• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
  • entries
    625
  • comments
    1446
  • views
    1006373

Epoch Optimizations and Garbage Collection

Sign in to follow this  
Followers 0
ApochPiQ

690 views

Following the Great Garbage Collection Debug Spree of the past few weeks, I've noticed a general trend towards bad performance in the realtime raytracer benchmark. At one point it was down to ~6.5FPS peak performance, which is just unacceptably bad.

Profiling revealed two main causes of this slowdown. One is the garbage collector invocations themselves; Epoch is designed to have a "greedy" collector which trips frequently and collects aggressively. Unfortunately, the cost of walking the stack and heap for live objects is nontrivial, so tripping the GC all the time leads to lower performance throughput.

It's worth noting that in standard apps like Era, the difference isn't even visible; I've yet to see Era hitch noticeably with the fixed garbage collector. But for something that wants to peg a CPU core and go flat-out as fast as possible, triggering collection cycles all the time is a bad idea.


The other major cause of performance woes is register spilling. Without going into painful amounts of detail, one of the requirements of LLVM's garbage collection support is that you can no longer store pointers to GC-able objects in CPU registers. Instead, all objects that might be collected must be stored in memory on the machine stack. Moreover, any time you want to update those values, it has to be done on the stack, so there is overhead in moving from register values back onto the stack every time you touch a variable that might be garbage collected.

This may not seem like much, but it turns out to be pretty painful in a benchmark that relies heavily on computations done on objects. Since every object can potentially be collected by the GC, every object's pointer must live on the stack at all times. This winds up being a substantial perf hit.


On the upside, both problems can be fixed with a single tactical strike. The key observation is that many Epoch functions never do anything that generates garbage. A typical Epoch program is comprised of many small, nearly-trivial functions; and if those functions can't possibly create any garbage, there's no purpose checking the GC when they finish.

As a bonus, if a function never needs to call the GC, and never calls any other functions which need the GC, we can turn off register spilling for that function!


The Shootout: Epoch vs. C++
Tonight I implemented a quick little function tag called [nogc]. This tag tells the JITter two things: first, the function cannot generate garbage, and therefore should not invoke the garbage collector; and secondly, because the GC is never invoked from a [nogc] function, register spilling can be disabled in the body of that function.

The results are duly impressive: the raytracer is back to running at ~26.5FPS on my dev machine - a full 4x speedup over the worst-case performance it was exhibiting earlier today.

But it gets even cooler.

Out of sheer curiosity, I wrote a port of the Epoch raytracer in C++. It's not optimal C++, to be fair, and cuts a few corners to make it function more similarly to the Epoch program. But, line for line, it does basically exactly what the Epoch version does.


The C++ port of the raytracer runs at only 23FPS.

4
Sign in to follow this  
Followers 0


0 Comments


There are no comments to display.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now