Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
Fetching contributors…

Cannot retrieve contributors at this time

199 lines (187 sloc) 7.454 kb
{-# LANGUAGE CPP, NamedFieldPuns, RecordWildCards, ScopedTypeVariables #-}
-- |
-- Module: Data.Pool
-- Copyright: (c) 2011 MailRank, Inc.
-- License: BSD3
-- Maintainer: Bryan O'Sullivan <bos@mailrank.com>
-- Stability: experimental
-- Portability: portable
--
-- A high-performance striped pooling abstraction for managing
-- flexibly-sized collections of resources such as database
-- connections.
--
-- \"Striped\" means that a single 'Pool' consists of several
-- sub-pools, each managed independently. A stripe size of 1 is fine
-- for many applications, and probably what you should choose by
-- default. Larger stripe sizes will lead to reduced contention in
-- high-performance multicore applications, at a trade-off of causing
-- the maximum number of simultaneous resources in use to grow.
module Data.Pool
(
Pool(idleTime, maxResources, numStripes)
, createPool
, withResource
) where
import Control.Applicative ((<$>))
import Control.Concurrent (forkIO, killThread, myThreadId, threadDelay)
import Control.Concurrent.STM
import Control.Exception (SomeException, catch, onException)
import Control.Monad (forM_, forever, join, liftM2, unless, when)
import Control.Monad.IO.Class (liftIO)
import Control.Monad.IO.Control (MonadControlIO, controlIO)
import Data.Hashable (hash)
import Data.List (partition)
import Data.Time.Clock (NominalDiffTime, UTCTime, diffUTCTime, getCurrentTime)
import Prelude hiding (catch)
import System.Mem.Weak (addFinalizer)
import qualified Data.Vector as V
-- | A single resource pool entry.
data Entry a = Entry {
entry :: a
, lastUse :: UTCTime
-- ^ Time of last return.
}
-- | A single striped pool.
data LocalPool a = LocalPool {
inUse :: TVar Int
-- ^ Count of open entries (both idle and in use).
, entries :: TVar [Entry a]
-- ^ Idle entries.
}
data Pool a = Pool {
create :: IO a
-- ^ Action for creating a new entry to add to the pool.
, destroy :: a -> IO ()
-- ^ Action for destroying an entry that is now done with.
, numStripes :: Int
-- ^ Stripe count. The number of distinct sub-pools to maintain.
-- The smallest acceptable value is 1.
, idleTime :: NominalDiffTime
-- ^ Amount of time for which an unused resource is kept alive.
-- The smallest acceptable value is 0.5 seconds.
--
-- The elapsed time before closing may be a little longer than
-- requested, as the reaper thread wakes at 1-second intervals.
, maxResources :: Int
-- ^ Maximum number of resources to maintain per stripe. The
-- smallest acceptable value is 1.
--
-- Requests for resources will block if this limit is reached on a
-- single stripe, even if other stripes have idle resources
-- available.
, localPools :: V.Vector (LocalPool a)
-- ^ Per-capability resource pools.
}
instance Show (Pool a) where
show Pool{..} = "Pool {numStripes = " ++ show numStripes ++ ", " ++
"idleTime = " ++ show idleTime ++ ", " ++
"maxResources = " ++ show maxResources ++ "}"
createPool
:: IO a
-- ^ Action that creates a new resource.
-> (a -> IO ())
-- ^ Action that destroys an existing resource.
-> Int
-- ^ Stripe count. The number of distinct sub-pools to maintain.
-- The smallest acceptable value is 1.
-> NominalDiffTime
-- ^ Amount of time for which an unused resource is kept open.
-- The smallest acceptable value is 0.5 seconds.
--
-- The elapsed time before destroying a resource may be a little
-- longer than requested, as the reaper thread wakes at 1-second
-- intervals.
-> Int
-- ^ Maximum number of resources to keep open per stripe. The
-- smallest acceptable value is 1.
--
-- Requests for resources will block if this limit is reached on a
-- single stripe, even if other stripes have idle resources
-- available.
-> IO (Pool a)
createPool create destroy numStripes idleTime maxResources = do
when (numStripes < 1) $
modError "pool " $ "invalid stripe count " ++ show numStripes
when (idleTime < 0.5) $
modError "pool " $ "invalid idle time " ++ show idleTime
when (maxResources < 1) $
modError "pool " $ "invalid maximum resource count " ++ show maxResources
localPools <- atomically . V.replicateM numStripes $
liftM2 LocalPool (newTVar 0) (newTVar [])
reaperId <- forkIO $ reaper destroy idleTime localPools
let p = Pool {
create
, destroy
, numStripes
, idleTime
, maxResources
, localPools
}
addFinalizer p $ killThread reaperId
return p
-- | Periodically go through all pools, closing any resources that
-- have been left idle for too long.
reaper :: (a -> IO ()) -> NominalDiffTime -> V.Vector (LocalPool a) -> IO ()
reaper destroy idleTime pools = forever $ do
threadDelay (1 * 1000000)
now <- getCurrentTime
let isStale Entry{..} = now `diffUTCTime` lastUse > idleTime
V.forM_ pools $ \LocalPool{..} -> do
resources <- atomically $ do
(stale,fresh) <- partition isStale <$> readTVar entries
unless (null stale) $ do
writeTVar entries fresh
modifyTVar_ inUse (subtract (length stale))
return (map entry stale)
forM_ resources $ \resource -> do
destroy resource `catch` \(_::SomeException) -> return ()
-- | Temporarily take a resource from a 'Pool', perform an action with
-- it, and return it to the pool afterwards.
--
-- * If the pool has an idle resource available, it is used
-- immediately.
--
-- * Otherwise, if the maximum number of resources has not yet been
-- reached, a new resource is created and used.
--
-- * If the maximum number of resources has been reached, this
-- function blocks until a resource becomes available.
--
-- If the action throws an exception of any type, the resource is
-- destroyed, and not returned to the pool.
--
-- It probably goes without saying that you should never manually
-- destroy a pooled resource, as doing so will almost certainly cause
-- a subsequent user (who expects the resource to be valid) to throw
-- an exception.
withResource :: MonadControlIO m => Pool a -> (a -> m b) -> m b
{-# SPECIALIZE withResource :: Pool a -> (a -> IO b) -> IO b #-}
withResource Pool{..} act = do
i <- liftIO $ ((`mod` numStripes) . hash) <$> myThreadId
let LocalPool{..} = localPools V.! i
resource <- liftIO . join . atomically $ do
ents <- readTVar entries
case ents of
(Entry{..}:es) -> writeTVar entries es >> return (return entry)
[] -> do
used <- readTVar inUse
when (used == maxResources) retry
writeTVar inUse $! used + 1
return $
create `onException` atomically (modifyTVar_ inUse (subtract 1))
ret <- controlIO $ \runInIO -> runInIO (act resource) `onException` (do
destroy resource `catch` \(_::SomeException) -> return ()
atomically (modifyTVar_ inUse (subtract 1)))
liftIO $ do
now <- getCurrentTime
atomically $ modifyTVar_ entries (Entry resource now:)
return ret
#if __GLASGOW_HASKELL__ >= 700
{-# INLINABLE withResource #-}
#endif
modifyTVar_ :: TVar a -> (a -> a) -> STM ()
modifyTVar_ v f = readTVar v >>= \a -> writeTVar v $! f a
modError :: String -> String -> a
modError func msg =
error $ "Data.Pool." ++ func ++ ": " ++ msg
Jump to Line
Something went wrong with that request. Please try again.