[Haskell-cafe] How to optimize a directory scanning?

Magicloud Magiclouds magicloud.magiclouds at gmail.com
Fri May 10 02:00:45 UTC 2019

I have asked this in Stackoverflow without getting an answer.
Wondering if people here could have some thoughts.

I have a function reading the content of /proc every second.
Surprisingly, its CPU usage in top is around 5%, peak at 8%. But same
logic in C or Rust just takes like 1% or 2%. Wondering if this can be
improved. /proc is virtual filesystem, so this is not related to HDD
performance. And I noticed this difference because my CPU is too old
(Core Gen2). On modern CPU, as tested by others, the difference is
barely noticeable.

import Control.Exception
import Control.Concurrent
import Control.Monad
import Data.Char
import Data.Maybe
import System.Directory
import System.FilePath
import System.Posix.Files
import System.Posix.Signals
import System.Posix.Types
import System.Posix.User
import System.IO.Strict as Strict

watch u limit0s limit0h = do
  listDirectory "/proc/" >>= mapM_ (\fp -> do
    isMyPid' <- maybe False id <$> wrap2Maybe (isMyPid fp u)
    wrap2Maybe (Strict.readFile ("/proc/" </> fp </> "stat")))
  threadDelay 1000000
  watch u limit0s limit0h
    wrap2Maybe :: IO a -> IO (Maybe a)
    wrap2Maybe f = catch ((<$>) Just $! f) (\(_ :: IOException) ->
return Nothing)
    isMyPid :: FilePath -> UserID -> IO Bool
    isMyPid fp me = do
      let areDigit = fp >= "0" && fp <= "9"
      isDir <- doesDirectoryExist $ "/proc/" </> fp
      owner <- fileOwner <$> getFileStatus ("/proc" </> fp)
      return $ areDigit && isDir && (owner == me)


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