On 10/30/2017 08:59 AM, Russell King - ARM Linux wrote:
On Fri, Oct 27, 2017 at 03:25:28PM -0700, Mark Salyzyn wrote:
Ensure monotonic and realtime are inline, small price to pay for
high volume common request.
Is this just based on a hunch, or is it based on proper measurement?
If proper measurement, where's the data? What CPU was it measured
with? How does this change affect other CPUs?
I was tested faster in the past. Story today is less conclusive and the
change is not worth it.
[TL;DR]
Code size in all cases is about 1/2 a 4K page, and change in size is not
that much in or out.
Originally coded to match assembler for arm64. I tested it when I was
first formulating the series and found a 2-4% improvement on arm
(Nexus6, backport to 3.10) and arm64 (Nexus 6P, backport to 3.18). But
that was (a technological) eon ago.
However, retested as-is, in and out, today side by side, clock_gettime
for CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_REALTIME, locked cores,
affinity to littles (0-3), 50M iterations, device cooled down for 15
minutes between (vdso64+vdso32) runs, 16 runs each averaged on a
Hikey960, 4.9 kernel, GCC 4.9 -O2 and I get a slightly different story
(with complete private patch stack that has vdso32):
vdso64
realtime: -4.8% (worse)
monotonic: +1.9% (better)
boottime: +3.2%
vdso32
realtime: +4.7% (better)
monotonic: +3.2%
boottime: +3.7%
The maximum deviation on the sample runs was in the order of +/-1%. I
can not explain (the highly repeatable anomaly) as to why vdso64
realtime is slower, yet vdso32 is equally faster. realtime is unique in
the set as common routine serves for both __vdso_clock_gettime and
__vdso_gettimeofday, and where I expected the gains (the hunch).
I have tried other combinations of forced inlines to try to cope with
the clock_gettime(CLOCK_REALTIME) speed, and determined it was almost
like a slippery tuning exercise. As such, I now come to the conclusion
that given the (small?) gains, it is better to trust the C compiler
(especially if this is used by a wider set of architectures) and drop
this patch (and its side effect for boottime) from the series.
It should be noted on the same test bench that the new C coded vdso64 is
+2.9% and +11% faster for realtime and monotonic respectively over the
hand coded assembler it is replacing. Additional props for the C
compiler doing the "right thing".
-- Mark
