This question came up:
You're searching for bottlenecks in your game, but nothing you're changing is making the game any faster, be it anything in the GPU pipeline or the CPU. Nothing is spiking, and the slowness appears to be distributed across everywhere. What do you do next?
I was flummoxed. Is it a trick question? When fixing perf issues, I always assume that this was the point at which you need to scale everything back. I don't think it's mem alloc, as that shows up in CPU perf.
I would have asked for more information. "Slow" is a poor indicator of bad performance and is a classification of a symptom rather than a symptom itself. For example, you might describe "slow" as being:
- Low frame rate
- Poor responsiveness to input
- High responsiveness and smooth framerate, but slow game mechanics (i.e.: the player and entities move smoothly but very slowly)
- In the case of networked games, apparent network lag
All of these problems have different potential causes and solutions:
- Low but consistent frame rate may be due to inefficiencies in your game loop. Simply running your favorite profiler may indicate that large amounts of time are spent in one particular piece of code. In a game I wrote, for instance, I discovered that low FPS was the result of a bad loop that calculated distances between entities multiple times without caching. In another game, I discovered that the data structure I was using to perform lookups against the terrain was
O(N) rather than O(1) (python stdlib...ick). You can't diagnose a problem you can't see, and profiling is the first line of defense.
- Poor responsiveness may be due to a number of things. If the FPS is high but the controls are sluggish to respond, the API that you're using to access the controls may simply be bad. Some controllers may have crappy drivers that can kill responsiveness. It might even be your game loop: you might simply not be checking for input from the controller frequently enough (perhaps you're not checking on every tick). In one of the aforementioned games, I had an issue where certain actions had a delayed effect: you'd use an item and the game would respond a half second or so later. It turned out that the issue was caused by the client making a full round-trip to the server to perform the action, verify that it happened, and wait for the server to broadcast back that the item was used. Simply having the behavior take place instantaneously on the client remedied the issue.
- Slow game mechanics might indicate that game constants simply aren't set high enough. If everything is smooth and beautiful but everything just moves very slowly, it's quite possible that default velocities or accelerations aren't turned up enough.
- Network lag can be caused by any number of things: the router you're connected to might be failing, the VPS you're developing against might be on a host that's being DDoSed, you might be using a protocol that's overly (but uniformly) chatty, or you're simply sending too much data over the wire. In a piece of simulation software I wrote in college, the computations were performed on some beefy computers in a lab, while the visualizations were being run on my MBP in my dorm. It turned out that the sheer amount of data that I was sending from the lab computers to my dorm was enough to overload the cheap network switches in the building and drop packets, resulting in horrible lag but perfectly reasonable log output.
So I guess the answer here is to have the interviewer describe the symptoms more fully. @Ali's answer is great, but it could be that there's a more nuanced problem at hand that requires some coaxing to diagnose.
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