问题
In my program I see some resident size increase. I suppose it is because of heap fragmentation.
So I am planning to use #pragma pack 1.
Will it reduce the heap fragmentation?
Will it be having some other overheads?
Shall I go for it or not?
回答1:
Packing structures probably won't have much affect on heap fragmentation. Heap fragmentation normally occurs when there is a repeating pattern of allocations and freeing of memory. There are two issues here, one issue is that the virtual address space gets fragmented, the other issue is that physical 4k pages end up with unused gaps, consuming increasing amounts of memory over time. Microsoft addresses the 4k page issue with it's .net framework that occasionally repacks memory, but does so by "pausing" a .net application during the repacks. I'm not sure how server apps that run 24 hours a day / 7 days a week deal with this without having to deal with the pauses, unless they occasionally fork off a new process to take over the server side and then close down the old process which would refresh the new process virtual address space with a new set of pages.
回答2:
There is a well proved technique called Memory pools. It is designed especially to reduce memory fragmentation and help with memory leaks. And it should be used in case where memory fragmentation became the bottleneck of the program functionality.
'pragma pack 1' isn't helpful to avoid heap fragmentation.
'pragma pack 1' is used to remove the padding bytes from structures to help with transmission of binary structures between programs.
回答3:
It's simply how the operating system works. When you free some memory you've allocated, it's not unmapped from the process memory map. This is kind of an optimization from the OS in case the process needs to allocate more memory again, because then the OS doesn't have to add a new mapping to the process memory map.
回答4:
#pragma pack N, tells the compiler to align members of structure in a particular way, with (N-1) bytes padding. For example, if N is 2, each char will occupy 2 bytes, one assigned, one padding. With N being 1, there will be no padding. This will have more fragmentation, as there would be odd number of bytes if the structure has say one char and one int, totaling 5 bytes.
Check: #pragma pack effect
回答5:
If you're specifically concerned about heap fragmentation then you might want to increase the structure packing. This would (occasionally) result in different structures being distributed between fewer different-sized buckets and reducing the likelihood of allocations leaving unusable gaps when they occupy the space of a previously-freed, slightly larger structure.
But this is unlikely to be your real concern. As another answer points out, the OS does not reclaim freed memory right away, and this can affect the apparent memory usage of the process.
来源：https://stackoverflow.com/questions/23487999/can-pragma-pack-1-be-helpful-to-avoid-heap-fragmentation