Implement timer coalescing (noticeably less CPU/power usage)

xmobar currently runs every monitor in its own thread. Monitors that do
periodic updates simply sleep and loop. This unfortunately leads to
these threads coming out of sync, and xmobar ends up waking up and
redrawing for every periodic monitor. In my case, that is 7 times per
second, which is enough for xmobar to be at the top of "top" with more
than 1% CPU usage, and to have a noticeable impact on battery life.

This commit adds a central timer coordination thread which makes sure
that periodic updates happen together and that we only redraw once
they're all done.

Together with PR #409, I managed to lower the idle power draw of my
laptop from 4W to 3W.

1 files changed, 130 insertions, 0 deletions

diff --git a/src/Xmobar/App/Timer.hs b/src/Xmobar/App/Timer.hs
new file mode 100644
index 0000000..d67695d
--- /dev/null
+++ b/src/Xmobar/App/Timer.hs
@@ -0,0 +1,130 @@
+------------------------------------------------------------------------------
+-- |
+-- Module: Xmobar.App.Timer
+-- Copyright: (c) 2019 Tomáš Janoušek
+-- License: BSD3-style (see LICENSE)
+--
+-- Maintainer: Tomáš Janoušek <tomi@nomi.cz>
+-- Stability: unstable
+--
+-- Timer coalescing for recurring actions.
+--
+------------------------------------------------------------------------------
+
+module Xmobar.App.Timer (doEveryTenthSeconds, withTimer) where
+
+import Control.Concurrent.Async (withAsync)
+import Control.Concurrent.STM
+import Control.Exception (bracket, bracket_)
+import Control.Monad (forever, forM, forM_, guard)
+import Data.IORef
+import Data.Int (Int64)
+import Data.Map (Map)
+import qualified Data.Map as M
+import Data.Time.Clock.POSIX (getPOSIXTime)
+import Data.Unique
+import System.IO.Unsafe (unsafePerformIO)
+
+newtype Timer = Timer (TVar Periods)
+
+type Periods = Map Unique Period
+
+data Period = Period { rate :: Int64, next :: Int64, tick :: TMVar (TMVar ()) }
+
+{-# NOINLINE timer #-}
+timer :: IORef (Maybe Timer)
+timer = unsafePerformIO (newIORef Nothing)
+
+now :: IO Int64
+now = do
+    posix <- getPOSIXTime
+    return $ floor (10 * posix)
+
+newPeriod :: Int64 -> IO (Unique, Period)
+newPeriod r = do
+    u <- newUnique
+    t <- now
+    v <- atomically newEmptyTMVar
+    let t' = t - t `mod` r
+    return (u, Period { rate = r, next = t', tick = v })
+
+-- | Perform a given action every N tenths of a second.
+--
+-- The timer is aligned with other timers to minimize the number of wakeups
+-- and unnecessary redraws.
+doEveryTenthSeconds :: Int -> IO () -> IO ()
+doEveryTenthSeconds r action = do
+    Just t <- readIORef timer
+    doEveryTenthSeconds' t r action
+
+doEveryTenthSeconds' :: Timer -> Int -> IO () -> IO ()
+doEveryTenthSeconds' (Timer periodsVar) r action = do
+    (u, p) <- newPeriod (fromIntegral r)
+    bracket_ (push u p) (pop u) $ forever $
+        bracket (atomically $ takeTMVar $ tick p)
+                (\doneVar -> atomically $ putTMVar doneVar ())
+                (const action)
+    where
+        push u p = atomically $ modifyTVar periodsVar (M.insert u p)
+        pop u = atomically $ modifyTVar periodsVar (M.delete u)
+
+-- | Start the timer coordination thread.
+withTimer :: (IO () -> IO ()) -> IO a -> IO a
+withTimer pauseRefresh action = do
+    periodsVar <- atomically $ newTVar M.empty
+    withAsync (timerLoop pauseRefresh periodsVar) $ \_ ->
+        bracket_
+            (writeIORef timer (Just (Timer periodsVar)))
+            (writeIORef timer Nothing) -- TODO: kill all periods?
+            action
+
+timerLoop :: (IO () -> IO ()) -> TVar Periods -> IO ()
+timerLoop pauseRefresh periodsVar = forever $ do
+    t <- now
+    toFire <- atomically $ do
+        periods <- readTVar periodsVar
+        writeTVar periodsVar (advanceTimers t periods)
+        return (timersToFire t periods)
+    pauseRefresh $ do
+        -- Fire timers ...
+        doneVars <- atomically $ forM toFire $ \p -> do
+            doneVar <- newEmptyTMVar
+            putTMVar (tick p) doneVar
+            return doneVar
+        -- ... and wait for them to avoid unnecessary redraws.
+        atomically $ forM_ doneVars takeTMVar
+    delayUntilNextFire periodsVar
+
+advanceTimers :: Int64 -> Periods -> Periods
+advanceTimers t = M.map advance
+    where
+        advance p | next p <= t = p { next = t - t `mod` rate p + rate p }
+                  | otherwise = p
+
+timersToFire :: Int64 -> Periods -> [Period]
+timersToFire t periods = [ p | p <- M.elems periods, next p <= t ]
+
+nextFireTime :: Periods -> Maybe Int64
+nextFireTime periods
+    | M.null periods = Nothing
+    | otherwise = Just $ minimum [ next p | p <- M.elems periods ]
+
+delayUntilNextFire :: TVar Periods -> IO ()
+delayUntilNextFire periodsVar = do
+    tMaybeNext <- fmap nextFireTime $ readTVarIO periodsVar
+    tNow <- now
+    delayVar <- case tMaybeNext of
+        Just tNext -> do
+            -- Work around the Int max bound: threadDelay takes an Int, we can
+            -- only sleep for so long, which is okay, we'll just check timers
+            -- sooner and sleep again.
+            let maxDelay = (maxBound :: Int) `div` 100000
+                delay = (tNext - tNow) `min` fromIntegral maxDelay
+                delayUsec = fromIntegral delay * 100000
+            registerDelay delayUsec
+        Nothing -> atomically $ newTVar False
+    atomically $ do
+        delayOver <- readTVar delayVar
+        tMaybeNext' <- fmap nextFireTime $ readTVar periodsVar
+        -- Return also if a new period is added (it may fire sooner).
+        guard $ delayOver || tMaybeNext /= tMaybeNext'