可以监视STM的争用级别吗?
有没有什么方法可以调查Clojure的STM交易是否正在重试以及以何种速度进行?
你可以观察到一个ref的history count
,它会表明它存在争用:
user=> (def my-ref (ref 0 :min-history 1))
#'user/my-ref
user=> (ref-history-count my-ref)
0
user=> (dosync (alter my-ref inc))
1
user=> (ref-history-count my-ref)
1
历史记录并不直接代表争用。 相反,它代表为维护并发读取而保留的过去值的数量。
历史记录的大小受限于min
和max
。 默认情况下,它们分别为0
和10
,但创建ref
时可以更改它们(请参见上文)。 由于min-history
默认为0
,所以通常不会看到ref-history-count
返回非零值,除非ref上存在争用。
请点击此处查看有关history count
更多讨论:https://groups.google.com/forum/?fromgroups#!topic/clojure/n_MKCoa870o
我认为clojure.core
没有任何办法来观察STM交易的速度。 当然,你可以做一些类似于@Chouser历史压力测试的东西:
(dosync
(swap! try-count inc)
...)
即在交易中增加一个计数器。 每次尝试交易时都会发生增量。 如果try-count
大于1
,则交易将被重试。
通过引入命名的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重试给定事务的时间。
(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))))
鉴于下面的例子...
(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
评估为:
;; {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}}, ...}
这个额外的实现要简单得多。
;; {thread-id retries-of-latest-tx}
(def tries (atom {}))
;; The max amount of tries any thread has performed
(def max-tries (atom 0))
(def ninc (fnil inc 0))
(def reporter (agent nil))
(defn report [_ tid]
(swap! max-tries #(max % (get @tries tid 0)))
(swap! tries update-in [tid] (constantly 0)))
(defmacro dosync [& body]
`(clojure.core/dosync
(swap! tries update-in [(.getId (Thread/currentThread))] ninc)
(commute commit-id inc)
(send reporter report (.getId (Thread/currentThread)))
~@body))
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