Microsoft working on new systems language

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

On Sunday, 29 December 2013 at 21:39:52 UTC, Walter Bright wrote:
> Since you can control if and when the GC runs fairly simply, 
> this is not any sort of blocking issue.

I agree, it is not a blocking issue. It is a cache-trashing 
issue. So unless the GC is cache-friendly I am concerned about 
using D for audio-visual apps. Granted, GC would be great for 
managing graphs in application logic (game AI, music structures 
etc). So I am not anti-GC per se.

Lets assume 4 cores, 4MB level 3 cache, 512+ MB AI/game-world 
data-structures. Lets assume that 50% CPU is spent on graphics, 
20% is spent on audio, 10% is spent on texture/mesh 
loading/building, 10% on AI and 10% is headroom (OS etc).

Ok, so assume you have 5 threads for simplicity:

thread 1, no GC: audio realtime hardware
thread 2/3, no GC: opengl "realtime" (designed to keep the GPU 
from starving)
thread 4, GC: texture/mesh loading/building and game logic

Thread 4 is halted during GC, but threads 1-3 keeps running 
consuming 70% of the CPU. Thread1-3 are tuned to keep most of 
their working set in cache level 3.

However, when the GC kicks in it will start to load 512+MB over 
the memory bus at fast pace. If there is one pointer per 32 bytes 
you touch all possible cache lines. So the the memory bus is 
under strain, and this pollutes cache level 3, which wipes out 
the look-up-tables used by thread 1&2 which then have to be 
loaded back into the cache over the memory bus... thread 1-3 
fails their deadline, you get some audio-visual defects and the 
audio/graphics systems compensate by reducing the load by cutting 
down on audio-visual features. After a while the audio-visual 
system detects that the CPU is under-utilized and turn the high 
quality features back on. But I feel there is a high risk of 
getting disturbing noticable glitches, if this happens every 10 
seconds it is going to be pretty annoying.

I think you need to take care, and have a cache-friendly 
GC-strategy tuned for real time. It is possible though. I don't 
deny it.