Adding Java and C++ to the MQTT benchmarks or: How I Learned to Stop Worrying and Love the Garbage Collector

[Ola Fosheim Grøstad" <ola.fosheim.grostad+dlang at gmail.com>]

On Thursday, 9 January 2014 at 13:51:09 UTC, Paulo Pinto wrote:
> That could possibly be achieved with a generational parallel GC.

Isn't the basic assumption in a generational GC that most free'd 
objects has a short life span and happened since the last 
collection?  Was there some assumption about the majority of 
inter-object pointers being within the same generation, too? So 
that you partition the objects in "train carts" and only have few 
pointers going between carts? I haven't looked at the original 
paper in a long time...

Anyway, if that is the assumption then it is generally not true 
for programs that are written for real time. Temporary objects 
are then allocated in pools or on the stack. Objects that are 
free'd tend to come from timers, events or because they have a 
lifespan (like enemies in a computer game).

I also dislike the idea of the GC locking cores down when it 
doesn't have to, so I don't think parallel is particularly 
useful. It will just put more pressure on the memory bus. I think 
it is sufficient to have a simple GC that only scans disjoint 
subsets (for that kind of application), so yes partitioned by 
type, or better: by reachability, but not by generation.

If the GC behaviour is predictable then the application can be 
designed to not trigger bad behaviour from the get go.