diff --git a/src/Development/Shake/Internal/Core/Build.hs b/src/Development/Shake/Internal/Core/Build.hs
index d25ee8153..6e812121c 100644
--- a/src/Development/Shake/Internal/Core/Build.hs
+++ b/src/Development/Shake/Internal/Core/Build.hs
@@ -102,7 +102,10 @@ buildOne global@Global{..} stack database i k r = case addStack i k stack of
     Right stack -> Later $ \continue -> do
         setIdKeyStatus global database i k (Running (NoShow continue) r)
         let go = buildRunMode global stack database r
-        fromLater go $ \mode -> liftIO $ addPool PoolStart globalPool $
+            priority = case r of
+              Nothing -> PoolStart
+              Just (execution -> t) -> PoolEstimate t (show k)
+        fromLater go $ \mode -> liftIO $ addPool priority globalPool $
             runKey global stack k r mode $ \res -> do
                 runLocked database $ do
                     let val = fmap runValue res
diff --git a/src/General/Pool.hs b/src/General/Pool.hs
index 6fe23a984..803674b7d 100644
--- a/src/General/Pool.hs
+++ b/src/General/Pool.hs
@@ -28,6 +28,8 @@ import qualified Data.Heap as Heap
 import qualified Data.HashSet as Set
 import Data.IORef.Extra
 import System.Random
+import Debug.Trace
+import GHC.Conc
 
 
 ---------------------------------------------------------------------
@@ -41,22 +43,15 @@ If any worker throws an exception, must signal to all the other workers
 data S = S
     {alive :: !Bool -- True until there's an exception, after which don't spawn more tasks
     ,threads :: !(Set.HashSet Thread) -- IMPORTANT: Must be strict or we leak thread stacks
-    ,threadsLimit :: {-# UNPACK #-} !Int -- user supplied thread limit, Set.size threads <= threadsLimit
     ,threadsCount :: {-# UNPACK #-} !Int -- Set.size threads, but in O(1)
     ,threadsMax :: {-# UNPACK #-} !Int -- high water mark of Set.size threads (accounting only)
     ,threadsSum :: {-# UNPACK #-} !Int -- number of threads we have been through (accounting only)
-    ,rand :: IO Int -- operation to give us the next random Int
-    ,todo :: !(Heap.Heap (Heap.Entry (PoolPriority, Int) (IO ()))) -- operations waiting a thread
     }
 
 
 emptyS :: Int -> Bool -> IO S
-emptyS n deterministic = do
-    rand <- if not deterministic then pure randomIO else do
-        ref <- newIORef 0
-        -- no need to be thread-safe - if two threads race they were basically the same time anyway
-        pure $ do i <- readIORef ref; writeIORef' ref (i+1); pure i
-    pure $ S True Set.empty n 0 0 0 rand Heap.empty
+emptyS n deterministic =
+    pure $ S True Set.empty 0 0 0
 
 
 data Pool = Pool
@@ -71,46 +66,37 @@ withPool (Pool var _) f = join $ modifyVar var $ \s ->
 withPool_ :: Pool -> (S -> IO S) -> IO ()
 withPool_ pool act = withPool pool $ fmap (, pure()) . act
 
-
-worker :: Pool -> IO ()
-worker pool = withPool pool $ \s -> pure $ case Heap.uncons $ todo s of
-    Nothing -> (s, pure ())
-    Just (Heap.Entry _ now, todo2) -> (s{todo = todo2}, now >> worker pool)
-
--- | Given a pool, and a function that breaks the S invariants, restore them.
---   They are only allowed to touch threadsLimit or todo.
---   Assumes only requires spawning a most one job (e.g. can't increase the pool by more than one at a time)
-step :: Pool -> (S -> IO S) -> IO ()
--- mask_ is so we don't spawn and not record it
-step pool@(Pool _ done) op = mask_ $ withPool_ pool $ \s -> do
-    s <- op s
-    case Heap.uncons $ todo s of
-        Just (Heap.Entry _ now, todo2) | threadsCount s < threadsLimit s -> do
-            -- spawn a new worker
-            t <- newThreadFinally (now >> worker pool) $ \t res -> case res of
-                Left e -> withPool_ pool $ \s -> do
-                    signalBarrier done $ Left e
-                    pure (remThread t s){alive = False}
-                Right _ ->
-                    step pool $ pure . remThread t
-            pure (addThread t s){todo = todo2}
-        Nothing | threadsCount s == 0 -> do
-            signalBarrier done $ Right s
-            pure s{alive = False}
-        _ -> pure s
-    where
-        addThread t s = s{threads = Set.insert t $ threads s, threadsCount = threadsCount s + 1
-                         ,threadsSum = threadsSum s + 1, threadsMax = threadsMax s `max` (threadsCount s + 1)}
-        remThread t s = s{threads = Set.delete t $ threads s, threadsCount = threadsCount s - 1}
-
+threshold :: Float
+threshold = 0.05
 
