Raise default number of workers

Now the default number of workers will be one less than the number
of cores on the system, with a minimum of 1 and a maximum of 3.

lib/Echidna/Types/Campaign.hs

@@ -3,10 +3,10 @@ module Echidna.Types.Campaign where
 import Control.Concurrent (ThreadId)
 import Data.Aeson
 import Data.Map (Map)
-import Data.Maybe (fromMaybe)
 import Data.Text (Text)
 import Data.Text qualified as T
 import Data.Word (Word8, Word16)
+import GHC.Conc (numCapabilities)
 
 import Echidna.ABI (GenDict, emptyDict, encodeSig)
 import Echidna.Types
@@ -211,9 +211,14 @@ defaultSymExecAskSMTIters :: Integer
 defaultSymExecAskSMTIters = 1
 
 -- | Get number of fuzzing workers (doesn't include sym exec worker)
--- Defaults to 1 if set to Nothing
+-- Defaults to (N-1) if set to Nothing, where N is Haskell's -N value,
+-- usually the number of cores, clamped between 1 and 3.
 getNFuzzWorkers :: CampaignConf -> Int
-getNFuzzWorkers conf = fromIntegral (fromMaybe 1 (conf.workers))
+getNFuzzWorkers conf = maybe defaultN fromIntegral conf.workers
+  where
+    n = numCapabilities
+    maxN = max 1 (n - 1) -- one dedicated thread for shrinking
+    defaultN = min 3 maxN -- capped at 3 by default
 
 -- | Number of workers, including SymExec worker if there is one
 getNWorkers :: CampaignConf -> Int

tests/solidity/basic/default.yaml

@@ -89,8 +89,8 @@ rpcUrl: null
 rpcBlock: null
 # Etherscan API key
 etherscanApiKey: null
-# number of workers
-workers: 1
+# number of workers. By default (unset) its value is the clamp of (# cores - 1) between 1 and 3
+workers: null
 # events server port
 server: null
 # whether to add an additional symbolic execution worker