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Date: Tue, 16 Oct 2012 19:37:47 +0800
From: Ni zhan Chen <nizhan.chen@...il.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
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 10/16/2012 07:28 PM, Kirill A. Shutemov wrote:
> 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.
Oh, I see, thanks for your patient. :-)
>
>>>>> 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. :-)
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