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

[David Feuer]

Pure speculation: are you paying for a lot of conversions between FilePath
(string) and C strings?

On Thu, May 9, 2019, 10:02 PM Magicloud Magiclouds <
magicloud.magiclouds at gmail.com> wrote:

> Hi,
> 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 <$> wrap2Maybe (isMyPid fp u)
>     wrap2Maybe (Strict.readFile ("/proc/" </> fp </> "stat")))
>   threadDelay 1000000
>   watch u limit0s limit0h
>   where
>     wrap2Maybe :: IO a -> IO (Maybe a)
>     wrap2Maybe f = catch ((<$>) 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)
>
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
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