3 ms·

Accurate timing is most useful when you are retro-programming a video game, or something else real time. How do you know if your game will run at 60 fps if the

by phire 5y ago

Accurate timing is most useful when you are retro-programming a video game, or something else real time. How do you know if your game will run at 60 fps if the execution times aren't accurate? > what CPU timing would you even emulate Ideally your emulator would support as many CPU + hardware configurations as possible, at many different speeds, so you can test as many as you want. But just one single accurate hardware configuration is better than none. At least then I can say "I programmed this game it runs on a Pentium III 550E, with a Riva TNT2" It's exactly the same when doing retro-programming on real hardware. If you only have one PC, then you can only confirm it's working on that exact same hardware configuration. But accurate emulators have advantages due to cost and ability to easily support multiple configurations.

AshamedCaptain 5y ago
> How do you know if your game will run at 60 fps if the execution times aren't accurate? Again, this is not a console. If you rely on a specific Pentium 3's instruction timings to reach 60 fps, your game is not going to reach 60fps _on any other PC_, not even if someone has an identical CPU, since any single other difference in hardware, configuration, or even layout of the filesystem is going to matter much more. You just can't get away with the same kinds of bug you can get away with in consoles, because even just trying ATI vs NVIDIA (or any two different brands of accelerator) is already going to be a completely different environment and timings, likely enough to trigger all those bugs (or at least more than different instruction timings will). i.e. even the simplest of emulators (incl. a virtualizer) with a runtime cap is going to suffice for the usecase of mildly estimating a framerate based on the CPU of some era. And there's very little value to increase the accuracy of such estimation since with so wildly varying PC hardware anything you can produce is going to be irrelevant anyway. (How to make a similar accurate-enough estimation of GPU performance is a different story).
- memetomancer 5y ago
  There is a world of difference between your inability to see the appeal/utility of a thing and that thing being actually worthless, as you are so stridently insisting here. For a mental exercise, just to prove it to yourself, why don't _you_ try examining all the reasons such accurate emulation might be desirable? The other poster already provided a reasonable motivation - preservation. But since this discussion started, you've only really come out swinging with disparagement. One has to wonder why you are putting so much energy into suppressing and trashing someone's hobby. Frankly it looks to me like you have some kind of preconceived and inflexible bias, or that you maybe trying to discover the appeal to this effort in a pointlessly adversarial way. Not a great look. If you really want people to think you have a smart, winning argument maybe try to show some understanding of both sides of the coin before floating your attempt at a clever and withering denouncement.
  - nitrogen 5y ago
    To add an example to your arguments, the C64 and PC/XT demo scenes show what can be done with cycle accurate retrocomputing. Especially 8088mph: https://youtu.be/yHXx3orN35Y https://youtu.be/yHXx3orN35Y
    - AshamedCaptain 5y ago
      Do you know a lot of demo scene binaries that only work with accurate instruction timings of a Pentium 3 ?
      - nitrogen 5y ago
        What's your fixation here? You're wasting a lot of people's time.
        deleted 5y ago
        [deleted]
        AshamedCaptain 5y ago
        Feel free to downvote.
  - anthk 5y ago
    PC games stopped being CPU cycle-bound long ago in mid-late 90's. The most issues you would have it's for high 486-Pentium I-II era games (specially the multimedia ones) which lots of them were speed bound, but for sure these games will be interpreted by ScummVM one day or another (Macromedia Director engine).
  - AshamedCaptain 5y ago
    > There is a world of difference between your inability to see the appeal/utility of a thing and that thing being actually worthless, as you are so stridently insisting here. I am asking a question, and answering "preservation" (which is not really the answer the poster made, since his goal is new development) without giving an actual concrete example of what behavior needs such accurate preservation kind of defeats the purpose of asking the question in the first place. If the answer is "for the sake of it" that is also fine. But I'm unaware of anything post-P3 that would really require cycle-level emulation, so I ask. Most PC emulators "draw the line" around that era for a reason, even the ones who wouldn't necessarily have performance problems with newer machines (like DosBox). > But since this discussion started, you've only really come out swinging with disparagement. One has to wonder why you are putting so much energy into suppressing and trashing someone's hobby. What do you think? Because not only I have the same hobby, my work is also related to this. I am most definitely not interested in thrashing it. It is also the _main thesis_ of this entire article, so why shouldn't we discuss it?
- phire 5y ago
