Explicit calls to the garbage collector.

[Jurriaan Hage]

LS.

I have a very memory intensive application. It seems that the timing of my application
depend very much on the precise setting of -H...M in the runtime system (-H2000M
seems to work best, computation time becomes a third of what I get when I pass no
-H option).  I conjecture that this good behaviour is the result of gc happening at the right time.
So I wondered: if I can one when is the right time, is it possible then to trigger
GC explicitly from within the Haskell code?

best,
Jur
 
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[Christopher Done]

I would also be interested to know this. A web server is an example of
a Haskell program that could force garbage collection at the end of
every request reply, especially a multi-threaded server where the
memory use is localized to threads. For long-running applications, a
GC at this point would be nice.

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[Joachim Breitner-2]

Hi,

Am Montag, den 07.05.2012, 15:33 +0200 schrieb Jurriaan Hage:
> I have a very memory intensive application. It seems that the timing of my application
> depend very much on the precise setting of -H...M in the runtime system (-H2000M
> seems to work best, computation time becomes a third of what I get when I pass no
> -H option).  I conjecture that this good behaviour is the result of gc happening at the right time.
> So I wondered: if I can one when is the right time, is it possible then to trigger
> GC explicitly from within the Haskell code?

there is performGC:
http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-Mem.html#v:performGC

Greetings,
Joachim

--
Joachim "nomeata" Breitner
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[Johan Tibell-2]

Hi,

On Mon, May 7, 2012 at 6:33 AM, Jurriaan Hage <[hidden email]> wrote:
> LS.
>
> I have a very memory intensive application. It seems that the timing of my application
> depend very much on the precise setting of -H...M in the runtime system (-H2000M
> seems to work best, computation time becomes a third of what I get when I pass no
> -H option).  I conjecture that this good behaviour is the result of gc happening at the right time.
> So I wondered: if I can one when is the right time, is it possible then to trigger
> GC explicitly from within the Haskell code?

You have to be very careful when benchmarking programs in garbage
collected languages. If you set the nursery size high enough the GC
might never run during your benchmark, but in a real program a big
nursery might perform worse than a smaller one. For any small GC bound
benchmark you can typically get it to run much faster by setting -A to
some ridiculous high number.

-- Johan

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[Johan Tibell-2]

On Mon, May 7, 2012 at 7:00 AM, Christopher Done
<[hidden email]> wrote:
> I would also be interested to know this. A web server is an example of
> a Haskell program that could force garbage collection at the end of
> every request reply, especially a multi-threaded server where the
> memory use is localized to threads. For long-running applications, a
> GC at this point would be nice.

In non-GCed language people often use an arena allocator in situations
like this. However, a semi-space collector (which is what we use) we
get that behavior for free. If your request is processed in less time
than the minor GC cycle, all your data will be dead by the time the GC
run and thus collecting it is free.

-- Johan

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[Simon Marlow-7]

On 07/05/2012 14:33, Jurriaan Hage wrote:
> LS.
>
> I have a very memory intensive application. It seems that the timing of my application
> depend very much on the precise setting of -H...M in the runtime system (-H2000M
> seems to work best, computation time becomes a third of what I get when I pass no
> -H option).  I conjecture that this good behaviour is the result of gc happening at the right time.
> So I wondered: if I can one when is the right time, is it possible then to trigger
> GC explicitly from within the Haskell code?

It is more likely that you are trading extra memory for better
performance, rather than triggering the GC at a good time.  GC is
basically a space/time tradeoff, see:

http://stackoverflow.com/questions/3171922/ghcs-rts-options-for-garbage-collection/3172704#3172704

If you think the program has points where residency is very low and it
would be good to trigger a GC, I would first confirm the hypothesis by
doing a heap profile.  GC can be triggered with System.Mem.performGC.

Cheers,
        Simon

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