Memory Barriers

Why Memory Barriers?

Because the CPU and/or the compiler can reorder the instructions written in program order. Modern processors and Compilers try to optimize the program by reordering the instructions all the time. But the observed effects (on load and stores on memory locations) are consistent.

Sequential Consistency

is defined as the result of any execution is the same as if the read and write operations by all processes were executed in some sequential order and the operations of each individual process appear in this sequence in the order specified by its program [Lamport, 1979].
which means:

  • The instructions executed by the same CPU in order as they were written.
  • all the threads should observe the effect of loads/stores to any shared variable.

Sequential consistency is very important especially in multi-threaded programs because when a thread changes the shared variable, the other threads should see the variable in a consistent and valid state.

Total Store Ordering

Modern processors have a buffer where they store the updates to the memory location  called as store buffer. The reason, updates do not go to main memory directly is writes to main memory are very slow and value from store buffer can be reused because of spatial locality.

Architectures with Strong Store Ordering guarantees: x86, SPARC
Architectures with weak Store Ordering guarantees: ARM, POWER, alpha etc.

Types of Barriers:

  • Compiler Barriers
    • This is to prevent compiler from reordering the instructions.
    • in Modern c++ (c++ 11) following compiler barriers are introduced:
    • A barrier can be applied by calling std::atomic_thread_fence() with argument for memory orders as  memory_order_relaxed, memory_order_consume, memory_order_acquire, memory_order_release, memory_order_acq_rel, memory_order_seq_cst etc.
  • Mandatory Barriers (Hardware)
    • Example instructions are mfence, lfence or sfence.
    • General barriers
    • Read-Write barriers
  • SMP (Simultaneous Multiprocessors) Conditional Barries:
    • This is used during multiprocessing example: used mainly in Linux Kernel.
  • Acquire/Release Barriers
  • Data Dependency Barriers
  • Devise (I/O) Barriers



Understanding glibc malloc


I always got fascinated by heap memory. Questions such as

How heap memory is obtained from kernel?
How efficiently memory is managed?
Is it managed by kernel or by library or by application itself?
Can heap memory be exploited?

were in my mind for quite some time. But only recently I got time to understand about it. So here I would like to share my fascination turned knowledge!! Out there in the wild, many memory allocators are available:

  • dlmalloc – General purpose allocator
  • ptmalloc2 – glibc
  • jemalloc – FreeBSD and Firefox
  • tcmalloc – Google
  • libumem – Solaris

Every memory allocator claims they are fast, scalable and memory efficient!! But not all allocators can be suited well for our application. Memory hungry application’s performance largely depends on memory allocator performance. In this post, I will only talk about ‘glibc malloc’ memory allocator. In future, I am hoping to cover up other memory allocators. Throughout…

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Linux Processes

process is a program in execution. Since linux represent everything in terms of files, currently running processes are also represented using /proc file system. The proc/<pid> contains open files (sockets), pending signals, process state, kernel data structures, threads of execution and data section.

Processes provides two types of virtualization

  • Virtual Processor
  • Virtual Memory

Processes are created on linux using fork() system call. When fork() is called, a process is created which is child of process who is called fork(). The reason it’s called as a child because it gets copy of resources (data, variables, code, pages, sockets etc) from it’s parent process.

The list of processes are stored in a doubly linked list called as task list. The process descriptor is type  struct task_struct defined in <linux/sched.h> .

Processes are identified by PID. PID is a defined by pid_t data structure which is an integer up to 32,768. Generally processes up to number 1024 is kernel processes and rest of them are user processes. When a new process is forked, the number is not sequential but its randomized in order to avoid being guesses. When number is processes reaches max limit, the number is reseted and starts from 1024. The limit of number of processes can be increased by setting value at proc/sys/kernel/pid_max.

Process creation

  • fork()
    • a process can create another process using fork() function.
    • It is implemented using clone()
    • fork(), vfork() and __clone() calls clone() which calls do_fork() which calls copy_process()
    • the process created is called as child process
    • Example of simple fork call:
      pid_t pid = fork();
    • if return value of fork() call is 0, fork is successful and control block is in child process.
       if (pid == 0)
            printf("child process");
    • if return value of fork() call is greater than 0, fork is successful and control block is in parent process process.
      else if (pid > 0)
            printf("parent process");
    • if return value of fork() is negative, the call is failed.
            printf("fork failed");
    • fork() creates a child process that is a copy of the current task. It differs from the parent only in its PID, its PPID (parent’s PID), and certain resources and statistics (e.g. pending signals) which are not inherited
    • Besides the open files, other properties of the parent are inherited by the child:
      • Real user ID, real group ID, effective user ID, and effective group ID
      • Supplementary group IDs
      • Process group ID
      • Session ID
      • Controlling terminal
      • The set-user-ID and set-group-ID flags
      • Current working directory
      • Root directory
      • File mode creation mask
      • Signal mask and dispositions
      • The close-on-exec flag for any open file descriptors
      • Environment
      • Attached shared memory segments
      • Memory mappings
      • Resource limits
  • exec():
    • The exec() family of function calls creates a new address space and loads a new program into the newborn child immediately after a fork.
    • There are following variations of exec calls
      • int execl(const char *path, const char *arg, ...
         /* (char *) NULL */);
      • int execlp(const char *file, const char *arg, ...
         /* (char *) NULL */);
      • int execle(const char *path, const char *arg, ...
         /*, (char *) NULL, char * const envp[] */);
      • int execv(const char *path, char *const argv[]);
      • int execvp(const char *file, char *const argv[]);
      • int execvpe(const char *file, char *const argv[],
         char *const envp[]);
Function pathname filename fd Arg list argv[] environ envp[]
execl * * *
execlp * * *
execle * * *
execv * * *
execvp * * *
execve * * *
fexecve * * *
(letter in name) p f l v e

