Messages in this thread | | | Date | Tue, 29 Jan 2013 09:12:03 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] [x86]: Compiler Option Os is better on latest x86 |
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* Valdis.Kletnieks@vt.edu <Valdis.Kletnieks@vt.edu> wrote:
> On Fri, 25 Jan 2013 09:11:01 -0500, ling.ma.program@gmail.com said: > > > Based on above reasons, we compiled linux kernel 3.6.9 with O2 and Os > > respectively. The results show Os improve performance netperf 4.8%, > > 2.7% for volano as below > > Am I allowed to NAK this? What the numbers given so far > *actually* show is 4.8% more instructions executed, *not* 4.8% > better performance.
cycles and elapsed time is down in both tests - the speedup seems statistically a wash in the first test and significant for the second workload.
the instruction count might be an artifact of byte wise versus word wise REP; MOV.
> I'm having a *very* hard time convincing myself that what > we're seeing isn't simply the expected behavior of loops *not* > being unrolled and similar non-optimizations done by -Os, so > more instructions get executed to do the same amount of work. > > Rather than "run for 10 seconds and count instructions", can > we "run for 50,000 syscalls and count clock time" or similar > that shows an *actual* improvement?
Look at the numbers, it counts a whole lot of other things as well beyond instructions - elapsed time being the most important one.
But more numbers never hurt.
Thanks,
Ingo
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