Re: [PATCH 00/07][RFC] i386: NUMA emulation

Magnus Damm
2005-10-03T02:08:37Z

On 10/1/05, Dave Hansen <haveblue@us.ibm.com> wrote:
> On Fri, 2005-09-30 at 16:33 +0900, Magnus Damm wrote:
> > These patches implement NUMA memory node emulation for regular i386 PC:s.
> >
> > NUMA emulation could be used to provide coarse-grained memory resource control
> > using CPUSETS. Another use is as a test environment for NUMA memory code or
> > CPUSETS using an i386 emulator such as QEMU.
>
> This patch set basically allows the "NUMA depends on SMP" dependency to
> be removed. I'm not sure this is the right approach. There will likely
> never be a real-world NUMA system without SMP. So, this set would seem
> to include some increased (#ifdef) complexity for supporting SMP && !
> NUMA, which will likely never happen in the real world.

Yes, this patch set removes "NUMA depends on SMP". It also adds some
simple NUMA emulation code too, but I am sure you are aware of that!
=)

I agree that it is very unlikely to find a single-processor NUMA
system in the real world. So yes, "[PATCH 02/07] i386: numa on
non-smp" adds _some_ extra complexity. But because SMP is set when
supporting more than one cpu, and NUMA is set when supporting more
than one memory node, I see no reason why they should be dependent on
each other. Except that they depend on each other today and breaking
them loose will increase complexity a bit.

> Also, I worry that simply #ifdef'ing things out like CPUsets' update
> means that CPUsets lacks some kind of abstraction that it should have
> been using in the first place. An #ifdef just papers over the real
> problem.

Maybe. CPUSETS has two bitmaps, one for cpus and one for mems. So
depending on SMP or NUMA seems logical to me. Regarding the #ifdef, it
was added because partition_sched_domain() is only implemented for
SMP. That symbol has no prototype or implementation when CONFIG_SMP is
not set. Maybe it is better to add an empty inline function in
linux/sched.h for !SMP?

> I think it would likely be cleaner if the approach was to emulate an SMP
> NUMA system where each NUMA node simply doesn't have all of its CPUs
> online.

Absolutely. And that removes the need for some of my patches. QEMU
runs SMP kernels. It is possible to run SMP kernels on UP hardware.
But there is of course a certain performance loss introduced by all
the SMP locks. I'd rather not force !SMP users to run SMP kernels if
they want coarse-grained memory resource control.

Thanks for your input!

/ magnus