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

[Paulo Pinto]

On Thursday, 9 January 2014 at 14:19:41 UTC, Ola Fosheim Grøstad 
wrote:
> 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...

That was just a suggestion. There are plenty of incremental GC 
algorithms to choose from.

>
> 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).

There are real time GCs controlling missile tracking systems.

Personally I find them a bit more real time than computer games.

On a game you might miss a few rendering frames, a GC induced
delay on a missile tracking system might turn out a bit ugly.

>
> 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.


Sure, the GC usage should not hinder the application's 
performance.

However, unless you target systems without an OS, you'll have 
anyway the OS making whatever it wants with the existing cores.

I never saw much control besides setting affinities.

--
Paulo