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Date: Tue, 16 Oct 2012 14:28:45 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Ni zhan Chen <nizhan.chen@...il.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
Andi Kleen <ak@...ux.intel.com>,
"H. Peter Anvin" <hpa@...ux.intel.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 00/10, REBASED] Introduce huge zero page
On Tue, Oct 16, 2012 at 07:13:07PM +0800, Ni zhan Chen wrote:
> On 10/16/2012 06:54 PM, Kirill A. Shutemov wrote:
> >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.
>
> Oh, I see, thanks for your quick response. Another one question below，
>
> >
> >>>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?
>
> Sorry for my unintelligent. Could you tell me which data I should
> care in this performance counter stats. The same question about the
> second benchmark counter stats, thanks in adance. :-)
I've missed relevant counters in this run, you can see them in the second
benchmark.
Relevant counters:
L1-dcache-*, LLC-*: shows cache related stats (hits/misses);
dTLB-*: shows data TLB hits and misses.
Indirect relevant counters:
stalled-cycles-*: how long CPU pipeline has to wait for data.
> >>>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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