Optimize Date plugin again (refresh timezone only once a minute)

This makes the Date plugin approximately twice as fast, and makes xmobar
up to about 5–10 % faster if Date is the only active plugin. (If more
expensive plugins like Network or MultiCpu are used, it doesn't make any
measurable difference.)

Micro-benchmark results on my HW:

    Date Benchmarks/Date                     mean 2.833 μs  ( +- 16.08 ns  )
    Date Benchmarks/DateZonedTime            mean 5.020 μs  ( +- 32.91 ns  )
    Date Benchmarks/DateWithTimeZone         mean 2.827 μs  ( +- 20.52 ns  )

(DateZonedTime is the original implementation and DateWithTimeZone is
the implementation we had since 0.34 which never refreshes timezone.)

Real-life measurements (done overnight on an idle laptop, with all
measured xmobars running in parallel to ensure comparable conditions;
xmobars configured to only display date and with rate 10 — once per
second):

    $ time timeout 6h xmobar .xmobarrc-DateZonedTime
    real    360m0,010s
    user    0m9,867s
    sys     0m4,644s

    (9.867 + 4.644) / (360 * 60) = 0.000672

    $ time timeout 6h xmobar .xmobarrc-Date
    real    360m0,008s
    user    0m9,535s
    sys     0m4,327s

    (9.535 + 4.327) / (360 * 60) = 0.000642

    $ time timeout 6h xmobar .xmobarrc-Date-10m
    real    360m0,010s
    user    0m9,780s
    sys     0m4,215s

    (9.780 + 4.215) / (360 * 60) = 0.000648

    $ time timeout 6h xmobar .xmobarrc-DateWithTimeZone
    real    360m0,006s
    user    0m9,658s
    sys     0m4,166s

    (9.658 + 4.166) / (360 * 60) = 0.000640

(.xmobarrc-Date-10m is the proposed implementation, but with timezone
refresh every 10 minutes instead of every 1 minute)

Interpretation of these results:

 * refreshing xmobar with just date takes around 650 μs
 * that is xmobar with just date uses around 0.065 % of CPU time
 * refreshing timezone takes additional cca 30 μs

When we only refresh timezone once a minute, these 30 μs become 0.5 μs
amortized, and that should be acceptable to even the most dedicated
perfectionist :-)

Fixes: a58e32f7c8af ("Revert "Optimize date plugin"")
Fixes: 878db3908060 ("Optimize date plugin")
Co-authored-by: Sibi Prabakaran <sibi@psibi.in>

1 files changed, 17 insertions, 4 deletions

diff --git a/src/Xmobar/Plugins/Date.hs b/src/Xmobar/Plugins/Date.hs
index 1cb0596..d7de2e9 100644
--- a/src/Xmobar/Plugins/Date.hs
+++ b/src/Xmobar/Plugins/Date.hs
@@ -17,22 +17,35 @@
 --
 -----------------------------------------------------------------------------
 
-module Xmobar.Plugins.Date (Date(..)) where
+module Xmobar.Plugins.Date (Date(..), date) where
 
 import Xmobar.Run.Exec
 
 #if ! MIN_VERSION_time(1,5,0)
 import System.Locale
 #endif
+import Data.IORef
 import Data.Time
+import Control.Concurrent.Async (concurrently_)
 
 data Date = Date String String Int
     deriving (Read, Show)
 
 instance Exec Date where
     alias (Date _ a _) = a
-    run   (Date f _ _) = date f
     rate  (Date _ _ r) = r
+    start (Date f _ r) cb =
+        -- refresh time zone once a minute to avoid wasting CPU cycles
+        withRefreshingZone 600 $ \zone ->
+            doEveryTenthSeconds r $ date zone f >>= cb
 
-date :: String -> IO String
-date format = fmap (formatTime defaultTimeLocale format) getZonedTime
+date :: IORef TimeZone -> String -> IO String
+date zoneRef format = do
+    zone <- readIORef zoneRef
+    fmap (formatTime defaultTimeLocale format . utcToZonedTime zone) getCurrentTime
+
+withRefreshingZone :: Int -> (IORef TimeZone -> IO ()) -> IO ()
+withRefreshingZone r action = do
+    zone <- newIORef =<< getCurrentTimeZone
+    let refresh = atomicWriteIORef zone =<< getCurrentTimeZone
+    concurrently_ (doEveryTenthSeconds r refresh) (action zone)