5 ms·

That’s one of the many reasons I think Windows NT kernel is generally better than Linux. Windows doesn’t do that. When you don’t have enough memory and not eno

That’s one of the many reasons I think Windows NT kernel is generally better than Linux. Windows doesn’t do that. When you don’t have enough memory and not enough page file space either, these allocation functions usually do fail returning nullptr.

milkey_mouse 5y ago
I've found overcommit useful for some scientific computing applications, but you're right it's not the most intuitive default. You can disable memory overcommit on Linux for the same behavior as Windows, if you want. https://www.kernel.org/doc/Documentation/vm/overcommit-accounting https://www.kernel.org/doc/Documentation/vm/overcommit-accou... Also, I found this post that suggests Windows technically does overcommit memory, but only for stacks(‽): https://superuser.com/questions/1194263/will-microsoft-windows-10-overcommit-memory https://superuser.com/questions/1194263/will-microsoft-windo...
- toast0 5y ago
  Disabling overcommit on Linux doesn't tend to enable library and utility developers to have used memory responsibly. So it can be hard to run a system with common software.
  - milkey_mouse 5y ago
    I keep it disabled on one of my systems for exactly that reason: I want to make sure my own code works with overcommit disabled, as it'll always be on Windows or OpenBSD.
tialaramex 5y ago
If you really want that you can just tell Linux not to overcommit. I can foresee your next question though, "Wait, why did I run out of memory? Stupid Linux, I have plenty of RAM".
- dodobirdlord 5y ago
  Well, it would be fair to call overcommit-less Linux stupid. There are a number of aspects of Linux that are only reasonable if you assume that memory overcommit is a thing, and would be radically stupid to include in an OS that does not overcommit by default. For example, fork(), which only makes sense with memory overcommit and shared copy-on-write pages. Process is using 50.1% of the physical memory? Guess it can’t spawn processes, since it has to fork() before it can exec() and there’s not enough memory to copy the process! I’m sure there are other horrendously degraded modes that Linux can theoretically operate in (read-only file system?, unreliable system clock?), but disabling overcommit, while possible, turns Linux into an exceedingly crummy OS.
dahart 5y ago
But this feature is also why malloc on Windows can take a long time to return - seconds or more if you malloc too much. I recently bumped into this problem on the GPU. Since GPU memory in the Windows Display Driver Model requires it to be backed by the host machine’s virtual memory, a malloc on the GPU when running low on space can end up searching & even defragging virtual memory to satisfy the request. Crazy! Just asking for RAM can cause you to land in heavy disk swap. I’ve seen cases of cudaMalloc taking minutes because of how Windows handles allocation, and the same is true of CPU mallocs.
- ayende 5y ago
  What would you rather have, in this case? Assuming that the work needs to be done? You can either do that on allocation (which you can predict, batch, etc) or you can do that when you need the next page to be used. Which is far harder to predict and work with.
- bscphil 5y ago
  > Since GPU memory in the Windows Display Driver Model requires it to be backed by the host machine’s virtual memory Wow, this seems really unintuitive to me, especially on a OS that doesn't overcommit. Is it as unintuitive as it sounds, or is there a good reason for this?
  - dahart 5y ago
    There are good reasons, not all of which I know or understand, but my basic mental model is that the backing is required for various scenarios where virtual memory might need to get relocated, either in response to a malloc or to something else. I’m told it’s complicated by having the GPU drive the display, and by scenarios involving multiple GPUs in a system. I’m guessing the description in the WDDM docs about DDI might be relevant to what’s happening under the covers in WDDM2? I’m not sure… :) https://docs.microsoft.com/en-us/windows-hardware/drivers/display/gpu-virtual-memory-in-wddm-2-0 https://docs.microsoft.com/en-us/windows-hardware/drivers/di...
  - Const-me 5y ago
    > is there a good reason for this? Multitasking. User can easily alt+tab into another program which also uses GPU. When many processes are using GPU concurrently, it's possible their combined VRAM use exceeds the amount of physical memory available on the GPU.

milkey_mouse 5y ago

I've found overcommit useful for some scientific computing applications, but you're right it's not the most intuitive default. You can disable memory overcommit on Linux for the same behavior as Windows, if you want. https://www.kernel.org/doc/Documentation/vm/overcommit-accounting https://www.kernel.org/doc/Documentation/vm/overcommit-accou... Also, I found this post that suggests Windows technically does overcommit memory, but only for stacks(‽): https://superuser.com/questions/1194263/will-microsoft-windows-10-overcommit-memory https://superuser.com/questions/1194263/will-microsoft-windo...

toast0 5y ago
Disabling overcommit on Linux doesn't tend to enable library and utility developers to have used memory responsibly. So it can be hard to run a system with common software.
- milkey_mouse 5y ago
  I keep it disabled on one of my systems for exactly that reason: I want to make sure my own code works with overcommit disabled, as it'll always be on Windows or OpenBSD.

tialaramex 5y ago

If you really want that you can just tell Linux not to overcommit. I can foresee your next question though, "Wait, why did I run out of memory? Stupid Linux, I have plenty of RAM".

dodobirdlord 5y ago
Well, it would be fair to call overcommit-less Linux stupid. There are a number of aspects of Linux that are only reasonable if you assume that memory overcommit is a thing, and would be radically stupid to include in an OS that does not overcommit by default. For example, fork(), which only makes sense with memory overcommit and shared copy-on-write pages. Process is using 50.1% of the physical memory? Guess it can’t spawn processes, since it has to fork() before it can exec() and there’s not enough memory to copy the process! I’m sure there are other horrendously degraded modes that Linux can theoretically operate in (read-only file system?, unreliable system clock?), but disabling overcommit, while possible, turns Linux into an exceedingly crummy OS.

dahart 5y ago

But this feature is also why malloc on Windows can take a long time to return - seconds or more if you malloc too much. I recently bumped into this problem on the GPU. Since GPU memory in the Windows Display Driver Model requires it to be backed by the host machine’s virtual memory, a malloc on the GPU when running low on space can end up searching & even defragging virtual memory to satisfy the request. Crazy! Just asking for RAM can cause you to land in heavy disk swap. I’ve seen cases of cudaMalloc taking minutes because of how Windows handles allocation, and the same is true of CPU mallocs.

ayende 5y ago
What would you rather have, in this case? Assuming that the work needs to be done? You can either do that on allocation (which you can predict, batch, etc) or you can do that when you need the next page to be used. Which is far harder to predict and work with.
bscphil 5y ago
> Since GPU memory in the Windows Display Driver Model requires it to be backed by the host machine’s virtual memory Wow, this seems really unintuitive to me, especially on a OS that doesn't overcommit. Is it as unintuitive as it sounds, or is there a good reason for this?
- dahart 5y ago
  There are good reasons, not all of which I know or understand, but my basic mental model is that the backing is required for various scenarios where virtual memory might need to get relocated, either in response to a malloc or to something else. I’m told it’s complicated by having the GPU drive the display, and by scenarios involving multiple GPUs in a system. I’m guessing the description in the WDDM docs about DDI might be relevant to what’s happening under the covers in WDDM2? I’m not sure… :) https://docs.microsoft.com/en-us/windows-hardware/drivers/display/gpu-virtual-memory-in-wddm-2-0 https://docs.microsoft.com/en-us/windows-hardware/drivers/di...
- Const-me 5y ago
  > is there a good reason for this? Multitasking. User can easily alt+tab into another program which also uses GPU. When many processes are using GPU concurrently, it's possible their combined VRAM use exceeds the amount of physical memory available on the GPU.