Re: [gentoo-user] Dual or Quad CPU complications?

[Grant]

> >> > So if I have 2 physical CPU's with 4 cores each and I enable SMP, I'm
> >> > using
> >> > 8 cores?  Can NUMA be either enabled or disabled when using more than
> >> > one
> >> > physical CPU, or is it required?
> >>
> >>
> >> NUMA is a hardware architecture. It's how you access memory on a
> >> hardware level: NUMA = Non Uniform Memory Access vs a UMA architecture
> >> of typical (old/legacy) SMP systems (UMA = Uniform Memory Access).
> >>
> >> In a UMA system, all the memory belongs to all the sockets. In a NUMA
> >> system, each socket has it's "own" local memory. In modern (x86-64)
> >> processors, each socket has it's own memory controller so each socket
> >> controls its own local memory. If one socket runs out of memory it can
> >> ask another socket to lend him some memory. In a UMA system, no socket
> >> has to ask since memory is global and belongs to all sockets so if one
> >> socket uses up all the memory ... the rest "starve". In NUMA, there's
> >> more control over who uses what (be it cores or RAM).
> >>
> >> If you have a modern dual or quad (or higher #) socket system ...
> >> you've got NUMA architecture and you can't get rid of it, it's
> >> hardware, not software.
> >
> > So I must enable CONFIG_NUMA for more than one physical CPU, and
disable it
> > for only one physical CPU?
>
>
> Yup. But ... Why would you want to disable a socket (CPU)? If you
> disable a socket (CPU) ... you lose the memory attached to that socket
> (CPU) not to mention you lose those cores ;)

Sure but it sounds like if my system only has one CPU socket, CONFIG_NUMA
should be disabled.

- Grant


> A better solution would be to use cgroups or numactl tools to pin a
> certain process to a set of cores and a memory region.
>
> If you really want to deactivate cores (but not the whole socket), you
can type:
>
>           echo 0 > /sys/devices/system/cpu/cpu1/online
>
> This would deactivate core #1. You can deactivate as many cores as you
> wish, except for core #0.
>
> This can be done without rebooting your server (aka during run time).
> Your memory will not be affected, but you will have less cores (and
> theoretically more memory bandwidth). I say "theoretically" because
> you always have to benchmark these things with YOUR application
> (remember logic NEVER applies to real life ;)
>
> If you want to check the # of cores you've got:
>
>      cat /proc/interrupts | grep CPU
>
> Other possibilities such as cat /proc/cpuinfo or dmesg, ... can be
> useful too for this: your choice, FLOSS gives you options.
>
> If you want to activate the previously deactivated core, you can run:
>
>           echo 1 > /sys/devices/system/cpu/cpu1/online
>
> Now ... be sure your core numbering is the expected core numbering.
> IOW, not all server vendors follow the same numbering scheme so core
> #1 in vendor A's server could be core #2 in vendor B's server. Never
> trust logic ;)
>
> As I mentioned previously: test/benchmark YOUR software. DON'T trust
> logic or generic benchmarks or web pages with results. Trust YOUR
> results only.
>
> HTH
>
>    Rafa