On Tue, Oct 16, 2012 at 05:53:07PM +0800, Ni zhan Chen wrote:
> >By hpa request I've tried alternative approach for hzp implementation (see
> >Virtual huge zero page patchset): pmd table with all entries set to zero
> >page. This way should be more cache friendly, but it increases TLB
> >pressure.
>
> Thanks for your excellent works. But could you explain me why
> current implementation not cache friendly and hpa's request cache
> friendly? Thanks in advance.
In workloads like microbenchmark1 you need N * size(zero page) cache
space to get zero page fully cached, where N is cache associativity.
If zero page is 2M, cache pressure is significant.
On other hand with table of 4k zero pages (hpa's proposal) will increase
pressure on TLB, since we have more pages for the same memory area. So we
have to do more page translation in this case.
On my test machine with simple memcmp() virtual huge zero page is faster.
But it highly depends on TLB size, cache size, memory access and page
translation costs.
It looks like cache size in modern processors grows faster than TLB size.
> >The problem with virtual huge zero page: it requires per-arch enabling.
> >We need a way to mark that pmd table has all ptes set to zero page.
> >
> >Some numbers to compare two implementations (on 4s Westmere-EX):
> >
> >Mirobenchmark1
> >==============
> >
> >test:
> > 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");
> > }
> >
> >hzp:
> > 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% )
> >
> >vhzp:
> > 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% )
>
> Could you tell me which data I should care in this performance
> counter. And what's the benefit of your current implementation
> compare to hpa's request?
>
> >
> >Mirobenchmark2
> >==============
> >
> >test:
> > 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");
> > }
> > }
> >
> >hzp:
> > 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% )
> >
> >vhzp:
> > 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% )
>
> For this case, the same question as above, thanks in adance. :-)
--
Kirill A. Shutemov
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