RE: [RFC] change non-atomic bitops method

[Wang, Yalin]

> -----Original Message-----
> From: Kirill A. Shutemov [mailto:kirill@xxxxxxxxxxxxx]
> Sent: Tuesday, February 03, 2015 9:18 AM
> To: Andrew Morton
> Cc: Wang, Yalin; 'arnd@xxxxxxxx'; 'linux-arch@xxxxxxxxxxxxxxx'; 'linux-
> kernel@xxxxxxxxxxxxxxx'; 'linux@xxxxxxxxxxxxxxxx'; 'linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx'
> Subject: Re: [RFC] change non-atomic bitops method
> 
> On Mon, Feb 02, 2015 at 03:29:09PM -0800, Andrew Morton wrote:
> > On Mon, 2 Feb 2015 11:55:03 +0800 "Wang, Yalin"
> <Yalin.Wang@xxxxxxxxxxxxxx> wrote:
> >
> > > This patch change non-atomic bitops,
> > > add a if() condition to test it, before set/clear the bit.
> > > so that we don't need dirty the cache line, if this bit
> > > have been set or clear. On SMP system, dirty cache line will
> > > need invalidate other processors cache line, this will have
> > > some impact on SMP systems.
> > >
> > > --- a/include/asm-generic/bitops/non-atomic.h
> > > +++ b/include/asm-generic/bitops/non-atomic.h
> > > @@ -17,7 +17,9 @@ static inline void __set_bit(int nr, volatile
> unsigned long *addr)
> > >  	unsigned long mask = BIT_MASK(nr);
> > >  	unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> > >
> > > -	*p  |= mask;
> > > +	if ((*p & mask) == 0)
> > > +		*p  |= mask;
> > > +
> > >  }
> >
> > hm, maybe.
> >
> > It will speed up set_bit on an already-set bit.  But it will slow down
> > set_bit on a not-set bit.  And the latter case is presumably much, much
> > more common.
> >
> > How do we know the patch is a net performance gain?
> 
> Let's try to measure. The micro benchmark:
> 
> 	#include <stdio.h>
> 	#include <time.h>
> 	#include <sys/mman.h>
> 
> 	#ifdef CACHE_HOT
> 	#define SIZE (2UL << 20)
> 	#define TIMES 10000000
> 	#else
> 	#define SIZE (1UL << 30)
> 	#define TIMES 10000
> 	#endif
> 
> 	int main(int argc, char **argv)
> 	{
> 		struct timespec a, b, diff;
> 		unsigned long i, *p, times = TIMES;
> 
> 		p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> 				MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1,
> 0);
> 
> 		clock_gettime(CLOCK_MONOTONIC, &a);
> 		while (times--) {
> 			for (i = 0; i < SIZE/64/sizeof(*p); i++) {
> 	#ifdef CHECK_BEFORE_SET
> 				if (p[i] != times)
> 	#endif
> 					p[i] = times;
> 			}
> 		}
> 		clock_gettime(CLOCK_MONOTONIC, &b);
> 
> 		diff.tv_sec = b.tv_sec - a.tv_sec;
> 		if (a.tv_nsec > b.tv_nsec) {
> 			diff.tv_sec--;
> 			diff.tv_nsec = 1000000000 + b.tv_nsec - a.tv_nsec;
> 		} else
> 			diff.tv_nsec = b.tv_nsec - a.tv_nsec;
> 
> 		printf("%lu.%09lu\n", diff.tv_sec, diff.tv_nsec);
> 		return 0;
> 	}
> 
> Results for 10 runs on my laptop -- i5-3427U (IvyBridge 1.8 Ghz, 2.8Ghz
> Turbo
> with 3MB LLC):
> 
> 				Avg		Stddev
> baseline			21.5351		0.5315
> -DCHECK_BEFORE_SET		21.9834		0.0789
> -DCACHE_HOT			14.9987		0.0365
> -DCACHE_HOT -DCHECK_BEFORE_SET	29.9010		0.0204
> 
> Difference between -DCACHE_HOT and -DCACHE_HOT -DCHECK_BEFORE_SET appears
> huge, but if you recalculate it to CPU cycles per inner loop @ 2.8 Ghz,
> it's 1.02530 and 2.04401 CPU cycles respectively.
> 
> Basically, the check is free on decent CPU.
> 
Awesome test, but you only test the one cpu which running this code,
Have not consider the other CPUs, whose cache line will be invalidate if
The cache is dirtied by writer CPU,
So another test should be running 2 thread on two different CPUs(bind to CPU),
One write , one read, to see the impact on the reader CPU.

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