On Sunday, 12 January 2014 at 10:40:50 UTC, Benjamin Thaut wrote:
Am 12.01.2014 11:27, schrieb Rainer Schuetze:
I think a moving collector is currently not feasible without
restricting
the language a lot, probably similar to safe D and more. I'm
not sure we
want that in general.
Could you give an example which part of the language would not
be doable with a moving collector? The only thing that comes to
my mind is unions and that problem can be solved by allowing
the user to specify manual scanning functions for structs or
classes containing unions.
Maybe I'm too pessimistic ;-) I guess moving in general could be
ok, I was thinking about segregating heaps by type
(shared/immutable/mutable) and moving data between them adds
restrictions. I'd like to be proven wrong.
Some thoughts regarding a moving collector:
- interfacing with C/C++ is problematic: even if a pointer is
passed on the stack, it is not guaranteed that this stack entry
is not modified by the called function. While this might cause
problems when collecting memory due to this being the last
reference to the data, it is much more likely that there are
still references, but moving pointers will definitely fail.
Having to explicitely pin every pointer passed to C functions
would be very expensive.
- if we have many pinned objects, compaction loses some of its
advantages (like fast allocation). If we keep allocating from
free lists of equal sized bins, fragmantation and memory overhead
is limited (though not small) without moving. Also, moving lot's
of data can also be expensive.
- interior pointers do not allow "threading" for pointer updates,
so it might get pretty expensive to update references to moved
objects
Also I don't think that we can create a GC which performs as
good as the one of Java or C# if we are not willing to make the
neccessary changes for a moving gc.
To compete with other GCs we'd probably need write barriers to
keep track of changed references (for concurrent operation or
generations). There just needs to be a way to avoid having to
rescan the full heap every time, it does not scale.
PS: my SSD with all the D stuff just died yesterday (was it a
failure of the disks GC?). I'll probably need some time to
recover from that...
