Memory Management Strategies

ITCS 343 Opera-ng System Principles retrospect Management Strategies Virtualizing Resources ? corporal Reality Di? erent Processes/Threads sh ar the same computer hardw be ? Need to mul-plex primary(prenominal)frame (Just ?nished scheduling) ? Need to mul-plex use of Memory (Today) ? Need to mul-plex disk and devices (later in term) ? The nab working state of a exhibit and/or kernel is de? ned by its selective information in computer storage (and biographys) ? Consequently, back endnot just let di? erent threads of assert use the same warehousing ? Probably dont regard di? erent threads to even fork up access to each others retrospect (protec-on) ?Physics two di? erent pieces of entropy cannot occupy the same loca-ons in memory ? Why worry about memory sharing? Memory Hierarchy of a Modern Computer System ? Take value of the principle of locality to ? Present as much memory as in the cheapest technology ? Provide access at speed o? ered by the fastest technology Proce ssor Control Second take Cache (SRAM) Main Memory (DRAM) Secondary retentiveness (Disk) Tertiary Storage (Tape) On-Chip Cache Registers 1s 100s Datapath Speed (ns) Size (bytes) 10s-? 100s Ks-? Ms 100s Ms 10,000,000s 10,000,000,000s (10s ms) (10s sec) Gs Ts Background ? schedule must be brought (from disk) into memory and placed deep down a subprogram for it to be run ? CPU can access directly to registers and important memory ? Register access in hotshot CPU time (or less) ? Main memory can take many cycles ? Cache sits amid main memory and CPU registers -? to reduce CPU idle .me and present the available data faster to access. ? Protec-on of memory ensures correct opera-on ? to protect the opera. ng system from access by user processes and, ? to protect user processes from one other. ? One simple implementa. on is through ft and limit registers Mul? stepProcessing of a plan for Execu-on ? Prepara-on of a program for execu-on involves components at ? Addresses can be bank to ?nal values anywhere in this path ? Dynamic Libraries ? Compile -me (i. e. gcc) ? touch base/Load -me (unix ld does link) ? Execu-on -me (e. g. dynamic libs) ? Depends on hardware support ? Also depends on opera-ng system ? Linking postponed un-l execu-on ? Small piece of code, stub, used to locate the appropriate memory-? occupant library rou-ne ? Stub replaces itself with the target of the rou-ne, and executes rou-ne Mul? step Processing of a Program or Execu-on ? User programs go through several steps to begin with being able to run. ? This mul? step processing of the program invokes ? The appropriate u-lity (the rectangle) ? Generates the involve module at each step (the circle) ? Basically, it is all about declare address mapping. Binding of Instruc9ons and Data to Memory ? Address binding of instruc-ons and data to memory addresses can happen at three di? erent stages ? Compile 9me If memory loca-on cognise a priori, absolute code can be generated must recompil e code if star-ng loca-on changes ? Load 9me Must generate relocatable ode if memory loca-on is not known at compile -me ? Execu9on 9me Binding delayed un-l run -me if the process can be moved during its execu-on from one memory segment to another. Need hardware support for address maps (e. g. , base and limit registers) ? Controlled overlap ? Address eccentric person ? Separate state of threads should not collide in sensible memory. Obviously, unexpected overlap causes chaos ? Conversely, would like the ability to overlap when want (for communica-on) ? A physical (absolute) address is a physical loca-on in main memory. ? A logical (virtual) address is a eference to a memory loca-on that is free of the physical organiza-on of memory. ? All memory references in user process are logical addresses. ? A rela-ve address is an example of logical address in which the address is expressed as a loca-on rela-ve to some known evince in the program (ex the beginning address). ? Transla-on ? Protec-on ? Ability to translate accesses from one address lacuna (virtual) to a di? erent one (physical) ? When transla-on exists, processor uses virtual addresses, physical memory uses physical addresses ? Side e? ects kindle be used to vacate overlap,Can be used to give uniform view of memory to programs ? hold back access to private memory of other processes ? Di? erent pages of memory can be given special behavior (Read Only, Invisible to user programs, etc). ? Kernel data protected from User programs ? Programs protected from themselves Base and Limit Registers ? Each process has a separate memory quadrangle (logical/user address space). ? A pair of base and limit registers de? ne the logical address space ? base register holds the smallest legal physical address ? limit register speci? es the size of the range of a process ? Could se base/limit for dynamic address transla9on (oBen called segmenta9on) ? Alter address of every preventative/store by adding base ? User al lowed to read/write within segment ? Accesses are rela9ve to segment so dont have to be relocated when program moved to di? erent segment ? User whitethorn have mul9ple segments available (e. g x86) ? Loads and stores include segment ID in opcode x86 Example mov esbx,ax. ? Opera9ng system moves around segment base pointers as required Mul-programming ? Problem Run mul-ple applica-ons in such a way that they are protected from one another ? Goals ?Isolate processes and kernel from one another ? Allow ?exible transla-on that ? Doesnt lead to fragmenta-on ? Allows easy sharing between processes ? Allows plainly part of process to be resident in physical memory ? (Some of the required) Hardware Mechanisms ? General Address Transla-on ? Dual Mode Opera-on ? Flexible Can ?t physical chunks of memory into arbitrary places in users address space ? Not limited to small number of segments ? Think of this as providing a large number (thousands) of ?xed-? sized segments (called pages) ? Prote c-on base involving kernel/user dis-nc-on