On Thu, 19 Nov 2009 12:01:25 -0500, dsimcha <dsim...@yahoo.com> wrote:
== Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s
article
dsimcha wrote:
> == Quote from Andrei Alexandrescu (seewebsiteforem...@erdani.org)'s
article
>> Yes, it will be because the book has a few failing unittests. In
fact, I
>> was hoping I could talk you or David into doing it :o).
>> Andrei
>
> Unfortunately, I've come to hate the MRU idea because it would fail
miserably for
> large arrays. I've explained this before, but not particularly
thoroughly, so
> I'll try to explain it more thoroughly here. Let's say you have an
array that
> takes up more than half of the total memory you are using. You try
to append to
> it and:
>
> 1. The GC runs. The MRU cache is therefore cleared.
>
> 2. Your append succeeds, but the array is reallocated.
>
> 3. You try to append again. Now, because you have a huge piece of
garbage that
> you just created by reallocating on the last append, the GC needs to
run again.
> The MRU cache is cleared again.
>
> 4. Goto 2.
This is not a matter of principles, but one of implementation. When you
GC, you can adjust the cache instead of clearing it.
Technically true, but what is a matter of principles is whether the
implementation
of arrays should be very tightly coupled to the implementation of the
GC. Fixing
this issue would have massive ripple effects throughout the already
spaghetti
code-like GC, and might affect GC performance. For every single object
the GC
freed, it would have to look through the MRU cache and remove it from
there if
present, too.
You perform the lookup via MRU cache (after mark, before sweep). I see it
as a single function call at the right place in the GC.
The point is that this **can** be done, but we probably don't **want** to
introduce this kind of coupling, especially if we want our GC model to
be sane
enough that people might actually come along and write us a better GC
one day.
What about implementing it as a hook "do this between mark and sweep"?
Then it becomes decoupled from the GC.
-Steve
