* Matteo Rizzo <matteorizzo@xxxxxxxxxx> wrote:
> On Fri, 15 Sept 2023 at 18:30, Lameter, Christopher
> <cl@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri, 15 Sep 2023, Dave Hansen wrote:
> >
> > > What's the cost?
> >
> > The only thing that I see is 1-2% on kernel compilations (and "more on
> > machines with lots of cores")?
>
> I used kernel compilation time (wall clock time) as a benchmark while
> preparing the series. Lower is better.
>
> Intel Skylake, 112 cores:
>
> LABEL | COUNT | MIN | MAX | MEAN | MEDIAN | STDDEV
> ---------------+-------+---------+---------+---------+---------+--------
> SLAB_VIRTUAL=n | 150 | 49.700s | 51.320s | 50.449s | 50.430s | 0.29959
> SLAB_VIRTUAL=y | 150 | 50.020s | 51.660s | 50.880s | 50.880s | 0.30495
> | | +0.64% | +0.66% | +0.85% | +0.89% | +1.79%
>
> AMD Milan, 256 cores:
>
> LABEL | COUNT | MIN | MAX | MEAN | MEDIAN | STDDEV
> ---------------+-------+---------+---------+---------+---------+--------
> SLAB_VIRTUAL=n | 150 | 25.480s | 26.550s | 26.065s | 26.055s | 0.23495
> SLAB_VIRTUAL=y | 150 | 25.820s | 27.080s | 26.531s | 26.540s | 0.25974
> | | +1.33% | +2.00% | +1.79% | +1.86% | +10.55%
That's sadly a rather substantial overhead for a compiler/linker workload
that is dominantly user-space: a kernel build is about 90% user-time and
10% system-time:
$ perf stat --null make -j64 vmlinux
...
Performance counter stats for 'make -j64 vmlinux':
59.840704481 seconds time elapsed
2000.774537000 seconds user
219.138280000 seconds sys
What's the split of the increase in overhead due to SLAB_VIRTUAL=y, between
user-space execution and kernel-space execution?
Thanks,
Ingo