  If someone was to decide to develop a game for a minimum performance requirement of a Pentium III at 500mhz and some GPU, for some reason (which would be a completely arbitrary choice, but that's the hobby, so roll with it) then the only way they can possibly check that out meets that minimum requirement is to test on a machine with that configuration. Either a real machine, or a accurate emulator. It doesn't matter if there is other PC hardware configurations out there with different preformance. A minimum requirements just means "I tested on this machine, and it meets the minimums." Ideally you should underspec your minimum requirement test machine so that your target audience can be reasonably expected meet it. You can't substitute in virtualization. That has zero chance in hell of providing a realistic estimate of performance, even if you paired it with an accurate gpu emulator. Modern CPUs simply have very different performance characters, instructions that might have huge stalls on the P3 might be extremely cheap under virtualization. Caches are also widely different sizes. If you use a proper, but inaccurate emulator, you get different issues. Even if it was tuned to provide a decent estimate of cpu performance over average code (and they are typically tuned to overestimate cpu preformance, because people playing games would rather framedrops from real hardware are not emulated), it's just an average that doesn't take into account things like cache misses and branch misspredicts. If you were to write code with a lot of cache misses or branch misspredicts, your inaccurate emulator would massively overestimate it's preformance compared to a real cpu. The various issues just add up and it becomes impossible to profile and optimise the game you are developing unless you have an accurate emulator. Other solutions will all point to different parts of the code being hot. Also, remember this is within the era when you might be still developing a game with a software renderer, and if not you still have to do vertex transform and lighting on the cpu. Personally I'm not that interested in accurate emulation of PC, the issues get a lot worse when it comes to developing games for 5th, 6th and maybe even 7th gen consoles. That's were my true interest in accurate OoO emulation lies. But I can see why someone might want accurate PC emulation too.
  - AshamedCaptain 5y ago
    > If someone was to decide to develop a game for a minimum performance requirement of a Pentium III at 500mhz and some GPU, for some reason (which would be a completely arbitrary choice, but that's the hobby, so roll with it) then the only way they can possibly check that out meets that minimum requirement is to test on a machine with that configuration. This just doesn't happen in modern PC development, save for heavyweights who can afford multiple identical hardware configurations (e.g. HPC clusters). I know I'm repeating myself, but the variety of configurations just makes this highly implausible. Sure, you can be some demoscene type of guy who decides to target specifically this configuration, but then you're literally targeting one processor out of hundreds, and per your own words, the fact that it works on the 500Mhz doesn't mean it will work with the same performance on the next generation or even on the 550Mhz variant. I guess this is obviously fine, but really stretching it. You'll quickly end up having something that only works on your machine, with the same starting disk image, etc. Even PCem doesn't fully simulate the x86 cache because there is no benefit to it, and that includes cores from eras which were much more sensitive to timing. Branch mispredictions? Forget about it. Most P3 software is going to run concurrently to some other software, anyway. I'm not saying that you don't need a cycle-accurate simulator to get real timings. I'm saying that with such a large divergence in configurations and environments, virtualization (or any other inaccurate emulator) is likely to provide a performance level that is quite accurately somewhere in the interval. Most specially since you will have actually calibrated it to that interval beforehand :) Now on consoles I can see the benefit. Consoles are lots of identical hardware, operating systems that tend to get out of the way, and the people who develop for them only test (for obvious reasons) on the console hardware itself or at most a developer edition which has the same hardware (for obvious reasons again). You can have a silent bug that depends on timing of a mispredicted branch or the relative speed between the bus accesses of two cores and _never_ notice it since your testing environment is exactly 1 device (such a bug would immediate flare on a PC on like the 2nd reboot). Whatever it is that you develop for any one such console, it is highly likely it will work on all the million other sold consoles. Consoles are practically designed to have reproduce-able environments. On the other hand you practically can emulate the entire x86 software catalog with emulators which _still_ have large differences in behavior at the actual instruction level compared to the hardware, so the instruction timing doesn't really seem important, and creating now some software that does depend on it seems .. complicated. As an anecdote, not long ago I was working on a x86 emulator, and to my horror I realized that the push/pop instructions were actually miscomputing the operand size on a rather common but not primary situation (long mode but with a 32-bit segment). The emulator was pushing the stack by double the amount it should, and pushing/popping the high dword of registers it shouldn't have clobbered. This was actually happening in some of the most critical operating system code out there (bootloaders, WoW, etc.) ... and yet the bug had been in the emulator for years and no one had been the wiser, booting 64-bit OSes just fine :)

