February 2017
Intermediate to advanced
368 pages
8h 21m
Japanese
Content preview from Optimized C++ ―最適化、高速化のためのプログラミングテクニック
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42
3 章 性能を測定する
3.4.2
コンピュータで時間を測る
コンピュータ上に時計を作るには、理想的には精度と真度に優れた、周期振動源とそこから
tick
を得るソフトウェアとが必要だ。時間計測用の専用マシンは簡単に作ることができるはずだ。しか
し、普通の現在のコンピュータアーキテクチャは、いい時計を提供することをあまり考えないで設
計されてきた。
PC
アーキテクチャと
Microsoft Windows
を使って問題を述べるが、
Linux
や組み
込みプラットフォームでも同じようなものだ。
PC
のクロック回路の水晶発振器は、通常、基本の正確度が百万回について
100
回、つまり
0.01
％、
1
日に約
8
秒だ。この正確度はデジタル腕時計をわずかに上回る程度で、まったく形式ば
らない測定結果が
2
、
3
％の正確度で得られるなら、性能測定には十分すぎる。安価な組み込み用
プロセッサのクロック回路はそれより正確度が劣るが、ほとんどの重大な問題は、周期振動源によ
るものではなく、プログラムで使うことができる信頼できる
tick
数をどうやって得るかという問
題の困難さによる。
3.4.2.1
tick
カウンタのハードウェア進化
IBM PC
にはソフトウェアで読めるクロックはあったが、ハードウェアの
tick
カウンタは元々
なかった。元の
Microsoft C
ランタイムライブラリは、
ANSI C
ライブラリをコピーしていて、
1970
年
1
月
1
日
00:00 UTC
からの秒数を返す関数
time_t time(time_t*)
を提供した。
time() ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.