Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR_SIZE should default to N

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Tue, Mar 22, 2011 at 3:27 AM, Ingo Molnar <mingo@elte.hu> wrote:
> >
> > If that situation has changed - if GCC has regressed in this area then a commit
> > changing the default IMHO gains a lot of credibility if it is backed by careful
> > measurements using perf stat --repeat or similar tools.
> 
> Also, please don't back up any numbers for the "-O2 is faster than
> -Os" case with some benchmark that is hot in the caches.
> 
> The thing is, many optimizations that make the code larger look really
> good if there are no cache misses, and the code is run a million times
> in a tight loop.
> 
> But kernel code in particular tends to not be like that. [...]

To throw some numbers into the discussion, here's the size versus speed 
comparison for 'hackbench 15' - which is more on the microbenchmark side of the 
equation - but has macrobenchmark properties as well, because it runs 3000 
tasks and moves a lot of data, hence thrashes the caches constantly:

     CONFIG_CC_OPTIMIZE_FOR_SIZE=y
     ----------------------------------------
     6,757,858,145 cycles                   #   2525.983 M/sec   ( +-   0.388% )
     2,949,907,036 instructions             #      0.437 IPC     ( +-   0.191% )
       595,955,367 branches                 #    222.759 M/sec   ( +-   0.238% )
        31,504,981 branch-misses            #      5.286 %       ( +-   0.187% )

        0.164320722  seconds time elapsed   ( +-   0.524% )


     # CONFIG_CC_OPTIMIZE_FOR_SIZE is not set
     ----------------------------------------
     6,061,867,073 cycles                   #   2510.283 M/sec   ( +-   0.494% )
     2,510,505,732 instructions             #      0.414 IPC     ( +-   0.243% )
       493,721,089 branches                 #    204.455 M/sec   ( +-   0.302% )
        38,731,708 branch-misses            #      7.845 %       ( +-   0.206% )

        0.148203574  seconds time elapsed   ( +-   0.673% )

They were perf stat --repeat 100 runs - repeated a couple of times to make sure 
it's all real. I have used GCC 4.6.0, a relatively recent compiler. (64-bit 
x86, typical .config, etc.)

The text size differences:

      text	   data	    bss	    dec	         filename
  -------------------------------------------------------------------------
   8809558	1790428	2719744	13319730	 vmlinux.optimize_for_size
  10268082	1825292	2727936	14821310	 vmlinux.optimize_for_speed

So by enabling CONFIG_CC_OPTIMIZE_FOR_SIZE=y, we get this total effect:

  -16.5% text size reduction
  +17.5% instruction count increase
  +20.7% branches executed increase
  -22.9% branch-miss reduction
  +11.5% cycle count increase
  +10.8% total runtime increase

A few observations:

 - the branch-miss reduction suggests that almost none of the new branches
   introduced by -Os generates a branch miss.

 - the cycles count increase is in line with the total runtime increase.

 - workloads where 16.5% more instruction cache footprint slows down the 
   workload by more than ~11% would win from enabling 
   CONFIG_CC_OPTIMIZE_FOR_SIZE=y.

Looking at these numbers i became more pessimistic about the usefulness of the 
current implementation of CONFIG_CC_OPTIMIZE_FOR_SIZE=y - it would need some 
*serious* icache thrashing to cause a larger than 11% slowdown, right?

I'm not sure what the best way would be to measure a realistic macro workloads 
where the kernel's instructions generate a lot of instruction-cache misses. 
Most of the 'real' workloads tend to be hard to measure precisely, tend to be 
very noisy and take a long time to run.

I could perhaps try to simulate them: i could patch a debug-only 'icache 
flusher' function into every system call, and compare the perf stat results - 
would that be an acceptable simulation of cache-cold kernel execution?

The 'icache flusher' would be something simple, like 10,000x 5-byte NOP 
instructions in a row, or so. This would slow things down immensely, but this 
particular slowdown is the same for both OPTIMIZE_FOR_SIZE=y and 
OPTIMIZE_FOR_SIZE=n.

Any better ideas?

	Ingo