Recently I've been profiling a lot v8's garbage collection. Surprising
truth is it's really slow when JS program does heavy computation. For
example, consider a straightforward n-body computation for 5 bodies, over
50 000 000 iterations [1].
When running this program in d8:
> time ./d8 ./nbody_plain_objects.js -- 50000000
-0.169075164
-0.169059907
./d8 ./nbody_plain_objects.js -- 50000000 *46.95s* user 0.07s system 99%
cpu 47.036 total
Now compare the same results with Mozilla's SpiderMonkey shell [2]:
> time ./js ./nbody_plain_objects.js 50000000
-0.169075164
-0.169059907
./js ./nbody_plain_objects.js 50000000 *20.27s* user 0.02s system 99% cpu
20.288 total
SpiderMonkey is more than two times faster! Why? Turns out V8 produces a
lot of numbers on the heap, when using plain javascript objects:
function Body(x,y,...) {
this.X = x;
this.Y = y; ...
}
This creates lots of garbage and slows down performance of the algorithm
significantly. The garbage collection could be avoided in V8, if program is
rewritten with use of Float64Arrays, and manual implementation of heap-like
structure. Doing so [3], puts v8 on the same speed level with SpiderMonkey:
> time ./d8 ./nbody_array.js -- 50000000
-0.169075164
-0.169059907
./d8 ./nbody_array.js -- 50000000 *21.45s* user 0.02s system 99% cpu
21.487 total
SpiderMonkey insignificantly suffers, but still shows decent results:
> time ./js ./nbody_array.js 50000000
-0.169075164
-0.169059907
./js ./nbody_array.js 50000000 *23.73s* user 0.02s system 99% cpu 23.749
total
I definitely could rewrite my programs with use of native arrays, but I
don't really think this scales well for the larger audience of programmers,
who are doing calculus in JS. Maybe I'm missing a technique which would let
me avoid GCs at all, with no need to rewrite programs? I would also like to
avoid imposing on my users a need to launch chrome (v8) with a special
flags...
PS: All tests are done on MacBook Pro, 2.4 GHz, Intel Core i5, with latest
x64.release build of V8, and latest nightly build of SpiderMonkey [2].
[1] https://gist.github.com/anvaka/5438615
[2] http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/
[3] https://gist.github.com/anvaka/5438702
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