Tuesday, March 2, 2010

Linux memory management study notes

32-bit architectures can reference 4 GB (2^32) of physical memory.
#Virtual Memory: User-space virtual space resides lower 3G, Kernel virtual space resides upper 1G
#Physical Memory: Three zones:
ZONE_DMA=0-16M, ZONE_NORMAL=16-896M  (896M-1024M kernel reserved ) and   ZONE_HIGHMEM=>1024M

32-bit architectures
Kernel has page tables  to map "virtual addresses" to "physical addresses"
The kernel virtual area  is mapped from  HIGH  1GB ( 3GB-4GB)  virtual space to LOW ( 1 GB)  physical RAM.

- RAM size is less than 896 MB
Liner mapping is possible from 1GB kernel address to 1GB  of physical RAM,which are ZONE_DMA and ZONE_NORMAL (not including 128M reserved space).
Kernel page tables must transform linear addresses starting from 0xc0000000 (3GB) into physical addresses starting from 0.

- RAM size is between 896 MB and 4096 MB
Dynamic remapping is done in the 128M reserved space, because ZONE_HIGHMEM zone includes page frames that cannot be directly accessed by the kernel through the linear mapping.

- RAM size is more than 4096 MB
dynamic rempapping with three-level paging model.
(With PAE capable hardware and hugemem Linux kernel , 32bit Linux can support up to 64G memory)

64-bit architectures
ZONE_HIGHMEM is empty, all are ZONE_NORMAL, no remapping needed.
#/proc/meminfo displays physical memory info
#32bit Kernel has both low and high memory

[ 32bit Kernel]$ cat /proc/meminfo 
MemTotal:      3897500 kB
MemFree:       3280456 kB
HighTotal:     3014592 kB
HighFree:      2685548 kB
LowTotal:       882908 kB
LowFree:        594908 kB

#64 bit kernel ZONE_NORMAL is huge, all fits in Lowtotal, so HighTotal is zero

[64bit Kernel]$ cat /proc/meminfo  
MemTotal:     37025752 kB
MemFree:      17509720 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:     37025752 kB
LowFree:      17423152 kB

#From kernel document

HighFree: Highmem is all memory above ~860MB of physical memory, Highmem areas are for use by userspace programs, or for the pagecache.  The kernel must use tricks to access this memory, making it slower to access than lowmem.

LowFree: Lowmem is memory which can be used for everything that highmem can be used for, but it is also available for the kernel's use for its own data structures.  Among many other things, it is where everything from the Slab is allocated.  Bad things happen when you're out of lowmem.

BOOK:Understanding the Linux Kernel By Daniel Pierre Bovet, Marco Cesatí
High Memory In The Linux Kernel
Kernel document

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