On Sun, Jan 24, 2016 at 12:46 AM, Thomas Koster
Using Criterion, I have been running benchmarks to measure the relative performance of STM and MVars for some simple transactions that I expect will be typical in my application. I am using GHC 7.10.2 and libraries as at Stackage LTS 3.2.
I have found that STM is faster than MVars in all my benchmarks, without exception. This seems to go against accepted wisdom [1][2][3]. I have not included my source code here to save space, but if you suspect that I am using MVars incorrectly, just say so and I will post my source code separately.
I have two questions:
1. When are MVars faster than STM? If the answer is "never", then when are MVars "better" than STM? (Choose your own definition of "better".)
2. When given two capabilities (+RTS -N2), MVars are suddenly an order of magnitude slower than with just one capability. Why?
On 24 January 2016 at 17:55, Christopher Allen
Could you post the code please?
module Main (main) where import Control.Concurrent.Async import Control.Concurrent.MVar import Control.Concurrent.STM import Control.Monad import Criterion.Main main = defaultMain [ bgroup "thrash" [ bench "MVar" $ whnfIO (thrashTest mvarNew mvarInc mvarGet), bench "TVar" $ whnfIO (thrashTest tvarNew tvarInc tvarGet) ] ] thrashTest :: IO a -> (a -> IO ()) -> (a -> IO b) -> IO b thrashTest new inc get = do var <- new threads <- replicateM 4 (async (replicateM_ 100000 $ inc var)) forM_ threads wait get var mvarNew :: IO (MVar Int) mvarNew = newMVar 0 mvarInc :: MVar Int -> IO () mvarInc var = modifyMVar_ var $ \ i -> return $! succ i mvarGet :: MVar Int -> IO Int mvarGet = readMVar tvarNew :: IO (TVar Int) tvarNew = newTVarIO 0 tvarInc :: TVar Int -> IO () tvarInc var = atomically $ do i <- readTVar var writeTVar var $! succ i tvarGet :: TVar Int -> IO Int tvarGet = readTVarIO -- Thomas Koster