[Haskell-cafe] Current state of garbage collection in Haskell
C K Kashyap
ckkashyap at gmail.com
Mon Jul 30 11:55:43 CEST 2012
Thank you so much Alexander and Thomas.
On Sun, Jul 29, 2012 at 11:59 PM, Thomas Schilling
<nominolo at googlemail.com>wrote:
> GHC does not provide any form of real-time guarantees (and support for
> them is not planned).
> That said, it's not as bad as it sounds:
> - Collecting the first (young) generation is fast and you can control
> the size of that first generation via runtime system (RTS) options.
> - The older generation is collected rarely and can be collected in
> - You can explicitly invoke the GC via System.Mem.performGC
> In a multi-threaded / multi-core program collecting the first
> generation still requires stopping all application threads even though
> only one thread (CPU) will perform GC (and having other threads help
> out usually doesn't work out due to locality issues). This can be
> particularly expensive if the OS decides to deschedule an OS thread,
> as then the GHC RTS has to wait for the OS. You can avoid that
> particular problem by properly configuring the OS via (linux boot
> isolcpus=... and taskset(8)). The GHC team has been working on a
> independent *local* GC, but it's unlikely to make it into the main
> branch at this time. It turned out to be very difficult to implement,
> with not large enough gains. Building a fully-concurrent GC is
> (AFAICT) even harder.
> I don't know how long the pause times for your 500MB live heap would
> be. Generally, you want your heap to be about twice the size of your
> live data. Other than that it depends heavily on the characteristics
> of you heap objects. E.g., if it's mostly arrays of unboxed
> non-pointer data, then it'll be very quick to collect (since the GC
> doesn't have to do anything with the contents of these arrays). If
> it's mostly many small objects with pointers to other objects, GC will
> be very expensive and bound by the latency of your RAM. So, I suggest
> you run some tests with realistic heaps.
> Regarding keeping up, Simon Marlow is the main person working on GHC's
> GC (often collaborating with others) and he keeps a list of papers on
> his homepage: http://research.microsoft.com/en-us/people/simonmar/
> If you have further questions about GHC's GC, you can ask them on the
> glasgow-haskell-users at haskell.org mailing list (but please consult the
> GHC user's guide section on RTS options first).
> On 29 July 2012 08:52, C K Kashyap <ckkashyap at gmail.com> wrote:
> > Hi,
> > I was looking at a video that talks about GC pauses. That got me curious
> > about the current state of GC in Haskell - say ghc 7.4.1.
> > Would it suffer from lengthy pauses when we talk about memory in the
> > of 500M +?
> > What would be a good way to keep abreast with the progress on haskell GC?
> > Regards,
> > Kashyap
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> Push the envelope. Watch it bend.
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