Re: [PATCH 1/1] x86_64: add config options to optimize for newer AMDprocessors

From: Borislav Petkov
Date: Thu Oct 03 2013 - 12:57:47 EST

On Thu, Oct 03, 2013 at 09:27:45AM -0700, Linus Torvalds wrote:
> On Thu, Oct 3, 2013 at 5:06 AM, Austin S Hemmelgarn
> <ahferroin7@xxxxxxxxx> wrote:
> > improved. Building kernel 3.12-rc2 with allmodconfig using 8 jobs on a FX-8320 takes
> >
> > 22 minutes and 57 seconds on a kernel with CONFIG_MK8,
> > 21 minutes and 35 seconds on a kernel with CONFIG_GENERIC, and
> > 19 minutes and 11 seconds on a kernel with CONFIG_PILEDRIVER.
>
> That's certainly noticeable. Surprisingly so. What makes MK8 so bad in
> particular, I wonder?
>
> Just out of interest, have you done any profiles on the kernel cost
> here to see what it is that makes such a big difference. Because
> normally on a kernel build, I see most of the overhead in path lookup.
> But that's only true for otherwise optimized builds that don't have
> system call auditing etc debugging that spreads the costs out over
> everything..

Yeah, I was having some doubts about the numbers above so I ran my own
benchmarking, machine is a Piledriver box:

vendor_id : AuthenticAMD
cpu family : 21
model : 2
model name : AMD FX(tm)-8350 Eight-Core Processor
stepping : 0

and I don't really see any of those improvements above. Actually,
-march=bdver2 is even slightly worse in comparison to mk8.

And the workload is of building a config specific to that machine but
allmodconfig looks very similar, the numbers being simply higher.

$ zgrep MK8 /proc/config.gz
CONFIG_MK8=y

/home/boris/bin/perf stat --repeat 10 -a --sync --pre /home/boris/kernel/pre-build-kernel.sh make -s -j64 bzImage

Performance counter stats for 'make -s -j64 bzImage' (10 runs):

1081808.628840 task-clock # 7.996 CPUs utilized ( +- 0.06% ) [100.00%]
1,203,753 context-switches # 0.001 M/sec ( +- 0.04% ) [100.00%]
48,748 cpu-migrations # 0.045 K/sec ( +- 0.59% ) [100.00%]
31,145,439 page-faults # 0.029 M/sec ( +- 0.00% )
3,836,736,801,500 cycles # 3.547 GHz ( +- 0.03% ) [100.00%]
957,386,966,493 stalled-cycles-frontend # 24.95% frontend cycles idle ( +- 0.06% ) [100.00%]
218,581,249,251 stalled-cycles-backend # 5.70% backend cycles idle ( +- 0.06% ) [100.00%]
2,466,632,641,972 instructions # 0.64 insns per cycle
# 0.39 stalled cycles per insn ( +- 0.00% ) [100.00%]
537,749,333,838 branches # 497.084 M/sec ( +- 0.00% ) [100.00%]
27,802,940,176 branch-misses # 5.17% of all branches ( +- 0.00% )

135.292843025 seconds time elapsed ( +- 0.06% )


$ zgrep PILEDRIVER /proc/config.gz
CONFIG_MPILEDRIVER=y

/home/boris/bin/perf stat --repeat 10 -a --sync --pre /home/boris/kernel/pre-build-kernel.sh make -s -j64 bzImage

Performance counter stats for 'make -s -j64 bzImage' (10 runs):

1085723.230470 task-clock # 7.996 CPUs utilized ( +- 0.10% ) [100.00%]
1,204,355 context-switches # 0.001 M/sec ( +- 0.10% ) [100.00%]
49,143 cpu-migrations # 0.045 K/sec ( +- 0.76% ) [100.00%]
31,196,575 page-faults # 0.029 M/sec ( +- 0.00% )
3,851,255,065,133 cycles # 3.547 GHz ( +- 0.02% ) [100.00%]
958,840,197,117 stalled-cycles-frontend # 24.90% frontend cycles idle ( +- 0.09% ) [100.00%]
220,260,399,411 stalled-cycles-backend # 5.72% backend cycles idle ( +- 0.04% ) [100.00%]
2,466,701,295,156 instructions # 0.64 insns per cycle
# 0.39 stalled cycles per insn ( +- 0.00% ) [100.00%]
537,992,040,195 branches # 495.515 M/sec ( +- 0.00% ) [100.00%]
27,860,290,286 branch-misses # 5.18% of all branches ( +- 0.00% )

135.784111961 seconds time elapsed ( +- 0.10% )

--
Regards/Gruss,
Boris.

Sent from a fat crate under my desk. Formatting is fine.
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