Neden g ++ ve clang arasında librt swap performansına bağlanıyor?

Question

Sanal işlev çağrısı overhead'i test etmek için bir bank koduyla @ tony-d'den this answer'u buldum. Onun (bu oran örneğin 2) daha iyi performans var

$ g++ -O2 -o vdt vdt.cpp -lrt 
$ ./vdt 
virtual dispatch: 150000000 0.128562 
switched: 150000000 0.0803207 
overheads: 150000000 0.0543323 
... 

ama sonra clang ile kontrol: Ben g++ kriter kullanıyor kontrol

$ clang++-3.7 -O2 -o vdt vdt.cpp -lrt 
$ ./vdt 
virtual dispatch: 150000000 0.462368 
switched: 150000000 0.0569544 
overheads: 150000000 0.0509332 
... 

Şimdi bu oran yaklaşık 70 kadar çıkıyor!

Sonra -lrt komut satırı argümanı fark ve googling biraz sonra yaklaşık librt Ben g++ ve clang için onsuz çalıştı: Gördüğünüz gibi

$ g++ -O2 -o vdt vdt.cpp 
$ ./vdt 
virtual dispatch: 150000000 0.4661 
switched: 150000000 0.0815865 
overheads: 150000000 0.0543611 
... 
$ clang++-3.7 -O2 -o vdt vdt.cpp 
$ ./vdt 
virtual dispatch: 150000000 0.155901 
switched: 150000000 0.0568319 
overheads: 150000000 0.0492521 
... 

, performans swaped vardır. Ben yaklaşık librt, bu clock_gettime ve diğer ilgili zaman hesaplaması (belki bu durumda beni yanlış düzeltmek am!) Için gereklidir bulundu ancak kod -lrt olmadan iyi derler, ve zaman I neyi doğru görünüyor kadarıyla

görmek.

Neden librt bağlanır/bağlanmıyor bu kodu o kadar çok etkiler? sistemimde ve derleyici hakkında

---

Bilgiler:

$ g++ --version 
g++-5 (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204 
Copyright (C) 2015 Free Software Foundation, Inc. 

$ clang++-3.7 --version 
Debian clang version 3.7.1-svn254351-1~exp1 (branches/release_37) (based on LLVM 3.7.1) 
Target: x86_64-pc-linux-gnu 
Thread model: posix 

$ uname -a 
Linux ****** 3.13.0-86-generiC#130-Ubuntu SMP Mon Apr 18 18:27:15 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux 

Answer 1

Ben -lrt belirtilmişse çünkü iyileştirici daha fazla veri ve sahip kütüphane ile bağlamaya çalışmak yerine, (oprimizer ile bağlı tahmin ediyorum

farklı şekilde optimize edebilir).

Farklılıklara gelince, g ++ (4.8.4) ile aynı sonuçları aldım ve -lrt olmadan, ama clang (3.4.-lubuntu3) arasında bir fark var. Ben folloving sonuçları ile perftools istatistik aracılığıyla bu çalıştırmayı denedi:

$ g++ -O2 -o vdt vdt.cpp -lrt && perf stat -d ./vdt 
virtual dispatch: 150000000 1.2304 
switched: 150000000 0.131782 
overheads: 150000000 0.0842732 
virtual dispatch: 150000000 1.13689 
switched: 150000000 0.137304 
overheads: 150000000 0.0854806 
virtual dispatch: 150000000 1.19261 
switched: 150000000 0.133561 
overheads: 150000000 0.0969093 

Performance counter stats for './vdt': 

     4068.861539 task-clock (msec)   # 0.961 CPUs utilized   
      1,068 context-switches   # 0.262 K/sec     
       0 cpu-migrations   # 0.000 K/sec     
       431 page-faults    # 0.106 K/sec     
    11,977,128,883 cycles     # 2.944 GHz      [40.18%] 
    6,088,274,331 stalled-cycles-frontend # 50.83% frontend cycles idle [39.92%] 
    3,984,855,636 stalled-cycles-backend # 33.27% backend cycles idle [39.98%] 
    6,581,309,599 instructions    # 0.55 insns per cycle   
              # 0.93 stalled cycles per insn [50.06%] 
    1,506,617,848 branches     # 370.280 M/sec     [50.12%] 
     303,871,937 branch-misses    # 20.17% of all branches   [49.88%] 
    2,708,080,460 L1-dcache-loads   # 665.562 M/sec     [49.94%] 
     559,844,530 L1-dcache-load-misses  # 20.67% of all L1-dcache hits [50.28%] 
       0 LLC-loads     # 0.000 K/sec     [40.05%] 
       0 LLC-load-misses   # 0.00% of all LL-cache hits [39.98%] 

     4.232477683 seconds time elapsed 

$ g++ -O2 -o vdt vdt.cpp && perf stat -d ./vdt 
virtual dispatch: 150000000 1.11517 
switched: 150000000 0.14231 
overheads: 150000000 0.0840234 
virtual dispatch: 150000000 1.11355 
switched: 150000000 0.130082 
overheads: 150000000 0.116934 
virtual dispatch: 150000000 1.16225 
switched: 150000000 0.13281 
overheads: 150000000 0.0798615 

