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

[Brandon Allbery]

...what?

Also, in C you'd stat() and check for -1 (not found_ or inspect the result
to see if it's what you want. But in Haskell this throws an exception
instead of producing a sane Either. so you either make multiple syscalls or
you have to catch an exception. So no matter what this ends up being higher
overhead than C or Rust.

On Thu, May 9, 2019 at 10:15 PM Magicloud Magiclouds <
magicloud.magiclouds at gmail.com> wrote:

> I could not tell, since those are some kind of "standard" functions of
> Haskell, right?
>
> On Fri, May 10, 2019 at 10:11 AM David Feuer <david.feuer at gmail.com>
> wrote:
> >
> > 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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>
>
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>
> And for G+, please use magiclouds#gmail.com.
> _______________________________________________
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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