你缺少一些关键信息:你使用哪个编译器?你用什么来衡量微基准的性能?你使用哪个标准库实现?
我的系统:
g++ (GCC) 4.9.1 20140903 (prerelease)
GLIBCXX_3.4.20
无论如何,我运行了您的示例,但首先保留向量的适当大小以消除内存分配开销。因此,有趣的是,我观察到了相反的一些有趣现象 - 与您看到的相反:
g++ -std=c++11 -O2 pair.cpp -o pair
perf stat -r 10 -d ./pair
Performance counter stats for './pair' (10 runs):
1647.045151 task-clock:HG (msec) # 0.993 CPUs utilized ( +- 1.94% )
346 context-switches:HG # 0.210 K/sec ( +- 40.13% )
7 cpu-migrations:HG # 0.004 K/sec ( +- 22.01% )
182,978 page-faults:HG # 0.111 M/sec ( +- 0.04% )
3,394,685,602 cycles:HG # 2.061 GHz ( +- 2.24% ) [44.38%]
2,478,474,676 stalled-cycles-frontend:HG # 73.01% frontend cycles idle ( +- 1.24% ) [44.55%]
1,550,747,174 stalled-cycles-backend:HG # 45.68% backend cycles idle ( +- 1.60% ) [44.66%]
2,837,484,461 instructions:HG # 0.84 insns per cycle
# 0.87 stalled cycles per insn ( +- 4.86% ) [55.78%]
526,077,681 branches:HG # 319.407 M/sec ( +- 4.52% ) [55.82%]
829,623 branch-misses:HG # 0.16% of all branches ( +- 4.42% ) [55.74%]
594,396,822 L1-dcache-loads:HG # 360.887 M/sec ( +- 4.74% ) [55.59%]
20,842,113 L1-dcache-load-misses:HG # 3.51% of all L1-dcache hits ( +- 0.68% ) [55.46%]
5,474,166 LLC-loads:HG # 3.324 M/sec ( +- 1.81% ) [44.23%]
<not supported> LLC-load-misses:HG
1.658671368 seconds time elapsed ( +- 1.82% )
对比:
g++ -std=c++11 -O2 tuple.cpp -o tuple
perf stat -r 10 -d ./tuple
Performance counter stats for './tuple' (10 runs):
996.090514 task-clock:HG (msec) # 0.996 CPUs utilized ( +- 2.41% )
102 context-switches:HG # 0.102 K/sec ( +- 64.61% )
4 cpu-migrations:HG # 0.004 K/sec ( +- 32.24% )
181,701 page-faults:HG # 0.182 M/sec ( +- 0.06% )
2,052,505,223 cycles:HG # 2.061 GHz ( +- 2.22% ) [44.45%]
1,212,930,513 stalled-cycles-frontend:HG # 59.10% frontend cycles idle ( +- 2.94% ) [44.56%]
621,104,447 stalled-cycles-backend:HG # 30.26% backend cycles idle ( +- 3.48% ) [44.69%]
2,700,410,991 instructions:HG # 1.32 insns per cycle
# 0.45 stalled cycles per insn ( +- 1.66% ) [55.94%]
486,476,408 branches:HG # 488.386 M/sec ( +- 1.70% ) [55.96%]
959,651 branch-misses:HG # 0.20% of all branches ( +- 4.78% ) [55.82%]
547,000,119 L1-dcache-loads:HG # 549.147 M/sec ( +- 2.19% ) [55.67%]
21,540,926 L1-dcache-load-misses:HG # 3.94% of all L1-dcache hits ( +- 2.73% ) [55.43%]
5,751,650 LLC-loads:HG # 5.774 M/sec ( +- 3.60% ) [44.21%]
<not supported> LLC-load-misses:HG
1.000126894 seconds time elapsed ( +- 2.47% )
正如您所见,对于我的情况来说,原因是前端和后端拖延周期的数量要高得多。
那么这是从哪里来的呢?我敢打赌这归结于一些失败的内联,类似于这里所解释的内容:std::vector performance regression when enabling C++11
确实,启用-flto
可以使结果平衡:
Performance counter stats for './pair' (10 runs):
1021.922944 task-clock:HG (msec)
63 context-switches:HG
5 cpu-migrations:HG
195,396 page-faults:HG
2,109,877,147 cycles:HG
1,098,031,078 stalled-cycles-frontend:HG
701,553,535 stalled-cycles-backend:HG
3,288,420,630 instructions:HG
672,941,736 branches:HG
660,278 branch-misses:HG
474,314,267 L1-dcache-loads:HG
19,481,787 L1-dcache-load-misses:HG
5,155,678 LLC-loads:HG
<not supported> LLC-load-misses:HG
1.025083895 seconds time elapsed ( +- 1.03% )
对于元组:
Performance counter stats for './tuple' (10 runs):
1018.980969 task-clock:HG (msec) # 0.999 CPUs utilized ( +- 0.47% )
8 context-switches:HG # 0.008 K/sec ( +- 29.74% )
3 cpu-migrations:HG # 0.003 K/sec ( +- 42.64% )
195,396 page-faults:HG # 0.192 M/sec ( +- 0.00% )
2,103,574,740 cycles:HG # 2.064 GHz ( +- 0.30% ) [44.28%]
1,088,827,212 stalled-cycles-frontend:HG # 51.76% frontend cycles idle ( +- 0.47% ) [44.56%]
697,438,071 stalled-cycles-backend:HG # 33.15% backend cycles idle ( +- 0.41% ) [44.76%]
3,305,631,646 instructions:HG # 1.57 insns per cycle
# 0.33 stalled cycles per insn ( +- 0.21% ) [55.94%]
675,175,757 branches:HG # 662.599 M/sec ( +- 0.16% ) [56.02%]
656,205 branch-misses:HG # 0.10% of all branches ( +- 0.98% ) [55.93%]
475,532,976 L1-dcache-loads:HG # 466.675 M/sec ( +- 0.13% ) [55.69%]
19,430,992 L1-dcache-load-misses:HG # 4.09% of all L1-dcache hits ( +- 0.20% ) [55.49%]
5,161,624 LLC-loads:HG # 5.065 M/sec ( +- 0.47% ) [44.14%]
<not supported> LLC-load-misses:HG
1.020225388 seconds time elapsed ( +- 0.48% )
所以记住,-flto
是你的朋友,而失败的内联可能会对大量模板化的代码产生极端影响。使用perf stat
查找正在发生的情况。
v.reserve(100000000)
,以使测试更加准确。 - Jonathan Potter-O3
会有什么不同。 - BЈовић