Linux Memory Management FAQ
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Linux Memory Management FAQ
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Re: Linux Memory Management FAQ
#2[deleted]
Re: Linux Memory Management FAQ
#3Much of this applies to other OSes with virtual memory also.
Re: Linux Memory Management FAQ
#4"Virtual addresses are the size of a CPU register. On 32 bit systems each process has 4 gigabytes of virtual address space all to itself, which is often more memory than the system actually has."
I guess this is not the most up-to-date document?
Re: Linux Memory Management FAQ
#5For anybody who's interested I also wrote up a whole bunch of notes on this at https://github.com/lorenzo-stoakes/linux-vm-notes and superceded by far more recent https://github.com/lorenzo-stoakes/linux-mm-notes
I have made a few patches into the mm subsystem some simply inspired by researching for the articles.
Re: Linux Memory Management FAQ
#6"Virtual addresses are the size of a CPU register. On 32 bit systems each process has 4 gigabytes of virtual address space all to itself, which is often more memory than the system actually has." I guess this is not the most up-to-date document?
I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked.
address sizes : 36 bits physical, 48 bits virtual
address sizes : 40 bits physical, 48 bits virtual
Re: Linux Memory Management FAQ
#7The Drepper series of article dates from 2007. Is it still relevant or has anything fundamental changed in memory handling in the last 13 years ?
Re: Linux Memory Management FAQ
#8"Virtual addresses are the size of a CPU register. On 32 bit systems each process has 4 gigabytes of virtual address space all to itself, which is often more memory than the system actually has." I guess this is not the most up-to-date document?
I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked. address sizes : 36 bits physical, 48 bits virtual address sizes : 40 bits physical, 48 bits virtual PS: I don't understand what this means, btw.
Your CPU can handle 39-bit physical memory addresses (up to 512 GB of physical memory), and 48-bit virtual addresses (256 TB). Your operating system maintains a mapping from virtual to physical addresses, usually arranging the map so that every process has a separate memory space. Pointers are all still 64 bits long though.
Re: Linux Memory Management FAQ
#9"Virtual addresses are the size of a CPU register. On 32 bit systems each process has 4 gigabytes of virtual address space all to itself, which is often more memory than the system actually has." I guess this is not the most up-to-date document?
I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked. address sizes : 36 bits physical, 48 bits virtual address sizes : 40 bits physical, 48 bits virtual PS: I don't understand what this means, btw.
It means that it can address 40-bits of address space worth of physical memory, but that virtual memory addresses can use 48 bits. Physical addresses are just your RAM bytes numbered 1 through whatever. Virtual address space is the address space of a process, which includes mapped physical memory, unmapped pages, guard pages, and other virtual memory tricks.
Re: Linux Memory Management FAQ
#10Earlier quoted context omitted.
I think there might be some more hardware-specific nuance here. e.g. /proc/cpuinfo says this on a couple of different x86_64 systems that I checked. address sizes : 36 bits physical, 48 bits virtual address sizes : 40 bits physical, 48 bits virtual PS: I don't understand what this means, btw.
Your CPU can handle 39-bit physical memory addresses (up to 512 GB of physical memory), and 48-bit virtual addresses (256 TB). Your operating system maintains a mapping from virtual to physical addresses, usually arranging the map so that every process has a separate memory space. Pointers are all still 64 bits long though.
In practice the actual available usable address space for userland is 64 TiB due to user/kernel split and the kernel maintaining a virtual mapping of the entire physical address space (minus I/O ranges) [0].
However newer incoming 5-level page intel chips [1] will allow up to 57 bits of address space, 128 PiB in theory though in practice 32 PiB of userland memory. See also [0] for discussion on practical limit for 5-page too!
[0]:https://github.com/lorenzo-stoakes/linux-mm-notes/blob/maste...