adjust benchmark expected time based on the host's number of cores

The goal of benchmarking the expensive hash is to get an accurate time
estimate for a single CPU, but argon2 uses 4 threads, so on a machine with
4 cores, it should only take a quarter as long.

1 files changed, 10 insertions, 4 deletions

diff --git a/ExpensiveHash.hs b/ExpensiveHash.hs
index f9a9548..4031ef3 100644
--- a/ExpensiveHash.hs
+++ b/ExpensiveHash.hs
@@ -19,6 +19,7 @@ import Data.Time.Clock
 import Control.DeepSeq
 import Control.Monad
 import Data.Monoid
+import GHC.Conc (getNumProcessors)
 
 -- | A hash that is expensive to calculate.
 --
@@ -46,7 +47,7 @@ expensiveHash (UseArgon2 cost opts) (Salt s) b = ExpensiveHash cost $
 		in sb <> B.replicate (8 - B.length sb ) 32
 
 benchmarkExpensiveHash :: Int -> ExpensiveHashTunable -> Cost op -> IO (BenchmarkResult (Cost op))
-benchmarkExpensiveHash rounds tunables expected = do
+benchmarkExpensiveHash rounds tunables@(UseArgon2 _ hashopts) expected = do
 	start <- getCurrentTime
 	forM_ [1..rounds] $ \_ -> do
 		let ExpensiveHash _ t = expensiveHash tunables
@@ -56,8 +57,15 @@ benchmarkExpensiveHash rounds tunables expected = do
 	end <- getCurrentTime
 	let diff = floor $ end `diffUTCTime` start
 	let actual = CPUCost $ Seconds diff
+	-- The expected cost is for a single core, so adjust it
+	-- based on the number of cores, up to a maximum of the number
+	-- of threads that the hash is configred to use.
+	numcpus <- fromIntegral <$> getNumProcessors
+	let maxthreads = Argon2.hashParallelism hashopts
+	let usedcpus = if numcpus > maxthreads then maxthreads else numcpus
+	let adjustedexpected = mapCost (`div` fromIntegral usedcpus) expected
 	return $ BenchmarkResult
-		{ expectedBenchmark = expected
+		{ expectedBenchmark = adjustedexpected
 		, actualBenchmark = actual
 		}
 
@@ -65,8 +73,6 @@ benchmarkTunables :: Tunables -> IO ()
 benchmarkTunables tunables = do
 	putStrLn "/proc/cpuinfo:"
 	putStrLn =<< readFile "/proc/cpuinfo"
-	putStrLn "Note that expected times are for 1 core machine."
-	putStrLn "If this machine has 2 cores, the actual times should be half the expected times."
 	putStrLn "Benchmarking 16 rounds of key generation hash..."
 	print =<< benchmarkExpensiveHash 16
 		(keyEncryptionKeyHash $ keyEncryptionKeyTunable tunables)