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

Mike Travis
2008-08-26T20:01:02Z

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). [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.)

Thanks,
Mike