Memory Management in Representative Operating Systems

1. Memory Management in Representative Operating Systems

2. Memory Management and Virtual Memory • Memory management requirements • Memory partitioning • Paging • Segmentation • Virtual memory with • Paging • Segmentation • Combined paging and segmentation • Virtual memory management policies and strategies CS-550: Memory Management in Representative Operating Systems

3. UNIX and Solaris Memory Management • Evolution • Early implementations of UNIX used variable partitioning and no virtual memory • Current implementations – SVR4 and Solaris – use paged virtual memory • Memory management schemes in SVR4 and Solaris • Paging system • Allocates page frames in main memory to processes • Allocates page frames to disk block buffers • Kernel memory allocator CS-550: Memory Management in Representative Operating Systems

4. UNIX and Solaris Memory Management (cont.) • Paging system • Data structures • Page table • One page table per process • One entry for each page in virtual memory for that process • Disk block descriptor • Has an entry associated with each page of a process • Each entry describes the disk copy of the virtual page • Page frame data table • Describes each frame of real memory • Indexed by frame number • Swap-use table • One table for each swap device • One entry for each page on the device CS-550: Memory Management in Representative Operating Systems

5. UNIX and Solaris Memory Management (cont.) CS-550: Memory Management in Representative Operating Systems

6. UNIX and Solaris Memory Management (cont.) CS-550: Memory Management in Representative Operating Systems

7. UNIX and Solaris Memory Management (cont.) • Paging system • Page replacement • Page frame data table used for page replacement • Lists are created within table using pointers (e.g., list of free frames available; when number of frames on this table drops below a threshold, the kernel will steal a number of pages to compensate) • Page replacement algorithm in SVR4 is a modified clock policy algorithm, the two-handed clock algorithm • It uses the reference bit in the page table entry for each page in memory that is eligible (not locked) to be swapped out • Bit is set to 0 when page is first brought in and set to 1 when page is referenced for read or write • The front-hand of the algorithm sweeps through the eligible pages and sets reference bits to 1 • The backhand, later, sweeps same list and checks the referenced bit: if 0, page is placed on the list to be paged out • Parameters: Scanrate and Handspead CS-550: Memory Management in Representative Operating Systems

8. UNIX and Solaris Memory Management (cont.) • Kernel memory allocator • Requirement • Kernel generates and destroys frequently small tables and buffers (e.g, file descriptor blocks) which require dynamic memory allocation • These tables and buffers are much smaller than typical machine page size: paging mechanism would be inefficient • Solution: SVR4 uses the lazy buddy system • Observation: demand for blocks of particular size varies slowly in time • Solution: defer coalescing blocks until it seems likely that it is needed and then coalesce as many blocks as possible • Strategy: try to maintain a pool of locally free blocks and only invoke coalescing if the number of free blocks exceeds a threshold • Criterion for coalescing: the number of locally free blocks of a given size should not exceed the number of allocated blocks of that size CS-550: Memory Management in Representative Operating Systems

9. Linux Memory Management • Characteristics • Shares many characteristics from the other UNIX implementations • Has some unique features and is quite complex • Linux virtual memory • Virtual memory addressing • Three-level page table structure • Page directory: one per active process, size of one page • Page middle directory • Page table • Virtual address space has four fields • Index into the page directory • Index into the page middle directory • Index into the page table • Offset within the selected page of memory • Page table structure is platform independent and was designed to accommodate the 64-bi Alpha processor CS-550: Memory Management in Representative Operating Systems

10. Linux Memory Management (cont.) • Linux virtual memory (cont.) • Virtual memory addressing (cont.) • Page allocation • A buddy system is used to improve the efficiency of reading and writing contiguous blocks of pages mapped into contiguous blocks of page frames • Kernel maintains a list of contiguous page frame groups of fixed size: 1, 2, 4, 8, 16, 32 page frames • For allocation and deallocation of pages in main memory, the available groups are split and merged using the buddy system • Page replacement algorithm • Variation of the clock algorithm: least frequently used policy • The use bit is replaced with an 8-bit age variable • Age variable incremented each time page is accessed • Periodically, Linux sweeps through the global page pool and decrements the age variable for each page • A page with a low age variable is a candidate for replacement CS-550: Memory Management in Representative Operating Systems

11. Linux Memory Management (cont.) • Kernel memory allocation • Based on the page allocation mechanism used for virtual memory management • Buddy algorithm is used to allocate and de-allocate kernel memory in units of one or more pages • Since the kernel needs small (smaller than a page) chunks of memory for short intervals,Linux uses slab allocation within an allocated page • Example: on a Pentium/x86 machine page size is 4 Kbytes and chunks can be allocated in sizes 32, 64, 128, 252, 508, 2040, and 4080 bytes • Slab allocator implementation • A set of linked lists are maintained, one for each size of chunk • Chunks can be split, aggregated, and moved between lists like in the buddy algorithm CS-550: Memory Management in Representative Operating Systems

12. Windows 2000 Memory Management • Characteristics • Designed to operate over a variety of platforms and use page sizes from 4 Kbytes to 64 Kbytes • Current platforms • Intel, PowerPC, and MIPS platforms have 4,096 bytes per page • DEC Alpha platforms have 8,192 bytes per page • W2K virtual address map • 2 Gbytes of available virtual address space for user (increased optionally to 3 Gbytes) • 2 Gbytes for the operating system (W2K Executive, microkernel, and device drivers) CS-550: Memory Management in Representative Operating Systems

13. Windows 2000 Memory Management (cont) CS-550: Memory Management in Representative Operating Systems

14. Windows 2000 Memory Management (cont.) • W2K paging • When a process is created, it can use up to 2 Gbytes (minus 128 Kbytes) of virtual space, divided in fixed-size pages • Page states: • Available: Pages not currently used by the process • Reserved: Set of contiguous pages set aside for the process, but not counted as part of process quota until used. Committed to to process when it needs to write into memory • Committed: Pages for which space on the paging file has been set aside (e.g., the disk file used when removing pages from memory) • Resident set management scheme • Variable allocation, local scope • When process first activated, given a default working set: if process references a page not in memory, one of the resident pages is swapped out • Working set of active processes is adjusted as follows: • When memory plentiful, active processes are allowed to grow • When memory becomes scarce, resident sets for each process are reduced CS-550: Memory Management in Representative Operating Systems