Re: [RFC] change non-atomic bitops method

Kirill A. Shutemov
2015-02-03T01:20:30Z

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@sonymobile.com> 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.

--
Kirill A. Shutemov