Linux Memory Management

MEMORY MANAGEMENTLinux kernel training, session 8

Eugeniu Roşca

ARM MMU. For docs see

http://infocenter.arm.com

3G/1G split

LINUX ADDRESS SPACES. FIST GLANCE

CONFIG_PAGE_OFFSET

cat /proc/vc-mem

LINUX ADDRESS SPACES. RASPBERRY PI.

Raspberry PI: Addresses in ARM Linux are:

• issued as virtual addresses by the ARM core, then

• mapped into a physical address by the ARM MMU, then

• mapped into a bus address by the ARM mapping MMU, and finally

• used to select the appropriate peripheral or location in RAM.

Address types in Linux, LDD3:1. virtual addresses = addresses allocated for user-space programs;

2. physical addresses = addresses used between the processor and the system’s memory;

In case of RasPi, translation done by ARM MMU.

3. bus addresses = addresses used between peripheral buses and memory.

Translation done by unit called IOMMU (in RasPi case: GPU MMU).

4. kernel logical addresses: normal address space of the kernel. One-to-one mapping to

physical addresses (+offset).

5. kernel virtual addresses: Also belong to kernel space only. But, not having one-to-one

mapping to physical. Cannot be represented by a void pointer.

ADDRESS TYPES. CORRELATION WITH LDD

GPU (VC)

MMU

BUS address

ARM classic

Raspberry PI

MMU. NORMAL CASE VS RASPBERRY PI.

/INCLUDE/LINUX/MMZONE.H

MEMORY ZONES. GENERIC CASE.


MEMORY ZONES. RASPBERRY PI CASE.

MEMORY ZONES. EXPLAINED.

- ZONE DMA:

- is memory in the lower (0-16 MB) physical memory range that certain ISA

devices require;

- At this point it exists for historical reasons => long time ago, there was

hardware that could only do DMA into this area of physical memory;

- ZONE DMA32:

- exists only in 64-bit Linux;

- it is the low 4 GBytes of memory, more or less;

- It exists because the transition to large memory 64-bit machines has created a

class of hardware that can only do DMA to the low 4 GBytes of memory

- ZONE NORMAL (for 32-bit machines):

- directly mapped by the kernel into the upper region of the linear address space;

- all RAM from 16 MB to 896 MB for complex and somewhat historical reasons;

- ZONE NORMAL (for 64-bit machines):

- all RAM from 4GB (or so) to the topmost limit;

- ZONE HIGHMEM

- exists ONLY on 32bit machines;

- all RAM above 896 MB;

- not directly mapped by the kernel;


KSWAPD. MEMORY ZONE WATERMARKS.

DOCUMENTATION / ARM / MEMORY.TXT

MEMORY LAYOUT ON ARM LINUX. 1.

DOCUMENTATION / ARM / MEMORY.TXT

MEMORY LAYOUT ON ARM LINUX. 2.

VMALLOC_START VMALLOC_END

PAGE_OFFSET high_memory

PKMAP_BSE PAGE_OFFSET - 1

MODULES_VADDR MODULES_END

TASK_SIZE - 1

Check Kernel

Definitions in

source code !

GOING DEEPER IN MEMORY

MANAGEMENT. STATIC

INCLUDE / LINUX / MMZONE.H

BUDDY ALLOCATOR.

- It is responsible for the management of the page allocations in the

entire system;

- It splits memory into pairs of 2n pages where n is in range

from 0 to MAX_ORDER;

- It stores information about the free blocks of pages in the array of

lists as follows:


BUDDY ALLOCATOR.

- Each list consists of free physically contiguous blocks of 2^i

memory pages, where i is the list number;

- Each of such blocks, except the block that consists of 1 page, can be

split into two halves and used as 2 blocks of a half size.

- So if no entries exist in the requested list, an entry from the next

upper list is broken into two separate clusters and one is returned

to the caller while the other one is added to the next lower list.

- On the other hand, every two blocks of memory of the same size,

which have common border (arranged in memory sequentially,

from the standpoint of physical addresses), may be united into the

single block of the bigger size.


BUDDY ALLOCATOR.

INTERNAL FRAGMENTATION VS EXTERNAL

FRAGMENTATION

External fragm: Space wasted outside of allocated memory blocks. Small

pieces of memory exist among the allocated blocks. However, a larger

allocation request cannot be fulfilled, because the free pieces are not

physically contiguous.

Internal fragm: Space wasted inside of allocated memory blocks because

of restriction on the allowed sizes of allocated blocks.

AVAILABLE KERNEL-SPACE MEMORY

ALLOCATION METHODS

MEMORY ALLOCATION API

GET PAGES DIRECTLY. BYPASS SLAB.








GET MEMORY THROUGH SLAB.

SLAB ALLOCATOR.

size-4194304 0 0 4194304 1 1024 : tunables 1 1 0 : slabdata 0 0 0




















kmem_cache 135 160 96 40 1 : tunables 120 60 0 : slabdata 4 4 0

cat /proc/slabinfo output:

SLAB ALLOCATOR.

SLAB ALLOCATOR.

KMALLOC ALLOCATOR.

KMALLOC ALLOCATOR.

KMALLOC ALLOCATOR.

VMALLOC ALLOCATOR.

/PROC INFORMATION

- /proc/iomem

- /proc/meminfo = see /Documentation/filesystems/proc.txt

- /proc/zoneinfo = see page_alloc.c

- /proc/vmstat/

- /proc/buddyinfo = debug External fragmentation. Lots of order 0

pages mean lots of fragmentation, (kernel can't even find two

adjacent aligned pages to merge into an 8kb order 1 chunk)

- /proc/sys/vm/* = see Documentation/sysctl/vm.txt

- /proc/pagetypeinfo = goal same as of buddyinfo (debug External

fragmentation)

- /proc/vmallocinfo = see /Documentation/filesystems/proc.txt

(iotable_init, ioremap, vmalloc)

- /proc/vc-mem/

Documents

Linux Memory Management