecture "586" or            │
  │ "Pentium" under "Processor family", the kernel will not work on 486     │
  │ architectures. Similarly, multiprocessor kernels for the "PPro"         │
  │ architecture may not work on all Pentium based boards.                  │
  │                                                                         │
  │ People using multiprocessor machines who say Y here should also say     │
  │ Y to "Enhanced Real Time Clock Support", below. The "Advanced Power     │
  │ Management" code will be disabled if you say Y here.                    │
  │                                                                         │
  │ See also the <file:Documentation/smp.txt>,                              │
  │ <file:Documentation/i386/IO-APIC.txt>,                                  │
  │ <file:Documentation/nmi_watchdog.txt> and the SMP-HOWTO available at    │
  │ <http://www.tldp.org/docs.html#howto>.                                  │
  │                                                                         │
  │ If you don't know what to do here, say N.                               │
  │                                                                         │
  │ Symbol: SMP [=y]                                                        │
  │ Prompt: Symmetric multi-processing support                              │
  │   Defined at arch/x86/Kconfig:174                                       │
  │   Location:                                                             │
  │     -> Processor type and features                                      │
  │   Selected by: X86_VOYAGER && <choice> && X86_32 && !BROKEN || X86_NUMA │

The CONFIG_SMP option is not set in the generic or huge non-smp kernels.