{-| Implemented here is a thread pool library on crack.
1.0 Introduction
Typically, a thread pool is a set of execution contexts that will execute
tasks from an input queue. Typically, thread pools are used to parallize
the processing of incoming work across all available CPUs without going
through the expense of starting a new thread for every new task.
In 'Control.Engine' you will find a somewhat unique implementation. The
'Engine' is not only a set of threads running a common mutator on the input
queue, producing an output queue, but also include hooks, task injection, and
state management.
Queues :: (Chan a)
From "Control.Concurrent.Chan".
Hooks :: (a -> IO Maybe a)
Hooks can be added and removed during execution without creating a new
engine. They allow the developer to modify tasks:
* prior to parallization (for sequential preprocessing)
* in parallel, prior to main mutation funciton
* in parallel, after mutation function
* post parallization (for sequential post processing)
State Management
The stateManager waits for any updates to the mutator state or hooks. If any
modifications are made then the new set of hooks (or state) is provided
to the workers. Correctness is handled by keeping the master copies as TVars
("Control.Concurrent.STM"). While the mutators and hooks read from an 'MVar'
("Control.Concurrent.MVar") to avoid contention.
The thinking here is that changing the hooks and state is a rare / low
contention action while the need for this information will be constant
and performance critical. How successful this stratagy is has yet to
be shown.
Injection
One injection point allows injection of a result that had no preceding
task. The second injector allows the initial hooks ('Input' hooks) to be
bypassed.
-}
module Control.Engine
(
-- * Main functions
initSimpleEngine
, initSimpleEngineIO
, initEngine
, Engine(..)
-- * Hooks
, Hook(..)
, HookLoc(..)
, addInputHook
, addOutputHook
, addPreMutateHook
, addPostMutateHook
, delInputHook
, delOutputHook
, delPreMutateHook
, delPostMutateHook
-- * Injectors
, injectPreMutator
, injectPostMutator
) where
import Control.Concurrent
import Control.Concurrent.MVar
import Control.Concurrent.STM
import Control.Concurrent.Chan
import Control.Monad
import Data.List (insert)
-- |An 'Engine' represents a pool of threads ready to execute tasks.
data Engine job result state =
Eng { chan1 :: Chan job
, chan2 :: Chan result
, tvInHook :: TVar [Hook state job]
, tvPreMutateHook :: TVar [Hook state job]
, tvPostMutateHook :: TVar [Hook state result]
, tvOutHook :: TVar [Hook state result]
, mvInHook :: MVar [Hook state job]
, mvPreMutateHook :: MVar [Hook state job]
, mvPostMutateHook :: MVar [Hook state result]
, mvOutHook :: MVar [Hook state result]
, state :: TVar state
}
-- |If all you want is a basic thread pool, this will work.
-- You should consider using "Control.ThreadPool" instead.
--
-- Evaluation of the result is forced using seq.
initSimpleEngine :: Int -> (job -> result) -> IO (Chan job, Chan result)
initSimpleEngine nr mutator = do
input <- newChan
output <- newChan
let m = const (return . Just . mutator)
initEngine nr (readChan input) (\o -> o `seq` writeChan output o) m ()
return (input, output)
-- |Simpler than calling 'initEngine', but it allows no state or interaction
-- with the hooks and injectors. No strictness is forced.
initSimpleEngineIO :: Int -> (job -> IO result) -> IO (Chan job, Chan result)
initSimpleEngineIO nr mutator = do
input <- newChan
output <- newChan
let m = (\_ j -> mutator j >>= return . Just)
initEngine nr (readChan input) (writeChan output) m ()
return (input, output)
-- |To initilize an engine you must provide:
--
-- * the number of threads
--
-- * an action that will get the input
--
-- * an action that will consume output
--
-- * a mutator function to perform on all inputs
--
-- * an initial state for the mutator function
--
-- No strictness is forced - be sure you force evaluation if wanted.
