Haskell中的读写器锁

donatello 发表于 Dev

多纳泰洛

我正在实现一个在内存中保存一些数据的Web应用程序。一些请求读取此数据以进行处理，而某些请求更新此数据。

在这种情况下，多个读取器可以同时对数据进行操作，但是写入器需要对内存中的数据进行独占访问。我想实现读写器锁来解决此问题。我还希望公平性属性，即按FIFO顺序处理锁上的服务员，以避免读写不足。

Haskell标准库似乎没有提供这种功能。我发现concurrency-extra提供了此功能，但该库似乎未维护（在LTS 3.22之后已从堆栈中删除）-其公平性对我而言并不明确。

我感到有点奇怪，在标准的haskell库和堆栈中没有读写器锁定库-读写器模式在许多软件中不是常见的吗？还是在Haskell中有一种完全不同的（也许是无锁的）首选方法？

编辑：更准确地说，在公平属性上，当写作者被阻止等待获取锁时，仅在写者获得并释放写锁之后才应允许后续的读锁定请求-与MVars公平属性类似-MVar具有FIFO财产

九州

最好的解决方案取决于读者/作家的关系，但是我认为您只能使用解决问题MVar。

让

import System.Clock
import Text.Printf
import Control.Monad
import Control.Concurrent
import Control.Concurrent.MVar

t__ :: Int -> String -> IO ()
t__ id msg = do
    TimeSpec s n <- getTime Realtime
    putStrLn $ printf "%3d.%-3d - %d %s" (s `mod` 1000) n id msg

reader :: MVar [Int] -> Int -> IO ()
reader mv id = do
    t__                            id $ "reader waiting"
    xs <- readMVar mv
    t__                            id $ "reader working begin"
    threadDelay (1 * 10^6)
    t__                            id $ "reader working ends, " ++ show (length xs) ++ " items"

writer :: MVar [Int] -> Int -> IO ()
writer mv id = do
    t__                            id $ "WRITER waiting"
    xs <- takeMVar mv
    t__                            id $ "WRITER working begin"
    threadDelay (3 * 10^6)
    t__                            id $ "WRITER working ends, " ++ show (1 + length xs) ++ " items"
    putMVar mv (id: xs)

main = do

    mv <- newMVar []
    forM_ (take 10 $ zipWith (\f id -> forkIO (f mv id)) (cycle [reader, reader, reader, writer]) [1..]) $ \p -> do
        threadDelay (10^5)
        p

    getLine

带输出

c:\tmp>mvar.exe +RTS -N20
486.306991300 - 1 reader waiting
486.306991300 - 1 reader working begin
486.416036100 - 2 reader waiting
486.416036100 - 2 reader working begin
486.525191000 - 3 reader waiting
486.525191000 - 3 reader working begin
486.634286500 - 4 WRITER waiting
486.634286500 - 4 WRITER working begin
486.743378400 - 5 reader waiting
486.852406800 - 6 reader waiting
486.961564300 - 7 reader waiting
487.070645900 - 8 WRITER waiting
487.179673900 - 9 reader waiting
487.288845100 - 10 reader waiting
487.320003300 - 1 reader working ends, 0 items
487.429028600 - 2 reader working ends, 0 items
487.538202000 - 3 reader working ends, 0 items
489.642147400 - 10 reader working begin
489.642147400 - 4 WRITER working ends, 1 items
489.642147400 - 5 reader working begin
489.642147400 - 6 reader working begin
489.642147400 - 7 reader working begin
489.642147400 - 8 WRITER working begin
489.642147400 - 9 reader working begin
490.655157400 - 10 reader working ends, 1 items
490.670730800 - 6 reader working ends, 1 items
490.670730800 - 7 reader working ends, 1 items
490.670730800 - 9 reader working ends, 1 items
490.686247400 - 5 reader working ends, 1 items
492.681178800 - 8 WRITER working ends, 2 items