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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Mon, 18 Sep 2023 19:39:43 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Matteo Rizzo <matteorizzo@google.com>
Cc:     "Lameter, Christopher" <cl@os.amperecomputing.com>,
        Dave Hansen <dave.hansen@intel.com>, penberg@kernel.org,
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        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC PATCH 00/14] Prevent cross-cache attacks in the SLUB
 allocator
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* Matteo Rizzo <matteorizzo@google.com> wrote:

> On Fri, 15 Sept 2023 at 18:30, Lameter, Christopher
> <cl@os.amperecomputing.com> 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