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

Andrea Arcangeli
2012-10-01T16:14:53Z

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:

virtual zeropage: char comparison seeking in 4G range 1000 times

Performance counter stats for './zeropage-bench2' (3 runs):

20624.051801 task-clock # 0.999 CPUs utilized ( +- 0.17% )
1,762 context-switches # 0.085 K/sec ( +- 1.05% )
1 CPU-migrations # 0.000 K/sec ( +- 50.00% )
4,221 page-faults # 0.205 K/sec
60,182,028,883 cycles # 2.918 GHz ( +- 0.17% ) [40.00%]
56,958,431,315 stalled-cycles-frontend # 94.64% frontend cycles idle ( +- 0.16% ) [40.02%]
54,966,753,363 stalled-cycles-backend # 91.33% backend cycles idle ( +- 0.10% ) [40.03%]
8,606,418,680 instructions # 0.14 insns per cycle
# 6.62 stalled cycles per insn ( +- 0.39% ) [50.03%]
2,142,535,994 branches # 103.885 M/sec ( +- 0.20% ) [50.03%]
115,916 branch-misses # 0.01% of all branches ( +- 3.86% ) [50.03%]
3,209,731,169 L1-dcache-loads # 155.630 M/sec ( +- 0.45% ) [50.01%]
264,297,418 L1-dcache-load-misses # 8.23% of all L1-dcache hits ( +- 0.02% ) [50.00%]
6,732,362 LLC-loads # 0.326 M/sec ( +- 0.23% ) [39.99%]
4,981,319 LLC-load-misses # 73.99% of all LL-cache hits ( +- 0.74% ) [39.98%]

20.649561185 seconds time elapsed ( +- 0.19% )

physical zeropage: char comparison seeking in 4G range 1000 times

Performance counter stats for './zeropage-bench2' (3 runs):

2719.512443 task-clock # 0.999 CPUs utilized ( +- 0.34% )
234 context-switches # 0.086 K/sec ( +- 1.00% )
0 CPU-migrations # 0.000 K/sec
4,221 page-faults # 0.002 M/sec
7,927,948,993 cycles # 2.915 GHz ( +- 0.17% ) [39.95%]
4,780,183,162 stalled-cycles-frontend # 60.30% frontend cycles idle ( +- 0.58% ) [40.14%]
2,246,666,029 stalled-cycles-backend # 28.34% backend cycles idle ( +- 3.59% ) [40.19%]
8,380,516,407 instructions # 1.06 insns per cycle
# 0.57 stalled cycles per insn ( +- 0.13% ) [50.21%]
2,095,233,526 branches # 770.445 M/sec ( +- 0.08% ) [50.24%]
24,586 branch-misses # 0.00% of all branches ( +- 11.77% ) [50.19%]
3,151,778,195 L1-dcache-loads # 1158.950 M/sec ( +- 0.01% ) [50.05%]
1,051,317,291 L1-dcache-load-misses # 33.36% of all L1-dcache hits ( +- 0.02% ) [49.96%]
1,049,134,961 LLC-loads # 385.781 M/sec ( +- 0.13% ) [39.92%]
6,222 LLC-load-misses # 0.00% of all LL-cache hits ( +- 35.68% ) [39.93%]

2.722077632 seconds time elapsed ( +- 0.34% )

NOTE: I used taskset -c 0 in all tests here to reduce the error (this
is also a NUMA system and AutoNUMA wasn't patched in for this test to
avoid the risk of rejects in "git am").

(it would have been prettier if I added the TLB data performance
counters, whatever too late ;)

So in this case the compute time increases 658% with the 2m virtual
page, and the 2M physical page wins by a wide margin.

So my preference is still for the physical zero page even if it wastes
2m-4k RAM and increases the compute time 6% in the worst case.

Thanks!
Andrea