Chapter 12. Kernel Memory Allocation

1. Chapter 12. Kernel Memory Allocation • Introduction • Resource Map Allocator • Simple Power-of-Two Free Lists • Mckusick-Karels Allocator • Buddy System • Mach-OSF/1 Zone Allocator • Solaris 2.4 Slab Allocator

2. Introduction • Page-level allocator • Paging system • Supports virtual memory system • Kernel memory allocator • Provides odd-sized buffers of memory to various kernel subsystems • Kernel frequently needs chunks of memory of various sizes usually for short periods of time

3. Introduction (cont) • Users of the kernel memory allocator • pathname translation routine • allocb( ) allocates STREAMs buffers of arbitrary size • In SVR4, the kernel allocates many objects (proc structures, vnodes, file descriptor blocks) dynamically when needed

4. Introduction (cont) physical memory Page-level allocator kernel memory allocator Paging system network buffers proc structures inodes, file descriptors user processes block buffer cache

5. Kernel Memory Allocator (KMA) • Functional requirements • Page-level allocator pre-allocates part of main memory to KMA, which must use this memory pool efficiently • KMA also have to monitor which part of its pool are allocated and which are free • Evaluation criteria • Utilization factor • KMA must be fast • Both average and worst-cast latency are important • Interaction with the paging system

6. Resource Map Allocator • Resource map • set of <base, size> for free memory • First fit, Best fit, Worst bit policies • Advantages • Easy to implement • Not restricted to memory allocation • Allocates the exact numbers of bytes requested • Client is not constrained to release the exact region it has allocated • Allocator coalesces adjacent free regions

7. Resource Map Allocator (cont) • Drawbacks • Map may become highly fragmented • As the fragmentation increases, so does the size of the map • Sorting for coalescing adjacent regions is expensive • Linear search to find a free region

8. Simple Power-of-Two Free List list headers free buffers allocated blocks 32 64 128 256 512 1024

9. Simple Power-of-Two Free List (cont) • Used frequently to implement malloc( ), and free( ) in the user-level C library • One-word header for each buffer • Advantages • Avoids the lengthy linear search of the resource map method • Eliminates the fragmentation problem

10. Simple Power-of-Two Free List (cont) • Drawbacks • Rounding of requests to the next power of two results in poor memory utilization • No provision for coalescing adjacent free buffer to satisfy larger requests • No provision to return surplus free buffers to the page-level allocator

11. Mckusick-Karels Allocator freelistaddr[ ] free buffers 32 allocated blocks 64 128 256 512 kmemsizes[ ] 32 512 64 F 32 128 F 32 32 256 64 F 2K F 128 freepages

12. Mckusick-Karels Allocator (cont) • Used in 4.4BSD, Digital UNIX • Requires that • a set of contiguous pages • all buffers belonging to the same page to be the same size • States of each page • Free • corresponding element of kmemsizes[ ] contains a pointer to the element for the next free page • Divided into buffers of a particular size • kmemsizes[ ] element contains the size

13. Mckusick-Karels Allocator (cont) • Part of a buffer that spanned multiple pages • kmemsizes[ ] element corresponding to the first page of the buffer contains the buffer size • Improvement over simple power-of-two • Faster, wastes less memory, can handle large and small request efficiently • Drawbacks • No provision for moving memory from one list to another • No way to return memory to the paging system

14. Buddy System • Basic idea • Combines free buffer coalescing with a power-of-two allocator • When a buffer is split, each buffer is called the buddy of the other • Advantages • Coalescing adjacent free buffers • Easy exchange of memory between the allocator and the paging system • Disadvantages • Performance degradation due to coalescing • Program interface needs the size of the buffer

15. Buddy System (e.g.) allocate(256)  allocate(128)  allocate(64) bitmap 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 free list headers 32 64 128 256 512 0 256 384 448 512 1023 B C D D’ A’ C’ B’ free in use A

16. Buddy System (e.g.)  allocate(128)  release(C, 128) bitmap 1 1 1 1 1 1 1 1 0 0 0 0 1 1 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 free list headers 32 64 128 256 512 0 256 384 448 512 640 768 1023 B C D D’ F F’ E’ C’ B’ E A A’

17. Buddy System (e.g.)  release(D, 64) bitmap 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 free list headers 32 64 128 256 512 0 256 512 640 768 1023 B B’ F F’ E’ C’ E A

18. SVR4 Lazy Buddy System • Basic idea • Defer coalescing until it becomes necessary, and then to coalesce as many buffers as possible • Lazy coalescing • N buffers, A buffers are active, L are locally free, G are globally free • N = A + L + G • slack = N - 2L - G = A - L

19. SVR4 Lazy Buddy System (cont) • Lazy state (slack = 0) • buffer consumption is in a steady state and coalescing is not necessary • Reclaiming state (slack = 1) • consumption is borderline, coalescing is needed • Accelerated state (slack = 2 or more) • consumption is not in a steady state, and the allocator must coalesce faster

20. Mach-OSF/1 Zone Allocator active zones free objects free_elements zone of zones structzone structzone structzone next_zone first_zone zone of zones structobj1 structobj1 structobj1 last_zone ... zone of zones structobjn struct objn struct objn

21. Mach-OSF/1 Zone Allocator (cont) • Basic idea • Each class of dynamically allocated objects is assigned to its own size, which is simply a pool of free objects of that class • Any single page is only used for one zone • Free objects of each zone are maintained on a linked list, headed by a struct zone • Allocation and release involve nothing more than removing objects from and returning objects to the free list • If an allocation request finds the free list empty, it asks the page-level allocator for alloc( ) more bytes

22. Mach-OSF/1 Zone Allocator (cont) • Garbage collection • Free pages can be returned to the paging system and later recovered for other zones • Analysis • Zone allocator is fast and efficient • No provision for releasing only part of the allocated object • Zone objects are exactly the required size • Garbage collector provides a mechanism for memory reuse

23. Hierarchical Allocator for Multiprocessors per-pages freelists Coalesce-to-Page layer Global layer global freelist bucket list target = 3 main freelist Per-CPU cache aux freelist target = 3

24. Solaris 2.4 Slab Allocator • Better performance and memory utilization than other implementation • Main issues • Object reuse • advantage of caching and reusing the same object, rather than allocating and initializing arbitrary chunks of memory • Hardware cache utilization • Most kernel objects have their important, frequently accessed fields at the beginning of the object

25. Solaris 2.4 Slab Allocator • Allocator footprint • Footprint of a allocator is the portion of the hardware cache and the translation lookaside buffer (TLB) that is overwritten by the allocator itself • Large footprint causes many cache and TLB misses, reducing the performance of the allocator • Large footprint: resource maps, buddy systems • Smaller footprint: Mckusick-Karels, zone • Design • slab allocator is a variant of the zone method and is organized as a collection of object caches

26. Solaris 2.4 Slab Allocator (cont) page-level allocator back end vnode cache proc cache mbuf cache msgb cache ... front end activevnodes activeprocs activembufs activemsgbs

27. Solaris 2.4 Slab Allocator (cont) • Implementation slab data coloring area (unused) unused space linked list of slabs in same cache free active free active active free active freelist NULL pointers extra space for linkage

28. Solaris 2.4 Slab Allocator (cont) • Analysis • Space efficient • Coalescing scheme results in better hardware cache and memory bus utilization • Small footprint (only one slab for most requests)