Re: [PATCH 0/3] Virtual huge zero page

Kirill A. Shutemov
2012-10-01T17:17:56Z

On Mon, Oct 01, 2012 at 06:14:37PM +0200, Andrea Arcangeli wrote:
> On Mon, Oct 01, 2012 at 04:49:48PM +0300, Kirill A. Shutemov wrote:
> > On Sat, Sep 29, 2012 at 04:37:37PM +0200, Andrea Arcangeli wrote:
> > > But I agree we need to verify it before taking a decision, and that
> > > the numbers are better than theory, or to rephrase it "let's check the
> > > theory is right" :)
> >
> > Okay, microbenchmark:
> >
> > % cat test_memcmp.c
> > #include <assert.h>
> > #include <stdlib.h>
> > #include <string.h>
> >
> > #define MB (1024ul * 1024ul)
> > #define GB (1024ul * MB)
> >
> > int main(int argc, char **argv)
> > {
> > char *p;
> > int i;
> >
> > posix_memalign((void **)&p, 2 * MB, 8 * GB);
> > for (i = 0; i < 100; i++) {
> > assert(memcmp(p, p + 4*GB, 4*GB) == 0);
> > asm volatile ("": : :"memory");
> > }
> > return 0;
> > }
> >
> > huge zero page (initial implementation):
> >
> > Performance counter stats for './test_memcmp' (5 runs):
> >
> > 32356.272845 task-clock # 0.998 CPUs utilized ( +- 0.13% )
> > 40 context-switches # 0.001 K/sec ( +- 0.94% )
> > 0 CPU-migrations # 0.000 K/sec
> > 4,218 page-faults # 0.130 K/sec ( +- 0.00% )
> > 76,712,481,765 cycles # 2.371 GHz ( +- 0.13% ) [83.31%]
> > 36,279,577,636 stalled-cycles-frontend # 47.29% frontend cycles idle ( +- 0.28% ) [83.35%]
> > 1,684,049,110 stalled-cycles-backend # 2.20% backend cycles idle ( +- 2.96% ) [66.67%]
> > 134,355,715,816 instructions # 1.75 insns per cycle
> > # 0.27 stalled cycles per insn ( +- 0.10% ) [83.35%]
> > 13,526,169,702 branches # 418.039 M/sec ( +- 0.10% ) [83.31%]
> > 1,058,230 branch-misses # 0.01% of all branches ( +- 0.91% ) [83.36%]
> >
> > 32.413866442 seconds time elapsed ( +- 0.13% )
> >
> > virtual huge zero page (the second implementation):
> >
> > Performance counter stats for './test_memcmp' (5 runs):
> >
> > 30327.183829 task-clock # 0.998 CPUs utilized ( +- 0.13% )
> > 38 context-switches # 0.001 K/sec ( +- 1.53% )
> > 0 CPU-migrations # 0.000 K/sec
> > 4,218 page-faults # 0.139 K/sec ( +- 0.01% )
> > 71,964,773,660 cycles # 2.373 GHz ( +- 0.13% ) [83.35%]
> > 31,191,284,231 stalled-cycles-frontend # 43.34% frontend cycles idle ( +- 0.40% ) [83.32%]
> > 773,484,474 stalled-cycles-backend # 1.07% backend cycles idle ( +- 6.61% ) [66.67%]
> > 134,982,215,437 instructions # 1.88 insns per cycle
> > # 0.23 stalled cycles per insn ( +- 0.11% ) [83.32%]
> > 13,509,150,683 branches # 445.447 M/sec ( +- 0.11% ) [83.34%]
> > 1,017,667 branch-misses # 0.01% of all branches ( +- 1.07% ) [83.32%]
> >
> > 30.381324695 seconds time elapsed ( +- 0.13% )
> >
> > On Westmere-EX virtual huge zero page is ~6.7% faster.
>
> Great test thanks!
>
> So the cache benefit is quite significant, and the TLB gains don't
> offset the cache loss of the physical zero page. My call was wrong...
>
> I get the same results as you did.
>
> Now let's tweak the benchmark to test a "seeking" workload more
> favorable to the physical 2M page by stressing the TLB.
>
>
> ===
> #include <assert.h>
> #include <stdlib.h>
> #include <string.h>
>
> #define MB (1024ul * 1024ul)
> #define GB (1024ul * MB)
>
> int main(int argc, char **argv)
> {
> char *p;
> int i;
>
> posix_memalign((void **)&p, 2 * MB, 8 * GB);
> for (i = 0; i < 1000; i++) {
> char *_p = p;
> while (_p < p+4*GB) {
> assert(*_p == *(_p+4*GB));
> _p += 4096;
> asm volatile ("": : :"memory");
> }
> }
> return 0;
> }

