Between the time you type in some C/C++ source code and the time it executes on a CPU, the memory interactions of that code may be reordered according to certain rules. Changes to memory ordering are made both by the compiler (at compile time) and by the processor (at run time), all in the name of making your code run faster. The cardinal rule of memory reordering, which is universally followed by
Spinlocks and Read-Write Locks Most parallel programming in some way will involve the use of locking at the lowest levels. Locks are primitives that provide mutual exclusion that allow data structures to remain in consistent states. Without locking, multiple threads of execution may simultaneously modify a data structure. Without a carefully thought out (and usually complex) lock-free algorithm, t
APPNEXUS MAIN APPNEXUS CAREERS AppNexus is the leader in real-time advertising technology, serving the most sophisticated clients in the ecosystem. Founded in 2007, we are consistently ranked as a top technology company in New York. Slides are at http://concurrencykit.org/presentations/lockfree_introduction/. Recently I’ve been interested in formalizing a course structure for the practical applica
Memory ordering is the order of accesses to computer memory by a CPU. Memory ordering depends on both the order of the instructions generated by the compiler at compile time and the execution order of the CPU at runtime.[1][2] However, memory order is of little concern outside of multithreading and memory-mapped I/O, because if the compiler or CPU changes the order of any operations, it must neces
STMはソフトウェアトランザクショナルメモリの略です。 ↓とりあえずwikipedia http://ja.wikipedia.org/wiki/%E3%82%BD%E3%83%95%E3%83%88%E3%82%A6%E3%82%A7%E3%82%A2%E3%83%88%E3%83%A9%E3%83%B3%E3%82%B6%E3%82%AF%E3%82%B7%E3%83%A7%E3%83%8A%E3%83%AB%E3%83%A1%E3%83%A2%E3%83%AA 日本でSTMの話題を検索すると「楽観的ロックでしょ？」といった発言を見かける事が多く、確かに実用的な手法の多くはロックベースだったりしていますが、正直なところロックベースな手法のSTMはデータベースでのトランザクションと似ているフシがあったりしてデータベースに詳しい人からするとそれほど驚くような手法ではない事が多いのです。その
マルチスレッドなどの環境下で時間経過によって勝手に変化する複数の変数を読みたい場面は多くあります。 しかしCPUは一度に一つしか値を読み書きできないので、簡単には出来ません。 何故なら読んでるそばから値が変動してでたらめな値を読むかも知れないからです。 変動してるものを読むのだから読む側が完璧じゃなくても仕方ないという妥協は有り得ますが 特定のある一瞬の時刻での状態を写真のようにパチリとフィルムに収められたら嬉しいですよね。 一番簡単なのはロックを取りながら読む事ですが、ロックは諸般の事情により使えない事とします。 前提 値が勝手に変動するという状況が分かりにくいと思うので、適当な例えとして卵の孵化を想像してみましょう。 「卵」→「ヒビ」→「ひよこ」 の順でランダムに勝手に進んで行きます。止めることは出来ませんし後戻りもしません。 こんな卵が複数ある孵化器の監視を任されたとします。卵の状態
Urban Airship Published on 29 Sep 2010 Like the idea of working on large scale problems? We’re always looking for talented engineers, and would love to chat with you – check it out! Note: Concurrency, as defined in this article, is the same as it is for The C10k problem: concurrent clients (or sockets). At Urban Airship we recently published a blog post about scaling beyond 500,000 concurrent sock
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