PCSX2 Documentation/VirtualAlloc on Linux: Difference between revisions
PCSX2 Documentation/VirtualAlloc on Linux (view source)
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''Originally written by Jake Stine'' | ''Originally written by Jake Stine'' | ||
Yes, there is a way to simulate Microsoft's VirtualAlloc behavior on Linux. Much searching of the internet did not reveal a satisfactory answer; only hints that when combined with some applied tests of my own yielded the following result: | Yes, there is a way to simulate Microsoft's VirtualAlloc behavior on Linux. Much searching of the internet did not reveal a satisfactory answer; only hints that when combined with some applied tests of my own yielded the following result: | ||
<source lang="cpp"> | |||
// to RESERVE memory in Linux, use mmap with a private, anonymous, non-accessible mapping. | // to RESERVE memory in Linux, use mmap with a private, anonymous, non-accessible mapping. | ||
// The following line reserves 1gb of ram starting at 0x10000000. | // The following line reserves 1gb of ram starting at 0x10000000. | ||
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int result3 = mprotect((void*)0x10000000, 0x100000, PROT_READ | PROT_WRITE); | int result3 = mprotect((void*)0x10000000, 0x100000, PROT_READ | PROT_WRITE); | ||
</source> | |||
When using mmap, you can create a simple uncommitted reservation of memory simply by specifying PROT_NONE on any anonymous mapping (in the world of mmap, anonymous means it has no associated file/pipe -- it's just a memory block). This is sufficient for reserving a large contiguous address range from being fragmented up by the likes of malloc. Granting the memory read and/or write privileges tells Linux to commit the memory (equivalent to VirtualAlloc with MEM_COMMIT). If there is not enough system memory to complete the call, it returns -1. | When using mmap, you can create a simple uncommitted reservation of memory simply by specifying PROT_NONE on any anonymous mapping (in the world of mmap, anonymous means it has no associated file/pipe -- it's just a memory block). This is sufficient for reserving a large contiguous address range from being fragmented up by the likes of malloc. Granting the memory read and/or write privileges tells Linux to commit the memory (equivalent to VirtualAlloc with MEM_COMMIT). If there is not enough system memory to complete the call, it returns -1. | ||
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Oddly enough, though, Linux makes it so that it isn't even necessary to bother with the above solution, via a strange little hacky technique called... | Oddly enough, though, Linux makes it so that it isn't even necessary to bother with the above solution, via a strange little hacky technique called... | ||
Over-committing Memory | ==Over-committing Memory== | ||
This 'feature' is enabled by default in most modern Linux kernels (anything 2.6 or newer). Basically all this means is that Linux will let programs commit a lot more RAM than is actually available to the operating system! Instead of performing a "strict contract" on commit that says "oh yes we absolutely have this much ram available", Linux looks at the ram and looks at the request, and makes some arbitrary judgement call on if the program will actually use that much ram or not. In other words, just because your call to malloc returned a valid non-NULL pointer doesn't mean there's actually anywhere near that much memory available to your app. It just means that Linux doesn't think you're going to use that much. | This 'feature' is enabled by default in most modern Linux kernels (anything 2.6 or newer). Basically all this means is that Linux will let programs commit a lot more RAM than is actually available to the operating system! Instead of performing a "strict contract" on commit that says "oh yes we absolutely have this much ram available", Linux looks at the ram and looks at the request, and makes some arbitrary judgement call on if the program will actually use that much ram or not. In other words, just because your call to malloc returned a valid non-NULL pointer doesn't mean there's actually anywhere near that much memory available to your app. It just means that Linux doesn't think you're going to use that much. | ||
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Another positive for the the above mmap / mprotect example is that it will also work well on Linux systems that have over-commit disabled, since it basically does what over-commit does but without the hacky "programs die instantly without error" part if the system runs out of physical memory. | Another positive for the the above mmap / mprotect example is that it will also work well on Linux systems that have over-commit disabled, since it basically does what over-commit does but without the hacky "programs die instantly without error" part if the system runs out of physical memory. | ||
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