4 ms·

Many physics simulations include internal checks (that must be there regardless of ECC, for algorithmic reasons) that would catch virtually all such issues. Hm

Many physics simulations include internal checks (that must be there regardless of ECC, for algorithmic reasons) that would catch virtually all such issues. Hmm, like what? I've run many simulations too and had the opposite experience. It's hard to implement checks because your algorithm defines how the simulation should permute over time. While a bit error could certainly crash the simulation, long running simulations can be resumed from checkpoints with minimal loss of time and the probability of a crash is still very low for most real workloads. Not really. Your algorithm determines how the simulation plays out, so you can't really notice small errors until much later. By then it's hard to roll back to an earlier state unless you're saving 64GB snapshots.

twic 9y ago
I don't know about checks, but any algorithm which works by iteratively converging on a solution, like Newton-Raphson and its bigger relatives, has quite a lot of bit-error tolerance built in.
- keldaris 9y ago
  That's right, any iterative solver method (which already encompasses most of the low level numerical solvers used in physics today) or, more generally, any method that algorithmically guarantees satisfying a particular mathematical constraint after each timestep (say, if you're enforcing fluid incompressibility at each timestep, most* single bitflips will not cause errors that would accumulate over time), etc. [*] There are exceptions in specific circumstances most of which would produce obviously wrong results or crash the simulation outright. However, I'm not claiming that single bitflips are never an issue for simulations, only that it happens much less often than people sometimes think.
  - sillysaurus3 9y ago
    If a one-bit error causes the sign bit to flip, your position can go from 129,121.34 to -129,121.34. Obviously this is rare, but it seems mistaken to say that the simulation's parameters can prevent errors arising from this. Also, it's not just physics simulations that are at risk. Voxel renderings are especially susceptible to one-bit errors.
    - keldaris 9y ago
      Not sure exactly what simulation you're referring to with the position example, in most of my code that would either be fixed in the next timestep or make the whole thing blow up immediately. Anyway, I haven't argued against using ECC memory in any of these comments. I've argued that you should know your needs and choose accordingly, and that ECC memory isn't free and therefore cannot be a universally optimal default for everyone. For much of my physics work I've found ECC memory not to be worth the cost at all. For other things, like most database servers (and perhaps voxel rendering, I don't have enough experience there to comment) ECC memory is clearly worth it. I just think people should invest some thought into the question before evangelizing for or against ECC in the general case.

twic 9y ago

I don't know about checks, but any algorithm which works by iteratively converging on a solution, like Newton-Raphson and its bigger relatives, has quite a lot of bit-error tolerance built in.

keldaris 9y ago
That's right, any iterative solver method (which already encompasses most of the low level numerical solvers used in physics today) or, more generally, any method that algorithmically guarantees satisfying a particular mathematical constraint after each timestep (say, if you're enforcing fluid incompressibility at each timestep, most* single bitflips will not cause errors that would accumulate over time), etc. [*] There are exceptions in specific circumstances most of which would produce obviously wrong results or crash the simulation outright. However, I'm not claiming that single bitflips are never an issue for simulations, only that it happens much less often than people sometimes think.
- sillysaurus3 9y ago
  If a one-bit error causes the sign bit to flip, your position can go from 129,121.34 to -129,121.34. Obviously this is rare, but it seems mistaken to say that the simulation's parameters can prevent errors arising from this. Also, it's not just physics simulations that are at risk. Voxel renderings are especially susceptible to one-bit errors.
  - keldaris 9y ago
    Not sure exactly what simulation you're referring to with the position example, in most of my code that would either be fixed in the next timestep or make the whole thing blow up immediately. Anyway, I haven't argued against using ECC memory in any of these comments. I've argued that you should know your needs and choose accordingly, and that ECC memory isn't free and therefore cannot be a universally optimal default for everyone. For much of my physics work I've found ECC memory not to be worth the cost at all. For other things, like most database servers (and perhaps voxel rendering, I don't have enough experience there to comment) ECC memory is clearly worth it. I just think people should invest some thought into the question before evangelizing for or against ECC in the general case.