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I was hoping someone had some schooling they could lay down about the whole HEAP and stack ordeal. I am trying to make a program that would attempt to create about 20,000 instances of just one union and if so some day I may want to implement a much larger program. Other than my current project consisting of a maximum of just 20,000 unions stored where ever c++ will allocate them do you think I could up the anti into the millions while retaining a reasonable return speed on function calls, approximately 1,360,000 or so? And how do you think it will handle 20,000?
Heap is an area used for dynamic memory allocation.
It's usually used to allocate space for variable collection size, and/or to allocate a large amount of memory. It's definitely not a CPU register(s).
Besides this I think there is no guarantee what heap is.
This may be RAM, may be processor cache, even HDD storage. Let OS and hardware decide what it will be in particular case.
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I am trying to code a relatively simple program to understand in which scenarios it would be more efficient and useful to use the heap.
I first read that you better store large objects on the heap.
So I created a std::vector on the heap and filled it with an insane amount of bytes, something like 18Gb. At some point, it threw a std::bad_alloc exception, then my OS (Linux mint) killed the process after the swap was full. But the result was the same with the stack, so I don't understand how it's better on the heap.
Maybe I lack creativity, but I cannot think of a design where I would absolutely need to use the heap, I can always pass a reference of my object on the stack and achieve the same memory usage efficiency. For every program I wrote, it was always more efficient to use the stack speed wise and was the same for memory usage.
So, in what scenario is it useful to use the heap regarding memory usage efficiency or speed?
You use the heap whenever you need more flexibility than the stack provides. If you need to allocate memory to return something from a function, then it can't be on the stack, because it would be freed when the function returned. And it can't be global, because you might call the same function more than once.
In C++, you might not always realize that you are using the heap, because classes like std::vector and std::string use the heap internally. You don't always have to write new yourself to use the heap.
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I'm working on software using a library with a huge memory usage (e.g. LightGBM).
I'm working on a data-science software which as the property to reduce RAM usage dynamically when data is not asked, and reload it from disk when necessary depending on our needs, kind of an advanced and configurable swap to sum up.
Therefore, when I call external code, we except that memory follows kind of the same requirements.
When working on huge dataset, memory usage can go way further available memory, the idea his to limit memory usage to avoid being stuck at 100% memory usage.
As soon as I don't want to modify memory management within LightGBM's code because it would mean choose a specific version and re-adapt code each time I want to update. In my software, can I programmatically restrict (and later release) physical RAM usage of my application, to force swapping?
Excepted pseudo-code:
some_function_before();
some_API::please_use_swap(/*threshold=*/16);
some_process_with_heavily_memory_usage();
some_API::end_requirement();
some_function_after();
If there is another approach to resolve this, I'll pick it of course.
Thanks.
There's such an API on Windows: SetProcessWorkingSetSize. You state how much physical RAM you want to use; the rest could be paged out.
As is normal, this is just a hint. Windows may decide that there's plenty of RAM and ignore your hint altogether.
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Preempt: (if anyone can link helpful articles explaining the stack and heap at a deep level, up to registers, would be much appreciated)
I am new to C++: I am trying to really grasp how memory management works. At this point, I understand any declaration of the type ObjClass obj; to have automatic duration within the scope it's declared in. Yet, ObjClass* obj = new ObjClass(); stores the obj pointer on stack, but assigns it an address of memory on the heap. What I'm wondering about is, in more complex programs, what design pattern is used to prevent stack overflow? I could see the storage on stack quickly exceeding 1mb. Is this achieved by making multiple smaller functions which run, use stack, and then automatically deallocate?
Related Q: As for global variables, I know they are held in "static" storage, yet am unsure how that works in the context of stack/heap. How is their memory allocated, and is there a small limit like the stack? Is heap close to the size of system RAM minus the OS-reserved memory?
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I'm still new to C++ but I catch on quick and also have experience in C#. Something I want to know is, what are some performance safe actions I can take to ensure that my game runs efficiently. Also, in which scenario am I likely to run out of memory on. 2k to 3k bullet objects on the stack, or heap? I think the stack is generally faster, but I heard that too much causes a stack overflow. That being said, how much is too much exactly?
Sorry for the plethora of questions, I just want to make sure I don't design a game engine in which it relies on good PCs in order to run well.
Firstly, program your game safely and only worry about optimizations like memory layout after profiling & debugging.
That being said, I have to dispel the myth that the stack is faster than the heap. What matters is cache performance.
The stack generally is faster for small quick accesses, because the stack usually already is in the cache. But when you are iterating over thousands of bullet objects on the heap, as long as you store them contiguously (e.g. std::vector, not std::list), everything should be loaded into the cache, and there should be no performance difference.
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Generally when do you allocate memory in C++, you have new/delete and virtualalloc, amongst a few other API calls, is the generally for dynamic allocation, but then we have vector and such so what are the common uses for allocating memory?
If you don't know, at compilation time, how many items you will need, your best option is to use dynamic allocation.
That way you can (hopefully) deal with all the input without wasting memory by reserving an humongous space with a big array.
// ...
int humongous[10000]; // I only expect 10 items, so this should be enough for creative users
// ...
If you want to deal with large memory (i.e memory that can't be allocated on stack) then you can use dynamic allocation.
As a general answer: "there may be cases where the memory needs of a program can only be determined during runtime. For example, when the memory needed depends on user input. On these cases, programs need to dynamically allocate memory, for which the C++ language integrates the operators new and delete."
source: http://www.cplusplus.com/doc/tutorial/dynamic/