Performance counter stats for './vdt': 

     4050.314222 task-clock (msec)   # 0.993 CPUs utilized   
       707 context-switches   # 0.175 K/sec     
       0 cpu-migrations   # 0.000 K/sec     
       402 page-faults    # 0.099 K/sec     
    12,213,599,260 cycles     # 3.015 GHz      [39.72%] 
    6,987,416,990 stalled-cycles-frontend # 57.21% frontend cycles idle [40.25%] 
    4,675,829,189 stalled-cycles-backend # 38.28% backend cycles idle [40.17%] 
    6,611,623,206 instructions    # 0.54 insns per cycle   
              # 1.06 stalled cycles per insn [50.54%] 
    1,505,162,879 branches     # 371.616 M/sec     [50.48%] 
     298,748,152 branch-misses    # 19.85% of all branches   [50.30%] 
    2,710,580,651 L1-dcache-loads   # 669.227 M/sec     [50.04%] 
     551,212,908 L1-dcache-load-misses  # 20.34% of all L1-dcache hits [49.86%] 
       3 LLC-loads     # 0.001 K/sec     [39.62%] 
       0 LLC-load-misses   # 0.00% of all LL-cache hits [40.01%] 

     4.080288324 seconds time elapsed 

$ clang++ -O2 -o vdt vdt.cpp -lrt && perf stat -d ./vdt 
virtual dispatch: 150000000 0.276252 
switched: 150000000 0.11926 
overheads: 150000000 0.0733678 
virtual dispatch: 150000000 0.249832 
switched: 150000000 0.0892711 
overheads: 150000000 0.117108 
virtual dispatch: 150000000 0.247705 
switched: 150000000 0.109486 
overheads: 150000000 0.0762541 

Performance counter stats for './vdt': 

     1347.887606 task-clock (msec)   # 0.989 CPUs utilized   
       222 context-switches   # 0.165 K/sec     
       0 cpu-migrations   # 0.000 K/sec     
       430 page-faults    # 0.319 K/sec     
    3,558,892,668 cycles     # 2.640 GHz      [42.47%] 
    1,316,787,839 stalled-cycles-frontend # 37.00% frontend cycles idle [42.61%] 
     438,592,926 stalled-cycles-backend # 12.32% backend cycles idle [40.57%] 
    6,388,507,180 instructions    # 1.80 insns per cycle   
              # 0.21 stalled cycles per insn [50.49%] 
    1,514,291,853 branches     # 1123.456 M/sec     [50.19%] 
     1,095,265 branch-misses    # 0.07% of all branches   [48.66%] 
    2,485,922,557 L1-dcache-loads   # 1844.310 M/sec     [47.99%] 
     577,213,257 L1-dcache-load-misses  # 23.22% of all L1-dcache hits [48.20%] 
       2 LLC-loads     # 0.001 K/sec     [40.51%] 
       0 LLC-load-misses   # 0.00% of all LL-cache hits [40.17%] 

     1.362403811 seconds time elapsed 

$ clang++ -O2 -o vdt vdt.cpp && perf stat -d ./vdt 
virtual dispatch: 150000000 1.0894 
switched: 150000000 0.0849747 
overheads: 150000000 0.0726611 
virtual dispatch: 150000000 1.03949 
switched: 150000000 0.0849843 
overheads: 150000000 0.0768674 
virtual dispatch: 150000000 1.07786 
switched: 150000000 0.0893431 
overheads: 150000000 0.0725624 

Performance counter stats for './vdt': 

     3667.235804 task-clock (msec)   # 0.993 CPUs utilized   
       356 context-switches   # 0.097 K/sec     
       0 cpu-migrations   # 0.000 K/sec     
       402 page-faults    # 0.110 K/sec     
    11,052,067,182 cycles     # 3.014 GHz      [39.98%] 
    5,346,555,173 stalled-cycles-frontend # 48.38% frontend cycles idle [40.10%] 
    3,480,506,097 stalled-cycles-backend # 31.49% backend cycles idle [40.09%] 
    6,351,819,740 instructions    # 0.57 insns per cycle   
              # 0.84 stalled cycles per insn [50.07%] 
    1,524,106,229 branches     # 415.601 M/sec     [50.17%] 
     299,296,742 branch-misses    # 19.64% of all branches   [50.05%] 
    2,393,484,447 L1-dcache-loads   # 652.667 M/sec     [49.93%] 
     554,010,640 L1-dcache-load-misses  # 23.15% of all L1-dcache hits [49.88%] 
       0 LLC-loads     # 0.000 K/sec     [40.33%] 
       0 LLC-load-misses   # 0.00% of all LL-cache hits [39.83%] 

     3.692786417 seconds time elapsed 

I (optimizer o tekrar) clang içinde dallanma tahmini (şube-özlüyor) bazı fark olduğunu görebilecekleri.