可以监视STM的争用级别吗？

问题描述

有什么方法来轮询Clojure的STM事务是否正在重试，以什么速度？

Is there any way to poll whether Clojure's STM transactions are being retried, and at what rate?

推荐答案

em>命名dosync块和提交计数（命名dosync成功的时间），可以很容易地跟踪线程已经重试给定事务的时间。

By introducing named dosync blocks and commit counts (the times a named dosync has succeeded), one can quite easily keep track of the times threads have retried a given transaction.

(def ^{:doc "ThreadLocal<Map<TxName, Map<CommitNumber, TriesCount>>>"}
  local-tries (let [l (ThreadLocal.)]
                (.set l {})
                l))

(def ^{:doc "Map<TxName, Int>"}
  commit-number (ref {}))

(def history ^{:doc "Map<ThreadId, Map<TxName, Map<CommitNumber, TriesCount>>>"}
  (atom {}))

(defn report [_ thread-id tries]
  (swap! history assoc thread-id tries))

(def reporter (agent nil))

(defmacro dosync [tx-name & body]
  `(clojure.core/dosync
    (let [cno# (@commit-number ~tx-name 0)
          tries# (update-in (.get local-tries) [~tx-name] update-in [cno#] (fnil inc 0))]
      (.set local-tries tries#)
      (send reporter report (.getId (Thread/currentThread)) tries#))
    ~@body
    (alter commit-number update-in [~tx-name] (fnil inc 0))))

p>

Given the following example...

(def foo (ref {}))

(def bar (ref {}))

(defn x []
  (dosync :x ;; `:x`: the tx-name.
          (let [r (rand-int 2)]
            (alter foo assoc r (rand))
            (Thread/sleep (rand-int 400))
            (alter bar assoc (rand-int 2) (@foo r)))))

(dotimes [i 4]
  (future
   (dotimes [i 10]
     (x))))

... @history 计算为：

;; {thread-id {tx-name {commit-number tries-count}}}
{40 {:x {3 1, 2 4, 1 3, 0 1}}, 39 {:x {2 1, 1 3, 0 1}}, ...}

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