# Synchronized threads, part II

For comparison, here is an implementation of multiple threads of which each attempt to perform as many steps as possible in 1 second.

```
> import Control.Monad
> import Control.Concurrent
> import Control.Concurrent.STM
> import Data.List
> import Data.IORef
> import System.Time
> import System.Environment
> import System.IO
> import System.Random
> import Text.Printf
> import Ratio
```

oneThread greedily attempts to loop through as many steps as possible
until one second has elapsed. Then it blocks while it waits for
the main thread to collect the previous result, so it can put
the new result in the TMVar. Every step it takes, it executes
the supplied function parameter `f`

.

```
> oneThread :: TMVar Int -> Int -> a -> (a -> a) -> IO ()
> oneThread mvar n v f = do
> TOD s ps <- getClockTime
> loop (fromIntegral s + ps%(10^12)) n n v
> where
> loop prevTime prevN n v = do
> TOD s ps <- getClockTime
> let now = fromIntegral s + ps%(10^12)
> tdiff = now - prevTime
> ndiff = fromIntegral $ n - prevN
> sps = floor (ndiff / tdiff)
> v' = f v
> if tdiff >= 1 then
> do atomically $ putTMVar mvar sps
> loop now n n v
> else v' `seq` loop prevTime prevN (n + 1) v'
```

nosync is akin to sync in that it is an STM action which collects
results from all the threads via the TMVars. Again, the key
portion is easy: `mapM takeTMVar mvars`

.

```
> nosync :: (Num a, Ord a) => [TMVar a] -> STM (a, a)
> nosync mvars = do
> vals <- mapM takeTMVar mvars
> return $ (maximum vals, sum vals)
```

```
> initialize :: Int -> a -> (a -> a) -> IO ([ThreadId], [TMVar Int])
> initialize k v f = do
> mvars <- atomically (forM [1..k]
> (\_ -> newEmptyTMVar))
> thds <- forM (zip mvars [1..k])
> (\(ch, n) -> forkIO (oneThread ch 0 v f))
> return (thds, mvars)
```

nosyncLoop waits for all the threads to place a value into their TMVar, which will happen after one second.

```
> nosyncLoop :: [TMVar Int] -> IO ()
> nosyncLoop mvars = do
> (best, sum) <- atomically $ nosync mvars
> printf "Best steps / second = %d; Sum steps / second = %d\n" best sum
> hFlush stdout
> nosyncLoop mvars
```

A computational time-waster to simulate "real work".

```
> computation l = let (v:l') = l
> in fact v `seq` l'
>
> fact n = product [1..n]
```

```
> main :: IO ()
> main = do
> args <- getArgs
> let n = case args of
> [] -> 10
> a:_ -> read a
> g <- newStdGen
> (_,mvars) <- initialize n (randomRs (500,600) g) computation
> nosyncLoop mvars
```

System is a 4-way Xeon 3.6GHz.

```
[mrd@system ~]$ ghc --make -O2 -threaded Unsync.lhs
```

[mrd@system ~]$ ./Unsync 1 +RTS -N1
Best steps / second = 3179; Sum steps / second = 3179
Best steps / second = 3181; Sum steps / second = 3181
Best steps / second = 3178; Sum steps / second = 3178
Best steps / second = 3175; Sum steps / second = 3175
Best steps / second = 3174; Sum steps / second = 3174
[mrd@system ~]$ ./Unsync 1 +RTS -N2
Best steps / second = 3142; Sum steps / second = 3142
Best steps / second = 3168; Sum steps / second = 3168
Best steps / second = 3174; Sum steps / second = 3174
Best steps / second = 3177; Sum steps / second = 3177
Best steps / second = 3172; Sum steps / second = 3172

[mrd@system ~]$ ./Unsync 5 +RTS -N1
Best steps / second = 635; Sum steps / second = 3071
Best steps / second = 638; Sum steps / second = 3094
Best steps / second = 668; Sum steps / second = 3080
Best steps / second = 669; Sum steps / second = 3184
Best steps / second = 751; Sum steps / second = 3181
[mrd@system ~]$ ./Unsync 5 +RTS -N2
Best steps / second = 1429; Sum steps / second = 5601
Best steps / second = 1434; Sum steps / second = 5647
Best steps / second = 1446; Sum steps / second = 5647
Best steps / second = 1413; Sum steps / second = 5647
Best steps / second = 1502; Sum steps / second = 5639
[mrd@system ~]$ ./Unsync 5 +RTS -N3
Best steps / second = 1912; Sum steps / second = 5792
Best steps / second = 2092; Sum steps / second = 5934
Best steps / second = 2107; Sum steps / second = 5938
Best steps / second = 1959; Sum steps / second = 5922
Best steps / second = 2068; Sum steps / second = 5960
[mrd@system ~]$ ./Unsync 5 +RTS -N4
Best steps / second = 1876; Sum steps / second = 7428
Best steps / second = 1865; Sum steps / second = 7402
Best steps / second = 1891; Sum steps / second = 7420
Best steps / second = 1895; Sum steps / second = 7581
Best steps / second = 1899; Sum steps / second = 7602

[mrd@system ~]$ ./Unsync 10 +RTS -N1
Best steps / second = 334; Sum steps / second = 2852
Best steps / second = 332; Sum steps / second = 3100
Best steps / second = 334; Sum steps / second = 3082
Best steps / second = 335; Sum steps / second = 3176
Best steps / second = 335; Sum steps / second = 3186
[mrd@system ~]$ ./Unsync 10 +RTS -N2
Best steps / second = 594; Sum steps / second = 5577
Best steps / second = 669; Sum steps / second = 5631
Best steps / second = 588; Sum steps / second = 5641
Best steps / second = 622; Sum steps / second = 5657
Best steps / second = 604; Sum steps / second = 5639
[mrd@system ~]$ ./Unsync 10 +RTS -N3
Best steps / second = 702; Sum steps / second = 5846
Best steps / second = 692; Sum steps / second = 5865
Best steps / second = 717; Sum steps / second = 5884
Best steps / second = 679; Sum steps / second = 5893
Best steps / second = 745; Sum steps / second = 5913
[mrd@system ~]$ ./Unsync 10 +RTS -N4
Best steps / second = 949; Sum steps / second = 7133
Best steps / second = 958; Sum steps / second = 7198
Best steps / second = 989; Sum steps / second = 7189
Best steps / second = 906; Sum steps / second = 7155
Best steps / second = 964; Sum steps / second = 7181

### Observations

Number of steps is proportional to number of processors, and inversely proportional to number of threads.

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