Re: Re[4]: [Haskell-cafe] GC'ing file handles and other resources

16 Apr 2008


      Thanks, both for the summary and for the link. Will try to go through it.

Regards,
Abhay

On Wed, Apr 16, 2008 at 12:37 PM, Bulat Ziganshin 
wrote:
...
Hello Abhay,
Wednesday, April 16, 2008, 10:51:07 AM, you wrote:
...
I am not saying that it should claim it as soon as it is unused;
all I am saying that as soon as a priority object becomes
unreferenced, it should be the first choice for collecting in the next
collect.
on the GC, all unreferenced objects are collected. there is no
difference which ones will be collected first - anyway program is
stopped until whole GC will be finished
...
Further I was under the impression (I may be wrong) that it uses a
generational GC and therefore scans allocated memory incrementally;
not the whole at a time. Please correct me if I am wrong.
yes, it uses generational GC. data are first allocated in small 256k block
and when it is filled, GC for this small block occurs. data that are
still alive then moved to the global heap. this minor GC doesn't scan
global heap. if it will do this, each minor GC will become as slow as
major ones
"Generational garbage collection for Haskell"
http://research.microsoft.com/~simonpj/Papers/gen-gc-for-haskell.ps.gzhttp://research.microsoft.com/%7Esimonpj/Papers/gen-gc-for-haskell.ps.gz
...
Regards,
Abhay
...
On Wed, Apr 16, 2008 at 11:55 AM, Bulat Ziganshin
 wrote:
 Hello Abhay,
Wednesday, April 16, 2008, 9:30:34 AM, you wrote:
i think it will not work with current ghc GC - it scans entire
 memory/nursery when garbage collected so anyway you will need to wait
 until next GC event occurs
...
...
Your mail gives me an idea, though I am not an iota familiar with
compiler/garbage collector internals. Can we have some sort of
internally maintained priority associated with allocated objects?
The garbage collector should look at these objects first when it
tries to free anything. The objects which hold other system
resources apart from memory, such as file handles, video memory, and
so on could be allocated as higher priority objects. Is such a thing
possible?
2008/4/16 Conal Elliott :
 Are Haskell folks satisfied with the practical necessity of
imperatively & explicitly reclaiming resources such as file handles,
fonts & brushes, video memory chunks, etc?  Doesn't explicit freeing
of these resources have the same modularity and correctness problems
as explicit freeing of system memory (C/C++ programming)?
I wrote a lovely purely functional graphics library that used video
memory to lazily compute and cache infinite-resolution images, and I
found that I don't know how to get my finalizers to run anytime soon
after video memory chunks become inaccessible.  Explicit freeing
isn't an option, since the interface is functional, not imperative
(IO).
I guess I'm wondering a few things:
...
* Are Haskell programmers generally content with imperative and
bug-friendly interfaces involving explicit freeing/closing of
resources?
* Do people assume that these resources (or handling them frugally)
aren't useful in purely functional interfaces?
 * Are there fundamental reasons why GC algorithms cannot usefully
apply to resources like video memory, file descriptors, etc?
* Are there resource management techniques that have the
flexibility, efficiency, and accuracy of GC that I could be using for
these other resources?
Thanks,
  - Conal
...
2008/4/14 Abhay Parvate :
 Hello,
...
In describing the Handle type, the GHC documentation says (in the
System.IO documentation):
...
GHC note: a Handle will be automatically closed when the garbage
collector detects that it has become unreferenced by the program.
However, relying on this behaviour is not generally recommended:
the garbage collector is unpredictable.  If possible, use explicit
an explicit hClose to close Handles when they are no longer
required.  GHC does not currently attempt to free up file
descriptors when they have run out, it is your responsibility to
 ensure that this doesn't happen.
...
But one cannot call hClose on Handles on which something like
hGetContents has been called; it just terminates the character list
at the point till which it has already read. Further the manual says
that hGetContents puts the handle in the semi-closed state, and
further,
A semi-closed handle becomes closed:
 if hClose is applied to it;  if an I/O error occurs when reading
an item from the handle;  or once the entire contents of the handle
has been read.
So do I safely assume here, according to the third point above,
that it's fine if I do not call hClose explicitly as far as I am
consuming all the contents returned by hGetContents?
...
Thanks,
Abhay
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--
 Best regards,
  Bulat                            mailto:Bulat.Ziganshin@gmail.com
...
--
Best regards,
 Bulat                            mailto:Bulat.Ziganshin@gmail.com