Ticket #1889 (closed bug: wontfix)
Regression in concurrency performance from ghc 6.6 to 6.8
|Reported by:||dons||Owned by:||simonmar|
|Keywords:||threads, concurrency, performance||Cc:||dons@…, sjanssen@…|
|Type of failure:||Runtime performance bug||Difficulty:||Unknown|
|Test Case:||Blocked By:|
While most Great Language Shootout benchmarks are running noticeably (10-20+ %) faster with GHC 6.8.1, benchmarks for concurrency seem to consistently be running slower, on a variety of architectures.
Attached are some example programs from the shootout where performance has worsened.
The thread-ring benchmark:
import Control.Monad import Control.Concurrent import System.Environment ring = 503 new l i = do r <- newEmptyMVar forkIO (thread i l r) return r thread :: Int -> MVar Int -> MVar Int -> IO () thread i l r = go where go = do m <- takeMVar l when (m == 1) (print i) putMVar r $! m - 1 when (m > 0) go main = do a <- newMVar . read . head =<< getArgs z <- foldM new a [2..ring] thread 1 z a
We can benchmark this as follows:
$ ghc-6.8.1 -O2 threadring.hs -o ghc68 -no-recomp $ ghc-6.6.1 -O2 threadring.hs -o ghc66 -no-recomp $ time ./ghc68 30000000 ; time ./ghc66 30000000 75 ./ghc68 30000000 4.98s user 0.05s system 100% cpu 5.030 total 75 ./ghc66 30000000 4.58s user 0.04s system 99% cpu 4.643 total
'chameneos' is another that has got slower.
$ time ./ghc66 7000000 ; time ./ghc68 7000000 14000000 ./ghc66 7000000 3.36s user 0.05s system 99% cpu 3.425 total 14000000 ./ghc68 7000000 3.75s user 0.06s system 99% cpu 3.815 total