Wpis ten przeleżał zdecydowanie za długo jako szkic…

Albo, by być w zgodzie z technikaliami, ośmioprocesowość. Otóż symuluję sobie działanie sieci peer-to-peer. Algorytmy sterujące zrobiły się trochę zasobożerne, więc już przy 100 maszynach w rzeczonej sieci czas w symulacji płynie wolniej niż na zegarze ściennym. Mając sporo godzinnych symulacji do zrobienia, niespodzianka, uruchamiam je równolegle. Przy okazji zyskałem możliwość pochwalenia się, co potrafi 8 wątków sprzętowych w i7 zainstalowanym w MacBook Pro.

Ośmiowątkowość

Nie będę się rozpisywał o tym, jak działają owe wątki sprzętowe. Ważna jest informacja, że procesor zawiera cztery równorzędne rdzenie. Każdy rdzeń zaś zawiera zdublowany pipeline (jak to właściwie powiedzieć po Polsku?), jednak tylko z jedną kompletną ALU. W efekcie w danym momencie wykonywana jest tylko jedna operacja. Zysk pojawia się, gdy potrzebny jest dostęp do pamięci, kiedy to jedna operacja czeka i można wykonywać drugą. Więcej poczytać można na Wikipedii. Tutaj zaprezentuję szybkie spojrzenie na to, jak to działa w moim przypadku.

Mam więc program który potrafi rozdzielać robotę między pulę procesów. Działania programu nie będę teraz opisywał, poza dwoma ważnymi szczegółami. Po pierwsze, procesy pracują zupełnie niezależnie, łączenie wyników ich działań następuje w masterze po zakończeniu obliczeń. Po drugie, można kontrolować ich liczbę z linii poleceń.

Bez owijania w bawełnę, oto wyniki:

$ echo yes | time ./streaming/script/run_series.py \
    streaming/config/prime.py out -C -p 1 -l 100000

Removing out, continue? 
./streaming/script/run_series.py 
streaming/config/prime.py out -C -p 1 -l  
178.37s user 0.22s system 100% cpu 2:58.52 total

$ echo yes | time ./streaming/script/run_series.py \
    streaming/config/prime.py out -C -p 4 -l 100000

Removing out, continue? 
./streaming/script/run_series.py 
streaming/config/prime.py out -C -p 4 -l  
200.00s user 0.31s system 357% cpu 56.065 total

$ echo yes | time ./streaming/script/run_series.py \
    streaming/config/prime.py out -C -p 8 -l 100000

Removing out, continue? 
./streaming/script/run_series.py 
streaming/config/prime.py out -C -p 8 -l  
338.27s user 0.57s system 684% cpu 49.516 total

Dodałem trochę białych znaków dla czytelności…

Jak widać HT działa. W tym konkretnym przypadku obliczenia wykorzystując HT idą 11.68% czasu krócej niż po prostu używając wszystkich rdzeni. Czysty efekt HT oceniałbym jednak trochę wyżej, z dwóch powodów. Po pierwsze, test wykonałem na laptopie, na którym działo się kilka innych rzeczy. Po drugie, z wyników można się domyślać, że każdy proces ma overhead. Odejmując czas procesora dla jednego procesu od czasu dla czterech procesów i dzieląc przez te trzy dodatkowe procesy, widzimy że wychodzi 7 dodatkowych sekund na proces. Gdyby odjąć te 7 sekund od ostatecznego czasu HT, wyszłoby nam około 25% zysku z użycia HT. Czyli z grubsza tyle, ile sugeruje Intel.

Komentarze

  • Caladan (2012-07-19 10:26:37): Pipeline w procesorze to potok. To ciekawe, może i ja zrobię test czasu wykonywania obliczeń przez mój soft w wersji n i 2*n wątkowej na i7-920 i i5-25xx?
  • Remigiusz ‘lRem’ Modrzejewski (2012-07-19 10:27:00): i5 chyba nie ma HT?
  • Caladan (2012-07-19 10:27:58): MacBook Air 13’ z 2011 ma core i5 i ma HT.
  • Caladan (2012-07-19 10:29:39): O taki: http://ark.intel.com/products/54620
  • Zal (2012-07-19 11:09:20): W naszym projekcie, który realizuje intensywne obliczenia na strumieniach audio-wideo przyrost wydajności z tytułu HT nigdy nie przekroczył 10%. Ale i tak lepiej mieć te 10%, niż ich nie mieć.
  • Caladan (2012-07-19 11:10:32): @Zal: pewnie dlatego, że robicie obliczeń dużo, a skoro jest tylko jedno ALU (a więc pewnie i jedna jednostka MMX/SSE), to czego się spodziewać? A co dokładnie robicie z tymi danymi AV?
  • Zal (2012-07-19 11:16:54): @Caladan: Kodowanie i dekodowania audio-wideo plus wszelkiego rodzaju analiza poszczególnych klatek pod kątem interesujących nas elementów. Przy czym staramy się oddzielić węzły obsługujące kodeki od węzłów dokonujących analizy. Musiałbym poszukać raportów z testów wydajnościowych, ale w ogólności, w naszym przypadku sporą rolę odgrywa również właściwe wykorzystanie cache i pamięci operacyjnej. Szczególnie jednak cache.
  • lrem (2012-07-19 12:10:19): No to jednak spora różnica wynika z problemu. Ja tutaj mam przede wszystkim masę dostępów do pamięci, prawie zerowa lokalność…
  • sprae (2012-07-19 17:26:34): Ja nie wiem czy można dziś mówić o potokach. Są 3 dekodery instrukcji, a jednostek wykonawczych jest 5 (2x ALU, FPU, LOAD, STORE). Wydaje mi się że dużo większą rolę gra w HT wyższa abstrakcja mapowania rejestrów. :-)
  • rozie (2012-07-19 22:51:56): Nawet i3 ma HT…
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