std.container.Array/RefCounted(T) leaking memory?
Jonathan M Davis
jmdavisProg at gmx.com
Wed Jan 12 16:29:30 PST 2011
On Wednesday, January 12, 2011 15:29:51 %u wrote:
> Sorry to bump this up, but is RefCounted(T) really leaking, or am I missing
> something? I would like to use this in my program, and I'm curious as to
> why no one responded, since if it's actually leaking, it would be an
> important issue.
There probably aren't all that many people who saw your post, and out of those
who did, there are probably very few - if any - who have actually done much with
RefCounted. It's fairly new.
There's at least one major bug on Array at the moment (
http://d.puremagic.com/issues/show_bug.cgi?id=4942 ). There are also several
bugs having to do with destructors at the moment which could be causing you
Now, even assuming that you're not seeing any problem with a destructor-related
bug and that you're not hitting a known bug with Array, there are three things
that you need to be aware of which would likely show high memory usage
1. Array uses an array internally, and there is some caching that goes on with
regards to arrays that has to do with appending. This means that if you're
dealing with large arrays, you could have several which haven't been garbage
collected yet simply because they're cached. Steven Schveighoffer has talked
about it in several posts, and he has done some work to improve the situation,
but I'm not sure that any of it has been in a release yet.
2. The garbage collector does not currently run in its own thread. IIUC, it only
gets run when you try and allocate memory. So, if you allocate a bunch of
memory, and then you never try and allocate memory again, no memory will be
collected, regardless of whether it's currently being used or not.
3. As I understand it, the current garbage collector _never_ gives memory back
to the OS. It will reclaim memory that you're not referencing any longer so that
it doesn't necessarily need to go grab more memory from the OS when you try and
allocate something, but once the garbage collector has gotten a block of memory
from the OS, it doesn't give it back. So, currently you will _never_ see the
memory usage of a D program go down, unless you're explicitly using malloc and
free instead of GC-allocated memory.
So, if you really want to be testing for leaks, you probably should testing for
short-lived small arrays in a loop with lots of iterations or something similar.
Testing a couple of large arrays will almost certainly mean that the memory
usage will be high and that it won't drop. On the other hand, if you keep
creating small arrays in a loop where they have no references outside the loop
and could be collected after they go out of scope, then you have lots of arrays
for the garbage collector to collect, and if it fails to properly collect them,
_then_ you'll see the memory usage continue to rise and show that there's a
leak. But at this point, using a few large arrays is almost certainly going to
look like it's leaking.
I'm sure that at some point D's garbage collector will improve so that these
issues don't exist or at least or definitely diminished, but fixing the GC is not
exactly at the top of the TODO list, so it's not currently as performant as
would be nice. Once more important stuff has been fixed though, it'll get its
- Jonathan M Davis
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