On Sat, 11 Feb 2012 14:24:53 -0500, H. S. Teoh <[email protected]>
wrote:
On Sat, Feb 11, 2012 at 11:02:43AM -0800, Jonathan M Davis wrote:
On Saturday, February 11, 2012 10:51:36 H. S. Teoh wrote:
> This brings up an interesting point: what does the GC do if you have
an
> array that's continually appended to, and also shrunk from the front?
> That is:
>
> ubyte[] buf;
> while ( ... ) {
> consumeData(buf[0]);
> buf = buf[1 .. $];
>
> ubyte x = getMoreData();
> buf ~= x;
> }
>
> Will such a program "leak memory" in the sense that the memory chunk
> allocated to buf will grow larger and larger, even though its
> initial segment is never accessed again? Or is the GC smart enough
> to only reallocate the needed size when the memory chunk is moved
> (at some point when it has no more space to append x)?
>
> If the GC is smart enough, then this kind of construct could be used
> instead of copying overlapping ranges.
The memory blocks don't grow. When an array no longer has enough room
to increase its size, and you append to it, a new block is allocated,
and the array is moved to there. Any slices which refer to the
original block still refer to it. And when a garbage collection cycle
is run and a memory block has no references to it, it's collected. The
only "leaking" that occurs is that once no more slices refer to a
particular portion of a memory block, that portion is no longer
accessible until the block has been recycled.
I know that. What I was asking is how the GC handles memory block
reallocations: does it copy the original block in its entirety, or just
the part referenced by the slice being appended to.
If you haven't read this article yet, you really should:
http://www.dsource.org/projects/dcollections/wiki/ArrayArticle
[...]
After reading this article, I think it's clear that only the portion of
the block referenced by buf gets copied, correct? And it gets copied to
the beginning of the new block, so there is no wasted space in the new
block and the old block will simply be GC'd. Cool.
Yes, most likely. However, there is one case where it doesn't --
extending pages.
If an array slice can be extended by consuming the next allocated page
(this is only for page-sized blocks and above), then instead of copying
the data, it will just mark the next page as part of the block, and
continue on. In this case, the block is not "copied" at all, so it cannot
deallocate the no-longer referenced data.
This is probably something that can be detected. For instance, if you are
appending to a slice of length 5 and its at the end of a 10,000 element
array, it makes little sense to not copy that data (allowing the original
block to be freed by the GC). But the situations are always different.
-Steve
