index.md (2286B)
1 +++ 2 title = 'The Memory Address Space' 3 +++ 4 # The Memory Address Space 5 introduce abstraction of address space - every program runs in its own address space 6 7 - base register operates dynamic relocation, controls where address space starts 8 - limit register decides maximum address in physical memory that you can access 9 10 how memory works with many programs: 11 12 - processes compete for memory partitions 13 - but how do you know the size of partitions? use dynamic partitions and swapping 14 15  16 17 - problem: swapping may lead to memory fragmentation (when you have a many separate small chunks of free memory) 18 - solution: memory compaction 19 - problem - need to allow extra room for process growth 20 - how much extra room? memory usage vs out-of-memory (OOM) risk 21 - what do when OOM (out of mana/memory)? 22 - kill process 23 - relocate process 24 - swap out 25 26 memory management 27 28 - which part of memory is allocated? 29 - divide memory in blocks (e.g. each block is 4 bytes) 30 - keep track of them using: 31 - bitmap 32 - divide memory in blocks. every bit in bitmap corresponds to a byte in memory. bit is 1 when the memory is allocated, 0 when it's free. 33 - in bitmap, allocation is super fast. but to find free memory, you need to slowly scan for a hole. 34 - linked list of unallocated memory 35 - allocation is slow af, so is deallocation 36 - holes sorted by address for fast coalescing 37 - how do you allocate? 38 - first fit: take first fitting hole (MINIX 3). simplest option. 39 - next fit: take next fitting hole. slower than first fit in practice. 40 - best fit: take best fitting hole. prone to fragmentation. 41 - worst fit: take worst fitting hole. poor performance in practice. 42 - quick fit: keep holes of different sizes. poor coalescing performance. 43 - buddy allocation scheme: improve quick fit's coalescing performance (Linux uses this). 44 - originally - area of 64 "chunks" (a unit chunk is whatever you want, on UNIX it's 4 kb) 45 - maintain *n* lists, one per size class 46 - you split on powers of 2 until you get the right size 47 - how do you find your buddies? shout eyyyyy buddy