-- All hooks start out empty.
initEngine :: (Eq st) => Int -> (IO job) -> (result -> IO ()) -> (st -> job -> IO (Maybe result)) -> st -> IO (Engine job result st)
initEngine nrWorkers input output mutator initialState = do
c1 <- newChan
c2 <- newChan
inputHooks <- newMVar []
outputHooks <- newMVar []
preMutatorHooks <- newMVar []
postMutatorHooks <- newMVar []
ms <- newMVar initialState
tv <- newTVarIO initialState
ch1tv <- newTVarIO []
ch2tv <- newTVarIO []
ch3tv <- newTVarIO []
ch4tv <- newTVarIO []
let engine = Eng c1 c2 ch1tv ch2tv ch3tv ch4tv inputHooks preMutatorHooks postMutatorHooks outputHooks tv
forkIO $ inputManager input c1 inputHooks ms
forkIO $ outputManager output c2 outputHooks ms
forkIO $ stateManager engine ms
forM_ [1..nrWorkers] $ \_ -> forkIO $ worker c1 preMutatorHooks ms mutator postMutatorHooks c2
return engine
worker :: Chan job -> MVar [Hook st job] -> MVar st -> (st -> job -> IO (Maybe result)) -> MVar [Hook st result] -> Chan result -> IO ()
worker c1 preMutatorHooks ms mutator postMutatorHooks c2 = forever $ worker'
where
worker' = do
-- Get next message, newest hooks, and state
msg <- readChan c1
preMH <- readMVar preMutatorHooks
postMH <- readMVar postMutatorHooks
st <- readMVar ms
-- run hook1, mutator, hook2
msg' <- runHooks preMH st msg
out <- runStage (mutator st) msg'
out' <- runStage (runHooks postMH st) out
case out' of
Nothing -> return ()
Just o -> writeChan c2 o
runStage :: (a -> IO (Maybe b)) -> Maybe a -> IO (Maybe b)
runStage _ Nothing = return Nothing
runStage stage (Just a) = stage a
stateManager :: (Eq st) => Engine job result st -> MVar st -> IO ()
stateManager eng ms = do
curr <- atomically $ do -- FIXME, clean up this redundant code - its ugly!
s <- readTVar (state eng)
ih <- readTVar (tvInHook eng)
eh <- readTVar (tvPreMutateHook eng)
th <- readTVar (tvPostMutateHook eng)
oh <- readTVar (tvOutHook eng)
return (s,ih, eh, th, oh)
updateState curr
where
updateState (s, ih, eh, th, oh) = do
new@(s', ih', eh', th', oh') <- atomically $ do
s' <- readTVar (state eng)
ih' <- readTVar (tvInHook eng)
eh' <- readTVar (tvPreMutateHook eng)
th' <- readTVar (tvPostMutateHook eng)
oh' <- readTVar (tvOutHook eng)
when (not $ s' == s && ih' == ih && eh' == eh && th' == th && oh' == oh) retry
return (s', ih', eh', th', oh')
when (s' /= s) (swapMVar ms s' >> return ())
when (ih' /= ih) (swapMVar (mvInHook eng) ih' >> return ())
when (eh' /= eh) (swapMVar (mvPreMutateHook eng) eh' >> return ())
when (th' /= th) (swapMVar (mvPostMutateHook eng) th' >> return ())
when (oh' /= oh) (swapMVar (mvOutHook eng) oh' >> return ())
updateState new
-- Input.hs
inputManager :: (IO msg) -> Chan msg -> MVar [Hook st msg] -> MVar st -> IO ()
inputManager input outChan hookMV stMV= forever $ input >>= handleMsg
where
handleMsg msg = do
hook <- readMVar hookMV
s <- readMVar stMV
new <- runHooks hook s msg
case new of
Just m -> writeChan outChan m
Nothing -> return ()
-- Output.hs
outputManager :: (result -> IO ()) -> Chan result -> MVar [Hook st result] -> MVar st -> IO ()
outputManager output msgChan hookMV stMV = forever $ do
m <- readChan msgChan
hook <- readMVar hookMV
s <- readMVar stMV
new <- runHooks hook s m
case new of
Just n -> output n
Nothing -> return ()
-- Hooks.hs
-- |A hook is simply a mutation on the task. The priority is used to order
-- hook execution (lower value priorites happen first). For accounting and to
-- remove old hooks the description field is used.
