Today • Memory Management Ø Virtual Memory Ø Segmentation, Paging • Project 5 Nov 30, 2018 Sprenkle - CSCI330 1 Review • What abstraction does virtual memory provide? • What requirements do we have for the VM, from the various stakeholders, so far? Aaron Bauer, University of Washington “Understanding Human Problem Solving in Complex Digital Environments” Talk at 4 p.m. (Reception at 3:45 p.m.) Nov 30, 2018 Sprenkle - CSCI330 2 1
Address Translation: Wish List • Map virtual addresses to physical addresses • Allow multiple processes OS OS Text to be in memory at once, Process 1 Data Process 3 but isolate them from Heap Process 2 each other • Determine which subset libc code of data to keep in Process 1 memory/move to disk • Allow the same physical Stack memory to be mapped in multiple process VASes Nov 30, 2018 Sprenkle - CSCI330 3 Review: Where should process P be placed? • First fit Ø Don’t spend time searching! OS • Best fit First Fit Process P A Ø It fits tightly! Ø Maybe no other process will fit in that spot • Worst fit Best Fit B Ø Leaves lots of space for another process Worst Fit C Nov 30, 2018 Sprenkle - CSCI330 4 2
(External) Fragmentation • No matter where it ends up, the remaining gaps get smaller OS • Large gaps are probably still usable, small ones likely aren’t A • Fragmentation: over time, we end up with small gaps that become more B difficult to use (eventually, wasted) • “External” because the gaps are between C allocated pieces Nov 30, 2018 Sprenkle - CSCI330 5 (External) Fragmentation • Suppose we put it here, and later, P asks for more memory? OS • What if there isn’t enough space… Ø Move P? Ø Move everybody to compact the address space? • This seems bad. Lots of tough problems P (placement, fragmentation) with no clear solutions. Nov 30, 2018 Sprenkle - CSCI330 6 3
Alternative Organization of PAS • Divide PAS into fixed- size pieces • Use memory translation to assign virtual OS OS addresses to physical locations • Every physical location is an equally good choice! Nov 30, 2018 Sprenkle - CSCI330 7 Address Translation: Wish List • Map virtual addresses to physical addresses • Allow multiple processes to OS OS be in memory at once, but Text Process 1 isolate them from each other Data Process 3 • Determine which subset of Heap Process 2 data to keep in libc code memory/move to disk Process 1 • Allow the same physical memory to be mapped in multiple process VASes Stack • Make it easier to perform placement in a way that reduces fragmentation Nov 30, 2018 Sprenkle - CSCI330 8 4
OS Perspective • Primary challenge: Which physical memory do we give to processes? • Other important considerations: Ø Protection : OS is resource gatekeeper, must isolate itself (and processes) Ø Performance : OS should map memory for best performance, as long as it doesn’t violate protection Nov 30, 2018 Sprenkle - CSCI330 9 Address Translation: Wish List • Map virtual addresses to physical addresses • Allow multiple processes to be OS OS in memory at once, but isolate Text Process 1 them from each other Data Process 3 • Determine which subset of Heap Process 2 data to keep in memory/move to disk libc code • Allow the same physical Process 1 memory to be mapped in multiple process VASes Stack • Make it easier to perform placement in a way that reduces fragmentation • Protection : OS is resource gatekeeper, must isolate itself (and processes) • Performance : OS should map memory for best performance, as long as it doesn’t violate protection Nov 30, 2018 Sprenkle - CSCI330 10 5
Recall: Context Switching Performance • Even though it’s fast, context switching is expensive: 1. time spent is 100% overhead 2. must invalidate other processes’ resources (caches, memory mappings) 0x0 Operating system 3. kernel must execute – it must be Text accessible in memory Data Heap • Solution to #3: Ø keep kernel mapped in every process Stack VAS Ø protect it to be inaccessible 0xFFFFFFFF Nov 30, 2018 Sprenkle - CSCI330 11 Hardware • Hardware and OS are symbiotic, often influence each other Ø Example: atomic instructions • Memory management is another important area of collaboration • Hardware goals: Ø Make translation fast Ø Give OS storage for and control over mappings Nov 30, 2018 Sprenkle - CSCI330 12 6