by phire 5y ago

AshamedCaptain 5y ago

> How do you know if your game will run at 60 fps if the execution times aren't accurate? Again, this is not a console. If you rely on a specific Pentium 3's instruction timings to reach 60 fps, your game is not going to reach 60fps _on any other PC_, not even if someone has an identical CPU, since any single other difference in hardware, configuration, or even layout of the filesystem is going to matter much more. You just can't get away with the same kinds of bug you can get away with in consoles, because even just trying ATI vs NVIDIA (or any two different brands of accelerator) is already going to be a completely different environment and timings, likely enough to trigger all those bugs (or at least more than different instruction timings will). i.e. even the simplest of emulators (incl. a virtualizer) with a runtime cap is going to suffice for the usecase of mildly estimating a framerate based on the CPU of some era. And there's very little value to increase the accuracy of such estimation since with so wildly varying PC hardware anything you can produce is going to be irrelevant anyway. (How to make a similar accurate-enough estimation of GPU performance is a different story).

memetomancer 5y ago
There is a world of difference between your inability to see the appeal/utility of a thing and that thing being actually worthless, as you are so stridently insisting here. For a mental exercise, just to prove it to yourself, why don't _you_ try examining all the reasons such accurate emulation might be desirable? The other poster already provided a reasonable motivation - preservation. But since this discussion started, you've only really come out swinging with disparagement. One has to wonder why you are putting so much energy into suppressing and trashing someone's hobby. Frankly it looks to me like you have some kind of preconceived and inflexible bias, or that you maybe trying to discover the appeal to this effort in a pointlessly adversarial way. Not a great look. If you really want people to think you have a smart, winning argument maybe try to show some understanding of both sides of the coin before floating your attempt at a clever and withering denouncement.
- nitrogen 5y ago
  To add an example to your arguments, the C64 and PC/XT demo scenes show what can be done with cycle accurate retrocomputing. Especially 8088mph: https://youtu.be/yHXx3orN35Y https://youtu.be/yHXx3orN35Y
  - AshamedCaptain 5y ago
    Do you know a lot of demo scene binaries that only work with accurate instruction timings of a Pentium 3 ?
    - nitrogen 5y ago
      What's your fixation here? You're wasting a lot of people's time.
      - deleted 5y ago
        [deleted]
      - AshamedCaptain 5y ago
        Feel free to downvote.
- anthk 5y ago
  PC games stopped being CPU cycle-bound long ago in mid-late 90's. The most issues you would have it's for high 486-Pentium I-II era games (specially the multimedia ones) which lots of them were speed bound, but for sure these games will be interpreted by ScummVM one day or another (Macromedia Director engine).
- AshamedCaptain 5y ago
  > There is a world of difference between your inability to see the appeal/utility of a thing and that thing being actually worthless, as you are so stridently insisting here. I am asking a question, and answering "preservation" (which is not really the answer the poster made, since his goal is new development) without giving an actual concrete example of what behavior needs such accurate preservation kind of defeats the purpose of asking the question in the first place. If the answer is "for the sake of it" that is also fine. But I'm unaware of anything post-P3 that would really require cycle-level emulation, so I ask. Most PC emulators "draw the line" around that era for a reason, even the ones who wouldn't necessarily have performance problems with newer machines (like DosBox). > But since this discussion started, you've only really come out swinging with disparagement. One has to wonder why you are putting so much energy into suppressing and trashing someone's hobby. What do you think? Because not only I have the same hobby, my work is also related to this. I am most definitely not interested in thrashing it. It is also the _main thesis_ of this entire article, so why shouldn't we discuss it?
phire 5y ago
If someone was to decide to develop a game for a minimum performance requirement of a Pentium III at 500mhz and some GPU, for some reason (which would be a completely arbitrary choice, but that's the hobby, so roll with it) then the only way they can possibly check that out meets that minimum requirement is to test on a machine with that configuration. Either a real machine, or a accurate emulator. It doesn't matter if there is other PC hardware configurations out there with different preformance. A minimum requirements just means "I tested on this machine, and it meets the minimums." Ideally you should underspec your minimum requirement test machine so that your target audience can be reasonably expected meet it. You can't substitute in virtualization. That has zero