Results on my machine:

vitual zeropage:
Performance counter stats for 'taskset -c 0 ./test_memcmp2' (5 runs):

27313.891128 task-clock # 0.998 CPUs utilized ( +- 0.24% )
62 context-switches # 0.002 K/sec ( +- 0.61% )
4,384 page-faults # 0.160 K/sec ( +- 0.01% )
64,747,374,606 cycles # 2.370 GHz ( +- 0.24% ) [33.33%]
61,341,580,278 stalled-cycles-frontend # 94.74% frontend cycles idle ( +- 0.26% ) [33.33%]
56,702,237,511 stalled-cycles-backend # 87.57% backend cycles idle ( +- 0.07% ) [33.33%]
10,033,724,846 instructions # 0.15 insns per cycle
# 6.11 stalled cycles per insn ( +- 0.09% ) [41.65%]
2,190,424,932 branches # 80.195 M/sec ( +- 0.12% ) [41.66%]
1,028,630 branch-misses # 0.05% of all branches ( +- 1.50% ) [41.66%]
3,302,006,540 L1-dcache-loads
# 120.891 M/sec ( +- 0.11% ) [41.68%]
271,374,358 L1-dcache-misses
# 8.22% of all L1-dcache hits ( +- 0.04% ) [41.66%]
20,385,476 LLC-load
# 0.746 M/sec ( +- 1.64% ) [33.34%]
76,754 LLC-misses
# 0.38% of all LL-cache hits ( +- 2.35% ) [33.34%]
3,309,927,290 dTLB-loads
# 121.181 M/sec ( +- 0.03% ) [33.34%]
2,098,967,427 dTLB-misses
# 63.41% of all dTLB cache hits ( +- 0.03% ) [33.34%]

27.364448741 seconds time elapsed ( +- 0.24% )

physical zeropage
Performance counter stats for 'taskset -c 0 ./test_memcmp2' (5 runs):

3505.727639 task-clock # 0.998 CPUs utilized ( +- 0.26% )
9 context-switches # 0.003 K/sec ( +- 4.97% )
4,384 page-faults # 0.001 M/sec ( +- 0.00% )
8,318,482,466 cycles # 2.373 GHz ( +- 0.26% ) [33.31%]
5,134,318,786 stalled-cycles-frontend # 61.72% frontend cycles idle ( +- 0.42% ) [33.32%]
2,193,266,208 stalled-cycles-backend # 26.37% backend cycles idle ( +- 5.51% ) [33.33%]
9,494,670,537 instructions # 1.14 insns per cycle
# 0.54 stalled cycles per insn ( +- 0.13% ) [41.68%]
2,108,522,738 branches # 601.451 M/sec ( +- 0.09% ) [41.68%]
158,746 branch-misses # 0.01% of all branches ( +- 1.60% ) [41.71%]
3,168,102,115 L1-dcache-loads
# 903.693 M/sec ( +- 0.11% ) [41.70%]
1,048,710,998 L1-dcache-misses
# 33.10% of all L1-dcache hits ( +- 0.11% ) [41.72%]
1,047,699,685 LLC-load
# 298.854 M/sec ( +- 0.03% ) [33.38%]
2,287 LLC-misses
# 0.00% of all LL-cache hits ( +- 8.27% ) [33.37%]
3,166,187,367 dTLB-loads
# 903.147 M/sec ( +- 0.02% ) [33.35%]
4,266,538 dTLB-misses
# 0.13% of all dTLB cache hits ( +- 0.03% ) [33.33%]

3.513339813 seconds time elapsed ( +- 0.26% )

--
Kirill A. Shutemov