 -- | Add a new task to the pool. See the top of the module for the relative ordering
 --   and semantics.
 addPool :: PoolPriority -> Pool -> IO a -> IO ()
-addPool priority pool act = step pool $ \s -> do
-    i <- rand s
-    pure s{todo = Heap.insert (Heap.Entry (priority, i) $ void act) $ todo s}
-
+addPool priority pool@(Pool _ done) act = 
+    withPool_ pool $ \s -> do
+      traceEventIO $ "Scheduling event with priority: " ++ show priority
+      t <- newThreadFinally l mcap act $ \t res -> do
+        traceEventIO $ show l ++ " done."
+        case res of
+          Left e -> withPool_ pool $ \s -> do
+            signalBarrier done $ Left e
+            pure (remThread t s){alive = False}
+          Right _ -> withPool_ pool $ \s -> do
+            let s' = remThread t s
+            when (threadsCount s' == 0) $
+              signalBarrier done $ Right s'{alive = False}
+            pure $ s'{alive = threadsCount s' /= 0}
+      pure (addThread t s)
+  where
+      addThread t s = s{threads = Set.insert t $ threads s, threadsCount = threadsCount s + 1
+                       ,threadsSum = threadsSum s + 1, threadsMax = threadsMax s `max` (threadsCount s + 1)}
+      remThread t s = s{threads = Set.delete t $ threads s, threadsCount = threadsCount s - 1}
+      mcap = case priority of
+        PoolEstimate t _ | t <= threshold -> Just 0
+        _ -> Nothing
+      l = case priority of
+        PoolEstimate _ s -> s
+        _ -> "Unknown"
 
 data PoolPriority
     = PoolException
@@ -118,15 +104,13 @@ data PoolPriority
     | PoolStart
     | PoolBatch
     | PoolDeprioritize Double
-      deriving (Eq,Ord)
+    | PoolEstimate { estimatedTime :: Float, label :: String }
+      deriving (Eq,Ord,Show)
 
 -- | Temporarily increase the pool by 1 thread. Call the cleanup action to restore the value.
 --   After calling cleanup you should requeue onto a new thread.
 increasePool :: Pool -> IO (IO ())
-increasePool pool = do
-    step pool $ \s -> pure s{threadsLimit = threadsLimit s + 1}
-    pure $ step pool $ \s -> pure s{threadsLimit = threadsLimit s - 1}
-
+increasePool pool = pure (pure ())
 
 -- | Make sure the pool cannot run out of tasks (and thus everything finishes) until after the cancel is called.
 --   Ensures that a pool that will requeue in time doesn't go idle.
@@ -139,7 +123,6 @@ keepAlivePool pool = do
         cancel
     pure $ signalBarrier bar ()
 
-
 -- | Run all the tasks in the pool on the given number of works.
 --   If any thread throws an exception, the exception will be reraised.
 runPool :: Bool -> Int -> (Pool -> IO ()) -> IO () -- run all tasks in the pool
diff --git a/src/General/Thread.hs b/src/General/Thread.hs
index bd5b32757..0e67c4ff0 100644
--- a/src/General/Thread.hs
+++ b/src/General/Thread.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE RankNTypes          #-}
 
 -- | A bit like 'Fence', but not thread safe and optimised for avoiding taking the fence
 module General.Thread(
@@ -14,6 +15,7 @@ import Control.Concurrent.Extra
 import Control.Exception
 import General.Extra
 import Control.Monad.Extra
+import GHC.Conc
 
 
 data Thread = Thread ThreadId (Barrier ())
@@ -25,15 +27,21 @@ instance Hashable Thread where
     hashWithSalt salt (Thread a _) = hashWithSalt salt a
 
 -- | The inner thread is unmasked even if you started masked.
-newThreadFinally :: IO a -> (Thread -> Either SomeException a -> IO ()) -> IO Thread
-newThreadFinally act cleanup = do
+newThreadFinally :: String -> Maybe Int -> IO a -> (Thread -> Either SomeException a -> IO ()) -> IO Thread
+newThreadFinally label mcap act cleanup = do
     bar <- newBarrier
-    t <- mask_ $ forkIOWithUnmask $ \unmask -> flip finally (signalBarrier bar ()) $ do
-        res <- try $ unmask act
+    t <- mask_ $ fork $ \unmask -> flip finally (signalBarrier bar ()) $ do
         me <- myThreadId
+        res <- try $ unmask act
         cleanup (Thread me bar) res
+    labelThread t $ label ++ labeltype
     pure $ Thread t bar
-
+  where
+    labeltype = maybe "(Free)" (\i -> "(Restricted to "++show i++")") mcap
+    fork :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
+    fork = case mcap of
+      Nothing -> forkIOWithUnmask
+      Just n -> forkOnWithUnmask n
 
 stopThreads :: [Thread] -> IO ()
 stopThreads threads = do