assistant: On Linux, the expensive transfer scan is run niced.

This is a compromise. I would like to nice every thread except for the
webapp thread, but it's not practical to do so. That would need every
thread to run as a bound thread, which could add significant overhead.
And any forkIO would escape the nice level.

Utility/Batch.hs

@@ -0,0 +1,40 @@
+{- Running a long or expensive batch operation niced.
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE CPP #-}
+
+module Utility.Batch where
+
+#if defined(__LINUX__) || defined(__ANDROID__)
+import Control.Concurrent.Async
+import System.Posix.Process
+#endif
+
+{- Runs an operation, at batch priority.
+ -
+ - This is done by running it in a bound thread, which on Linux can be set
+ - to have a different nice level than the rest of the program. Note that
+ - due to running in a bound thread, some operations may be more expensive
+ - to perform. Also note that if the action calls forkIO or forkOS itself,
+ - that will make a new thread that does not have the batch priority.
+ -
+ - POSIX threads do not support separate nice levels, so on other operating
+ - systems, the action is simply ran.
+ -}
+batch :: IO a -> IO a
+#if defined(__LINUX__) || defined(__ANDROID__)
+batch a = wait =<< batchthread
+  where
+  	batchthread = asyncBound $ do
+		setProcessPriority 0 maxNice
+		a
+#else
+batch a = a
+#endif
+
+maxNice :: Int
+maxNice = 19