data Hook st msg = Hk
{ hkFunc :: st -> msg -> IO (Maybe msg)
, hkPriority :: Int
, hkDescription :: String
}
instance Eq (Hook m s) where
(Hk _ p d) == (Hk _ p' d') = p == p' && d == d'
instance Ord (Hook a s) where
(Hk _ p _) `compare` (Hk _ p' _) = p `compare` p'
instance Show (Hook a s) where
show (Hk _ p d) = d ++ " Priority = " ++ (show p)
showsPrec _ (Hk _ p d) = (++) ("Hk { hkFunc = undefined, p = " ++ (show p) ++ " , hkDescription = " ++ d ++ " } ")
data HookLoc = InputHook | PreMutateHook | PostMutateHook | OutputHook deriving (Eq, Ord, Show)
runHooks :: [Hook st msg] -> st -> msg -> IO (Maybe msg)
runHooks hooks st m = foldM apply (Just m) hooks
where
apply Nothing f = return Nothing
apply (Just a) f = (hkFunc f st) a
-- |Adds a hook that will be performed in serial on all jobs added to
-- the input queue.
addInputHook :: Engine job result state -> Hook state job -> IO ()
addInputHook e h = atomically $ do
readTVar (tvInHook e) >>= writeTVar (tvInHook e) . insert h
-- |Adds a hook that will be performed in serial on all results
-- before they are added to the output queue.
addOutputHook :: Engine job result state -> Hook state result -> IO ()
addOutputHook e h = atomically $ do
readTVar (tvOutHook e) >>= writeTVar (tvOutHook e) . insert h
-- |Adds a hook that will be performed in parallel before the main mutator
-- function.
addPreMutateHook :: Engine job result state -> Hook state job -> IO ()
addPreMutateHook e h = atomically $ do
readTVar (tvPreMutateHook e) >>= writeTVar (tvPreMutateHook e) . insert h
-- |Adds a hook that will be performed in parallel after the main mutator
-- function.
addPostMutateHook :: Engine job result state -> Hook state result -> IO ()
addPostMutateHook e h = atomically $ do
readTVar (tvPostMutateHook e) >>= writeTVar (tvPostMutateHook e) . insert h
-- |Deletes all input hooks matching the provided desciption
delInputHook :: Engine j r s -> String -> IO ()
delInputHook e s = atomically $ do
readTVar (tvInHook e) >>= writeTVar (tvInHook e) . filter ( (/= s) . hkDescription)
-- |Deletes all pre-mutate hooks matching the provided desciption
delPreMutateHook :: Engine j r s -> String -> IO ()
delPreMutateHook e s = atomically $ do
readTVar (tvPreMutateHook e) >>= writeTVar (tvPreMutateHook e) . filter ( (/= s) . hkDescription)
-- |Deletes all post-mutate hooks matching the provided desciption
delPostMutateHook :: Engine j r s -> String -> IO ()
delPostMutateHook e s = atomically $ do
readTVar (tvPostMutateHook e) >>= writeTVar (tvPostMutateHook e) . filter ( (/= s) . hkDescription)
-- |Deletes all output hooks matching the provided desciption
delOutputHook :: Engine j r s -> String -> IO ()
delOutputHook e s = atomically $ do
readTVar (tvOutHook e) >>= writeTVar (tvOutHook e) . filter ( (/= s) . hkDescription)
-- Inject.hs
-- | Allows adding tasks that bypass the input hooks.
injectPreMutator :: Engine j r s -> j -> IO ()
injectPreMutator eng i = writeChan (chan1 eng) i
-- | Allows bypassing the mutator, meaning a 'result' can be produced without a task.
-- This still hits the output hooks.
injectPostMutator :: Engine j r s -> r -> IO ()
injectPostMutator eng o = writeChan (chan2 eng) o