Programming languages and performance

[Laeeth Isharc via Digitalmars-d]

thanks for the links and colour, Walter and HST

> But at the end of the day, the programmer has to know how to 
> write
> cache-efficient code. No matter how the language/compiler tries 
> to be
> smart and do the Right Thing(tm), poorly-laid out data is 
> poorly-laid
> out data, and you're gonna incur cache misses all over the 
> place.
> Cache-unfriendly algorithms are cache-unfriendly algorithms, 
> and no
> smart language design / smart optimizer is gonna fix that for 
> you. You
> have to know how to work with the modern cache hierarchies, how 
> to lay
> out data for efficient access, and how to write cache-friendly
> algorithms.

> While Phobos is making good progress at being allocation-free, 
> it still
> has a ways to go. And it doesn't help that the current D GC 
> isn't that
> great, when you do have to allocate -- I've managed to get 
> 30-40%
> performance improvements just by turning off the default 
> collection
> schedule and triggering collections myself at more strategic 
> intervals.

Would love to see an article sometime on efficient programming in 
D - both cache efficiency and how to make the GC your friend.  (I 
get the basic idea of data driven design, but not yet the 
subtleties of cache efficient code and I am sure many other 
newcomers to D must be in a similar position).

I found the same thing as you describe with a monster CSV import 
(files are daily, but data needs to be organized by symbol to be 
useful).

> Not having to box things is a big win IMO, though. Boxing of 
> POD types
> in Java/C# just screams "inefficient" to me... can you imagine 
> all that
> extra, needless indirection wreaking havoc on the CPU cache and 
> cache
> predictions?

There was an interesting post on Lambda the ultimate by Mike Pall 
(sp?  The Lua guy) in which he said certain eyesight decisions in 
Python meant much harder to ever make Python fast, and one of the 
pypy guys agreed with him.  (It was more than just boxing).

I am not in favour of extrapolating trends mindlessly, but I 
wonder what the world looks like In five or ten years should the 
gap between processor perf and memory latency continue to widen 
at similar rates given continued growth in data set sizes.


Laeeth.