# DATA 1. Data in Software and Computing 1. Perspectives on Data 1. Virtual Memory --- ## Data in Software and Computing --- ### What Is Data? * Generally: factual information (such as measurements or statistics) used as a basis for reasoning, discussion, or calculation * Computer science: information in digital form that can be transmitted or processed ---- ### Data Examples * Anatomy coursework - data regarding the human body * Census - data regarding population * Your Facebook profile - data regarding yourself * Your browser history - data regarding preferences --- ### What Is Data? (Computer Science)  ---- ### What Is Data? (Computer Science)  ---- ### What We Want * Optimal performance * retrieve and store should be as fast as possible * Optimal use of memory space * once data is not used, memory is freed immediately * minimize the time when memory is reserved but not used * data should occupy the minimal required space * Security * data correctness * data isolation --- ## Perspectives of Data * A Programmer's Perspective on Data * Programming Language Perspective * Hardware Perspective * Operating System Perspective --- ### A Programmer's Perspective on Data * Data = variables * Operations: declare/read/write * Variables are stored in memory, so depending on the language, you can also: * allocate memory * deallocate memory ---- ### Example `demo/variables/c_vars.c` ---- ### Performance: Depends on ... * number of memory copies - `demo/performance/copy.c` * degree of reuse of memory - `demo/performance/reuse.c` * number of memory allocations/deallocations - `demo/performance/alloc.c` - `demo/performance/alloc.c` * [Quiz](quiz/alloc.md) * cache friendliness of the program's behavior * hardware specifics ---- ### Space usage: Depends on ... * how we store data - use appropriate types for variables - `demo/space_usage/types.c` - `#pragma pack(1)` - `demo/space_usage/pragma.c` - union - demo/space_usage/union.c * how soon memory is freed ---- ### Security * Buffer overflow - `demo/security/bo.c` * NUL-terminated strings - `demo/security/nts.c` * Integer overflow - `demo/security/io.c` ---- ### Trade-offs * Python - performance for security, ease of use * D - can switch between both worlds ---- ### Example * Python - `demo/variables/d_vars.d` * D - `demo/variables/python_vars.d` * [Quiz](quiz/pl.md) ---- ### Who manages memory? * You (the programmer) - C/C++, D * The programming language - Python, Java, D, Rust * A library implementation - C/C++, D * The operating system - for all languages --- ### Stages of data  --- ### Programming Language Perspective  --- ### Hardware perspective  ---- ### Hardware Perspective  ---- ### How is it Mapped?  --- ### Operating System Perspective ---- ### Storage Units - v1  ---- ### Storage Units - v1 * Each user needs to manually verify that a storage box is free * The user needs to know the exact id of the box * Extra security measures need to be implemented so that a user does not end up opening another users box * We expect the users to govern the situation * Users = programs, storage unit = memory ---- ### Storing Units - v2  ---- ### Storage Units - part 2 * An automated system handles the requests and assigns each user a `userId` and a virtual `unitId` * The system assigns a physical box to each pair of (`userId`, `unitId`) * Each user will start counting its units from 0 to N * The user does not handle physical boxes ---- ### Storing Memory * How do computers use memory? - v1 or v2? - `demo/proc_storage/` --- ## Virtual Memory --- ### Virtual Memory  ---- ### Virtual Memory * Programs work with virtual memory as if it was physical memory * Each process can address the entire address space (potentially, more than the size of RAM) => simplicity * Each process is encapsulated (cannot have access to another process's memory) => security * Penalties: extra hardware and software needed, extra work is done when accessing memory ---- ### How Do We Store Data? * When we rent a storage unit, its size is fixed * Even if we store a coin, we still get a moderately large storage unit * The same goes for memory ---- ### Pages/Frames  ---- ### Pages/Frames * A page is a virtual memory unit * A frame is a physical memory unit * A page/frame size is typically 4 KB * Virtual pages are mapped to physical frames * The mapping is stored in a page table * Each page has access rights ---- ### An Example - How does it work? ```c int *arr = malloc(2000 * sizeof(int)); arr[1500] = 7; ``` ---- ### An Example - How does it work?  ---- ### What About Physical Memory? * Should we also allocate physical memory upfront? * What if the program does not end up using that memory? * Maybe we should allocate only when a memory location is actually used * We mark allocated pages that don't have a corresponding frame with (U) ---- ### An Example - How does it work?  ---- ### An Example - How does it work? * The requested address is checked to see if it was allocated (virtually) * If not => _segmentation fault_ * If yes, then we must retrieve it * Wait, about about physical frames? ---- ### What About Physical Memory? (part 2)  ---- ### What About Physical Memory? (part 2) * This strategy is called **demand paging** * We only allocate physical frames once they are needed * The pages that have been allocated, along with their mapping status, are stored in a ledger, called the page table * `demo/alloc_physical/` ---- ### An Example - How does it work?  ---- ### An Example - How does it work? * Subsequent accesses to virtual addresses that have been mapped to a physical frame simply check that the virtual address is valid * No need to allocate any other physical frames ---- ### An Example - How does it work?  ---- ### Benefits of Virtual Memory * Each process can have a different memory mapping * Physical RAM can be mapped into multiple processes at once - shared memory * Memory regions can have access permissions - read, write, execute * [Quiz](quiz/vm-draw.md) --- ### Page Faults * Occur when a virtual address access cannot be performed * Examples: - A valid virtual address is accessed, but no physical frame is allocated - An invalid virtual address is accessed ---- ### Page Fault Handlers * Page faults are handled by pre-registered OS routines: **page fault handlers** * A hardware unit generates a page fault that is treated by the operating system * Once a page fault is addressed, the instruction that caused it is re-executed (assuming the OS was able to treat the page fault) --- ### Implementation of Virtual Memory  ---- ### CPU * The CPU always works with virtual addresses * When a load is made, the MMU intercepts the address ---- ### Memory Management Unit (MMU) * hardware component that checks whether the virtual address is valid * stores the mappings (**page tables**) between virtual and physical addresses ---- ### The Page Table * maps virtual (page) addresses to physical (frame) addresses * is typically stored in memory (RAM) * For faster access, modern systems use ... ---- ### Translation Lookaside Buffers (TLB) * a cache of the page table * two memory accesses (access to page table and access to actual data) are reduced to a TLB cache access and a memory access ---- ### Page Fault Triggers * If the virtual address is part of the page table, i.e. there is a physical counterpart address for it, the MMU forwards the physical address to the memory subsystem * If not, a page fault is triggered ---- ### The Operating System * had previously registered a page fault handler * the page fault causes the execution of the page fault handler (part of the operating system) ---- ### The Page Fault Handler * verifies the virtual address is valid, i.e. it was allocated; issues memory exception (i.e. _segmentation fault_) if not * if the address is valid, verifies if there is an already allocated physical frame for the virtual page; allocates it if not * verifies if the access rights allow this access * updates the page table with the latest mappings * reruns the instruction that caused the page fault ---- ### The Memory Subsystem * works only with physical addresses * the MMU will forward only physical addresses, after it resolves the virtual address mapping * [Quiz](quiz/swap.md) --- ### Swapping * What happens if a process requires more memory than the available physical RAM? * Frames that haven't been used in a while are evicted on the disk * Once those frames are required they are brought up in RAM * However, a large penalty will be incurred * Typically, systems will freeze once the swap is used ---- ### Swapping  ---- ### Swapping  * [Quiz](quiz/swap.md) --- ### Case Study: Index Out Of Bounds ---- ### What is the result of running the following code? ```c void main() { /* some code, may contain allocations */ int *arr = malloc(2000 * sizeof(int)); arr[2005] = 10; } ``` * [Quiz](quiz/index-out-of-bounds.md) --- ### OS Implementation Of Virtual Memory ---- ### OS Data Structures  * [Quiz](quiz/sdfsd) ---- ### OS Interface To Virtual Memory * `/proc/<PID>/mem` - access to virtual memory - `demo/proc_mem/` * `/proc/<PID>/page_map` - access to page mappings - `demo/proc_pagemap/` * `/dev/mem` - access to physical memory - `demo/proc_pagemap/` --- ## Final Quiz * [Quiz](quiz/vm-final.md) --- ## The End