Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

Nick Piggin
2008-08-27T06:54:32Z

On Wednesday 27 August 2008 06:01, Mike Travis wrote:
> Dave Jones wrote:
> ...
>
> > But yes, for this to be even remotely feasible, there has to be a
> > negligable performance cost associated with it, which right now, we
> > clearly don't have. Given that the number of people running 4096 CPU
> > boxes even in a few years time will still be tiny, punishing the common
> > case is obviously absurd.
> >
> > Dave
>
> I did do some fairly extensive benchmarking between configs of NR_CPUS =
> 128 and 4096 and most performance hits were in the neighborhood of < 5% on
> systems with 8 cpus and 4GB of memory (our most common test system).

5% is a pretty nasty performance hit... what sort of benchmarks are we
talking about here?

I just made some pretty crazy changes to the VM to get "only" around 5
or so % performance improvement in some workloads.

What places are making heavy use of cpumasks that causes such a slowdown?
Hopefully callers can mostly be improved so they don't need to use cpumasks
for common cases.

Until then, it would be kind of sad for a distro to ship a generic x86
kernel and lose 5% performance because it is set to 4096 CPUs...

But if I misunderstand and you're talking about specific microbenchmarks to
find the worst case for huge cpumasks, then I take that back.

> [But
> changing cpumask_t's to be pointers instead of values will likely increase
> this.] I've tried to be very sensitive to this issue with all my previous
> changes, so convincing the distros to set NR_CPUS=4096 would be as painless
> for them as possible. ;-)
>
> Btw, huge count cpu systems I don't think are that far away. I believe the
> nextgen Larabbee chips will be geared towards HPC applications [instead of
> just GFX apps], and putting 4 of these chips on a motherboard would add up
> to 512 cpu threads (1024 if they support hyperthreading.)

It would be quite interesting if they make them cache coherent / MP capable.
Will they be?