std.parallelism amap not scaling?

[safety0ff]

Hello,
I tried converting a c++/go ray tracing benchmark [1] to D [2].

I tried to using std.parallelism amap to implement parallelism, 
but it does not seem to scale in the manner I expect.

By running the program with different numbers of threads in the 
thread pool, I got these results (core i7 sandy bridge, 4 core 
+HT):

Threads			1	2	3	4	5	6	7	8
Real time 
(s)		34.14	26.894	21.293	20.184	19.998	25.977	34.15	39.404
User time 
(s)		62.84	65.182	65.895	70.851	78.521	111.012	157.448	173.074
System time (s)		0.27	0.562	1.276	1.596	2.178	4.008	6.588	8.652
System 
calls		155808	224084	291634	403496	404161	360065	360065	258661
System calls error	39643	80245	99000	147487	155605	142922	142922 
108454

I got these measurements using latest DMD/druntime/phobos, 
compiled with "-O -release -inline -noboundscheck"

I used time and strace -c to measure:
e.g.:
time ./main -h=256 -w=256 -t=7 > /dev/null
strace -c ./main -h=256 -w=256 -t=7 > /dev/null

What I also noticed in the task manager is that no matter what I 
did, I could not get the utilization to go anywhere close to 99% 
(unlike the C++ program in [1].)

My interpretation of these results is that std.parallelism.amap 
does significant communication between threads which causes 
issues with scaling.

[1] https://github.com/kid0m4n/rays
[2] https://github.com/Safety0ff/rays/tree/master/drays