Address Translation: Wish List • Map virtual addresses to physical addresses OS OS • Allow multiple processes to be in Text Process 1 memory at once, but isolate them Data Process 3 from each other Heap Process 2 • Determine which subset of data to libc code keep in memory/move to disk • Allow the same physical memory to Process 1 be mapped in multiple process VASes Combination of hardware • Make it easier to perform Stack and OS, working together. placement in a way that reduces fragmentation In hardware, MMU: • Map addresses quickly with a little Memory Management Unit HW help Nov 30, 2018 Sprenkle - CSCI330 13 How does each benefit from having a logical memory abstraction? • The user Ø Memory size, protection • The programmer Ø Shared libraries • The compiler Ø Placement of data • The OS / OS designer Ø Memory placement, fragmentation • The hardware / hardware designer Ø Just makes it more complex BUT can help OS Nov 30, 2018 Sprenkle - CSCI330 14 7
Awkward transition PROJECT 5 Nov 30, 2018 Sprenkle - CSCI330 15 Project 5 • Combines processes and memory management • proc.c Ø Implementations of process management and memory management Nov 30, 2018 Sprenkle - CSCI330 16 8
proc.h Data Structures: memoryMap • Allow one process to be loaded into each segment memoryMap Ø Valid Segments: 0x2000, … 0x9000 0 Ø (0x0000 reserved for interrupt vector, USED 0x1000 reserved for kernel) 1 USED • Memory segment map: 2 FREE 3 FREE Ø Each index corresponds 4 USED to one memory segment. 5 FREE • segment = (index+2)*0x1000 6 FREE • index = (segment/0x1000)-2 7 FREE Ø Marked as: • FREE or USED Nov 30, 2018 Sprenkle - CSCI330 17 proc.h Data Structures: PCB • Process Control Block: Constants: struct truct PCB PCB DEFUNCT char name[7] STARTING int state RUNNING int segment int stackPointer READY struct PCB *next BLOCKED struct PCB *prev Nov 30, 2018 Sprenkle - CSCI330 18 9
proc.h Data Structures: pcbPool • PCB Pool struct truct PCB PCB char name[7]: "\0" int state: DEFUNCT int segment: 0x0000 int stackPointer 0x0000 pcbPool struct PCB *next NULL struct PCB *prev NULL 0 1 struct PCB 2 3 . 4 . 5 . 6 7 struct PCB Nov 30, 2018 Sprenkle - CSCI330 19 proc.h Data Structures • struct PCB *running The currently running process struct PCB pcbPool struct truct PCB PCB 0 char name[7]: "uprog2\0" int state: RUNNING 1 int segment: 0x3000 2 int stackPointer 0xFF00 3 struct PCB *next NULL struct PCB *prev NULL 4 5 . . 6 . 7 struct PCB Nov 30, 2018 Sprenkle - CSCI330 20 10
proc.h Data Structures • Ready Queue readyTail struct PCB Ø Doubly-linked list state: READY next: NULL prev: pcbPool struct PCB state: READY 0 next: . 1 . prev: . 2 struct PCB 3 state: READY 4 next: readyHead prev: NULL 5 . 6 . . 7 struct PCB Nov 30, 2018 Sprenkle - CSCI330 21 proc.h Data Structures: running • Initially the running process will be the idle process struct PCB *running idleProc struct truct PCB PCB char name[7]: "IDLE\0" int state: READY If no processes in int segment: 0x1000 int stackPointer 0x???? the ready queue, struct PCB *next NULL run idle process struct PCB *prev NULL Nov 30, 2018 Sprenkle - CSCI330 22 11
Equally awkward translation back VM à PHYSICAL ADDRESS TRANSLATION Nov 30, 2018 Sprenkle - CSCI330 23 Address Translation: Wish List • Map virtual addresses to physical addresses OS OS • Allow multiple processes to be in Text Process 1 memory at once, but isolate them Data Process 3 from each other Heap Process 2 • Determine which subset of data to libc code keep in memory/move to disk • Allow the same physical memory to Process 1 be mapped in multiple process VASes Combination of hardware • Make it easier to perform Stack and OS, working together. placement in a way that reduces fragmentation In hardware, MMU: • Map addresses quickly with a little Memory Management Unit HW help Nov 30, 2018 Sprenkle - CSCI330 24 12
Simple (Unrealistic) Translation Example • Process P 2 ’s virtual addresses don’t align with physical memory’s addresses 0 base P 2 • Consider: P 2 wants to access base + address 0x1000 P 1 • Determine offset from physical address 0 to 0 P 3 P 2 start of P 2 P 2max Phy max Ø store in base Nov 30, 2018 Sprenkle - CSCI330 25 Generalizing • Problem: process may not fit in one contiguous region 0 P 2 Base? + Base? + … Base? + P 2 ? 0 … P 2 P 2 P 2max Phy max Nov 30, 2018 Sprenkle - CSCI330 26 13
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