https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Game_Engines_2021W_Lecture_5 2026-04-08T03:15:38Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Game_Engines_2021W_Lecture_5&diff=22864&oldid=prev 2021-01-26T17:25:05Z

<p>Created page with &quot;==Notes== &lt;pre&gt; Lecture 5 --------- - project proposal, see class wiki How will technical scope be graded? - clean design - doable implementation in time given - testing...&quot;</p> <p><b>New page</b></p><div>==Notes==<br /> <br /> &lt;pre&gt;<br /> Lecture 5<br /> ---------<br /> - project proposal, see class wiki<br /> <br /> How will technical scope be graded?<br /> - clean design<br /> - doable implementation in time given<br /> - testing your understanding of Godot<br /> - so, a mix of ambition and reason<br /> <br /> Game engines - memory management<br /> <br /> - does Godot use malloc/standard new? NO<br /> - why not?<br /> - memory allocation is inherently expensive<br /> - how memory is managed has significant performance implications<br /> - so, makes sense to make some effort to do it &quot;right&quot; for any<br /> performance-oriented application<br /> <br /> the memory hierarchy<br /> - systems have a mix of small, fast, low latency memory and slow,<br /> high latency memory<br /> <br /> registers<br /> TLB &lt;- virtual memory<br /> on-CPU cache (L1, L2)<br /> external CPU cache (L3)<br /> main memory<br /> SSD<br /> (Spinning) Hard Drives<br /> Tape<br /> <br /> Every memory access starts with a virtual address but must end with a physical address<br /> - TLB is a cache of virtual-&gt;physical mappings at the page level<br /> (every 4K of memory)<br /> - OS&#039;s manage page tables which define the full virtual-&gt;physical mapping,<br /> most modern chips have hardware walk the page tables to get TLB entries<br /> <br /> Part of why memory management is expensive is it ultimately requires<br /> manipulating TLBs and page tables<br /> - so first rule of memory management in userspace is to ask the kernel<br /> to do it as infrequently as possible<br /> - reduce, reuse, recycle!<br /> <br /> Classic malloc isn&#039;t shy about going to the OS for memory<br /> - yes, allocates in chunks<br /> - but, not so good at recycling<br /> - because of memory fragmentation<br /> <br /> Memory fragmentation means memory is used less efficiently<br /> - you get &quot;holes&quot; in memory that can&#039;t be properly used<br /> <br /> Example:<br /> - Let&#039;s say you do a bunch of 1K allocations, that are randomly allocated then freed<br /> - after doing this for a while, you&#039;ll have memory with randomly placed 1K holes, the allocations that you later de-allocated<br /> - then, you need 4K, or 8K, or...1MB<br /> - you can have that amount available, but in scattered 1K chunks<br /> - so, you have to go ask the OS for more memory :-(<br /> - or, you move things around to compact memory (this is basically garbage<br /> collection)<br /> <br /> - the best strategies to avoid memory fragmentation make assumptions about<br /> patterns of allocation<br /> - thus, if you know how your application allocates memory, then<br /> you can probably develop a way to allocate memory that minimizes<br /> fragmentation<br /> <br /> Cache-friendly allocation<br /> - more to do with how you organize data structures wrt access patterns<br /> - should organize them so your &quot;working set&quot; at any time fits into the caches<br /> <br /> To get best performance, you want small data structures<br /> - so data structures can all be packed into cache<br /> but small data structures means more allocations/deallocations<br /> - which hurt performance<br /> <br /> Core data type in Godot is Variant<br /> - can be used to represent almost anything<br /> - note, it allows for standard type checking to be completely<br /> bypassed (but Godot has its own mechanisms)<br /> <br /> By having a small data structure that is uniform in size but can represent<br /> all kinds of data, it makes the memory allocation problem easier<br /> - just managing uniform-sized variant&#039;s!<br /> - different kinds of data structures will get their own pools in general<br /> <br /> Note, not the most efficient in terms of space<br /> - some Variant types are smaller than 20 bytes<br /> - but still can improve performance<br /> <br /> Always trade-offs between time and space<br /> <br /> Variant is interesting for reasons beyond allocator efficiency<br /> - why implement your own type system?!<br /> &lt;/pre&gt;<br /> <br /> Also see [https://docs.godotengine.org/en/stable/classes/class_variant.html Godot&#039;s documentation on Variant].</div>

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