Class size and performance

[Daniel Lewis]

Unknown W. Brackets Wrote:

> For some code I'm writing, I have a parser that creates instances of a 
> class.  For a test case I have, it creates 728,050 instances.  That's a 
> lot, and takes a lot of time, but this is life.
> 
> I've got a few ideas on optimizing it, but while testing some of them 
> out I noticed something interesting.  If I comment out a single, unused 
> from commented out code, member... I get double the speed.
> 
> The number and types of members do not matter, just their size.
> 
> I wrote a reduced test case.  To compile it, use either of these two 
> commands:
> 
> dmd -version=fast -O -inline -run weird.d
> dmd -version=slow -O -inline -run weird.d
> 
> The same effect happens with gdc.  It also happens with optimization and 
> inlining on or off (they make no difference.)
> 
> import std.stdio, std.perf;
> 
> class TestClass
> {
> 	ubyte[56] member1;
> 
> 	version (slow)
> 		ubyte[4] member2;
> }
> 
> void main()
> {
> 	// This is just how many my real example used.
> 	const test_instances = 728_050;
> 
> 	// Let's store them for accuracy to the problem.
> 	TestClass[] example = null;
> 	example.length = test_instances;
> 
> 	auto counter = new PerformanceCounter();
> 
> 	counter.start();
> 	for (int i = 0; i < test_instances; i++)
> 		example[i] = new TestClass();
> 	counter.stop();
> 
> 	PerformanceCounter.interval_type ms = counter.microseconds();
> 
> 	writefln("%f seconds", cast(double) ms / 1_000_000.0);
> }
> 
> Also of note, if I put a custom allocator on to determine the size, it's 
> slow until I remove another 4 bytes of members.
> 
> Is there some sort of threshold in the gc?  Why is this happening?
> 
> Thanks,
> -[Unknown]

Sir, I'm not sure about the GC, but I can say that your program probably doesn't need to create 728,000 objects to parse an input stream.

I'm writing a parser too at the moment; it's a rather experimental, oddly written one though.  It [should] perform tokenization and parsing in a single O(1) pass for the ECMAScript language.

Take a look if you're interested;
http://dsource.org/projects/walnut/browser/branches/1.9/source/interpreter.d

Regards,
Dan