chance in hell of providing a realistic estimate of performance, even if you paired it with an accurate gpu emulator. Modern CPUs simply have very different performance characters, instructions that might have huge stalls on the P3 might be extremely cheap under virtualization. Caches are also widely different sizes. If you use a proper, but inaccurate emulator, you get different issues. Even if it was tuned to provide a decent estimate of cpu performance over average code (and they are typically tuned to overestimate cpu preformance, because people playing games would rather framedrops from real hardware are not emulated), it's just an average that doesn't take into account things like cache misses and branch misspredicts. If you were to write code with a lot of cache misses or branch misspredicts, your inaccurate emulator would massively overestimate it's preformance compared to a real cpu. The various issues just add up and it becomes impossible to profile and optimise the game you are developing unless you have an accurate emulator. Other solutions will all point to different parts of the code being hot. Also, remember this is within the era when you might be still developing a game with a software renderer, and if not you still have to do vertex transform and lighting on the cpu. Personally I'm not that interested in accurate emulation of PC, the issues get a lot worse when it comes to developing games for 5th, 6th and maybe even 7th gen consoles. That's were my true interest in accurate OoO emulation lies. But I can see why someone might want accurate PC emulation too.
- AshamedCaptain 5y ago
  > If someone was to decide to develop a game for a minimum performance requirement of a Pentium III at 500mhz and some GPU, for some reason (which would be a completely arbitrary choice, but that's the hobby, so roll with it) then the only way they can possibly check that out meets that minimum requirement is to test on a machine with that configuration. This just doesn't happen in modern PC development, save for heavyweights who can afford multiple identical hardware configurations (e.g. HPC clusters). I know I'm repeating myself, but the variety of configurations just makes this highly implausible. Sure, you can be some demoscene type of guy who decides to target specifically this configuration, but then you're literally targeting one processor out of hundreds, and per your own words, the fact that it works on the 500Mhz doesn't mean it will work with the same performance on the next generation or even on the 550Mhz variant. I guess this is obviously fine, but really stretching it. You'll quickly end up having something that only works on your machine, with the same starting disk image, etc. Even PCem doesn't fully simulate the x86 cache because there is no benefit to it, and that includes cores from eras which were much more sensitive to timing. Branch mispredictions? Forget about it. Most P3 software is going to run concurrently to some other software, anyway. I'm not saying that you don't need a cycle-accurate simulator to get real timings. I'm saying that with such a large divergence in configurations and environments, virtualization (or any other inaccurate emulator) is likely to provide a performance level that is quite accurately somewhere in the interval. Most specially since you will have actually calibrated it to that interval beforehand :) Now on consoles I can see the benefit. Consoles are lots of identical hardware, operating systems that tend to get out of the way, and the people who develop for them only test (for obvious reasons) on the console hardware itself or at most a developer edition which has the same hardware (for obvious reasons again). You can have a silent bug that depends on timing of a mispredicted branch or the relative speed between the bus accesses of two cores and _never_ notice it since your testing environment is exactly 1 device (such a bug would immediate flare on a PC on like the 2nd reboot). Whatever it is that you develop for any one such console, it is highly likely it will work on all the million other sold consoles. Consoles are practically designed to have reproduce-able environments. On the other hand you practically can emulate the entire x86 software catalog with emulators which _still_ have large differences in behavior at the actual instruction level compared to the hardware, so the instruction timing doesn't really seem important, and creating now some software that does depend on it seems .. complicated. As an anecdote, not long ago I was working on a x86 emulator, and to my horror I realized that the push/pop instructions were actually miscomputing the operand size on a rather common but not primary situation (long mode but with a 32-bit segment). The emulator was pushing the stack by double the amount it should, and pushing/popping the high dword of registers it shouldn't have clobbered. This was actually happening in some of the most critical operating system code out there (bootloaders, WoW, etc.) ... and yet the bug had been in the emulator for years and no one had been the wiser, booting 64-bit OSes just fine :)