#2891: threadDelay appears to use excessive CPU in GHCi
---------------------------+------------------------------------------------
Reporter: JeremyShaw | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.3
Severity: normal | Resolution: wontfix
Keywords: | Difficulty: Unknown
Testcase: | Os: Linux
Architecture: x86 |
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Changes (by simonmar):
* status: new => closed
* difficulty: => Unknown
* resolution: => wontfix
Comment:
Ok, this program repeatedly waits for one second. When this is run with
the threaded RTS, here's what happens:
* the program calls threadDelay
* 1/3 of a second later, the RTS notices that the program is idle and
does
a GC
* after 1 second, the program wakes up again, etc.
so we get a full GC every second, which accounts for the 2% CPU usage: in
GHCi there's about 5Mb of stuff in the heap when idle. You'll see the
same effect when compiling the program with `-threaded`, but there the
heap will be almost empty, so the GC won't take much time. Without
`-threaded` there is no idle-time GC.
You can turn off the idle-time GC with the `-I0` RTS option, e.g.
{{{
ghci +RTS -I0
}}}
The idle-time GC isn't there just for performance reasons, it's also for
detecting deadlock. If the program has deadlocked, the only way to detect
that (and send `BlockedIndefinitely` exceptions) is to do a GC, but we
want to wait until the program looks idle before initiating the GC, hence
the current behaviour.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2891#comment:1>
GHC <http://www.haskell.org/ghc/>
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