#15153: GHC uses O_NONBLOCK on regular files, which has no effect, and blocks the
runtime
-------------------------------------+-------------------------------------
Reporter: nh2 | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.6.1
Component: Runtime | Version: 8.2.2
System |
Keywords: | Operating System: Linux
Architecture: | Type of failure: Runtime
Unknown/Multiple | performance bug
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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This is the outcome of https://mail.haskell.org/pipermail/ghc-
devs/2018-May/015749.html
Reading through the code of
[http://hackage.haskell.org/package/base-4.11.1.0/docs/src/GHC.IO.FD.html#rea...
readRawBufferPtr] the first line jumped to my eye:
{{{
| isNonBlocking fd = unsafe_read -- unsafe is ok, it can't block
}}}
This looks suspicious.
On Linux, if `fd` is a a descriptor to a regular file (on disk, a
networked filesystem, or a block device), then `O_NONBLOCK` will have no
effect, yet `unsafe_read` is used which will block the running OS thread.
You can read more about `O_NONBLOCK` not working on regular files on Linux
here:
* https://www.nginx.com/blog/thread-pools-boost-performance-9x/
* https://stackoverflow.com/questions/8057892/epoll-on-regular-files
* https://jvns.ca/blog/2017/06/03/async-io-on-linux--select--poll--and-
epoll/
*
https://groups.google.com/forum/#!topic/comp.os.linux.development.system/K
-fC-G6P4EA
And indeed, the following program does NOT keep printing things in the
printing thread, and instead blocks for 30 seconds:
{{{
module Main where
import Control.Concurrent
import Control.Monad
import qualified Data.ByteString as BS
import System.Environment
main :: IO ()
main = do
args <- getArgs
case args of
[file] -> do
forkIO $ forever $ do
putStrLn "still running"
threadDelay 100000 -- 0.1 s
bs <- BS.readFile file
putStrLn $ "Read " ++ show (BS.length bs) ++ " bytes"
_ -> error "Pass 1 argument (a file)"
}}}
when compiled with
{{{
~/.stack/programs/x86_64-linux/ghc-8.2.2/bin/ghc --make -O -threaded
blocking-regular-file-read-test.hs
}}}
on my Ubuntu 16.04 and on a 2GB file like
{{{
./blocking-regular-file-read-test /mnt/images/ubuntu-18.04-desktop-
amd64.iso
}}}
And `strace -f -e open,read` on it shows:
{{{
open("/mnt/images/ubuntu-18.04-desktop-amd64.iso",
O_RDONLY|O_NOCTTY|O_NONBLOCK) = 11
read(11,