Process 0 and 1

Process 0 is ancestor of all processes which is also known as swapper process. This process is build from scratch during boot up. Process 0 initializes all the data structures needed by kernel, enables interrupts, and creates init process which is process 1.init process, invoked by the kernel at the end of the bootstrap procedure. It is responsible for bringing up the system after the kernel has been bootstrapped. init usually reads the system-dependent initialization files (/etc/rc* files or /etc/inittab and the files in /etc/init.d) and brings the system to a certain state. Process 1 never dies. It is a normal user process, not a system process within the kernel, but runs with superuser privileges.

State of a Process

Process can be any one of following states:

    • Process is being executed on CPU or waiting
    • Process is sleeping/suspended.
    • Process is in sleeping state but signal to these processes will not change state of the process.
    • Process being stopped.
    • Process is being debugged
    • Child process is being removed because parent process issued wait4() or waitpid()
    • Child process being terminated but parent has not issued wait() or waitpid()

Copy on Write – an optimization on fork()

All the data and resources that belong to the parent are not copied to child. This is done is steps and as needed. This is an optimization in order to delay copying data as needed. The another reason is in calls such as exec(), none of the parent data is actually needed, there should not be a need to copy any pages from parent.

Process Switch

During process switch the registers are saved in kernel memory area for process and loaded back when process resumes on CPU. This is was done using far jmp assembly instruction on x86. On linux 2.6 onwards software is used for context switching instead of hardware context switching. Software context switch is safer and faster.


Processes work on time slicing basis giving user the illusion that only the user has access to the CPU and the memory is unlimited. The scheduling of processes depend upon scheduling policy or algorithm. The scheduling algorithm must schedule in such a way that processes should have fast response, good throughput, limited use of resources and honor the priorities.

Linux processes are preemptive. A process with high priority than low priority runs first by interruption of low priority process is one is running. There are following classes of Linux processes:


Scheduling factors for a process

  • static priority: between 100 (high priority) to 139 (low priority)
  • nice value: -20 (high priority) to 19 (low priority)
  • dynamic priority: between 100 (high priority) to 139 (low priority)
  • real-time priority: 1 (highest priority) to 99 (lowest priority)

Kernel Preemption

In non-preemptive kernels, kernel code runs until completion. That is, the scheduler cannot reschedule a task while it is in the kernel: kernel code is scheduled cooperatively, not preemptively. Kernel code runs until it finishes (returns to user-space) or explicitly blocks.

The Linux kernel (since 2.6), unlike most other Unix variants and many other operating systems, is a fully preemptive kernel. It is possible to preempt a task at any point, so long as the kernel is in a state in which it is safe to reschedule.


Linux threads are light weight processes i.e. Linux does not make any difference between threads and processes. They share resources with parent like heap space, file descriptors, sockets  but each thread gets own stack.

  • each thread contain TCB- a thread control block which contains following
    • Thread Identifier:Unique id (tid) is assigned to every new thread
    • Stack pointer: Points to thread’s stack in the process
    • Program counter: Points to the current program instruction of the thread
    • State of the thread (running, ready, waiting, start, done)
    • Thread’s register values
    • Pointer to the Process control block (PCB) of the process that the thread lives on
  • threads are also created with clone() system call like other processes except for some flags to clone like

Address Space of a Process

The address space of a process consist of linear addresses the process is allowed to use. The linear address spaces are different per process. The linear addresses are managed in form of Memory Regions. Following are examples of system calls to manage memory regions:

Syscall usage
brk()/sbrk() changing the heap size of process
execve Load a new program in the memory in the currently running process
_exit() exit the current process and delete the memory space occupied
fork() create a new (child) process, copying/referencing page tables from parent process
mmap(), mmap2() creates memory mapped file, enlarge the memory address space of the process.
mremap() Expand or shrink memory region
munmap() destroys previously mmaped memory area and shrinks memory space of the process
shmat() Attaches new memory region to the process
shmdt() Detaches new memory region to the process

List all processes

Since linux uses proc file system to represent processes, listing all the processes can be done using ls /proc/

Screen Shot 2018-03-04 at 11.24.18 PM

listing /proc/<PID>/

Screen Shot 2018-03-04 at 11.28.01 PM

The directories and files listed here represent resources like file descriptors /fd/ etc

Memory Descriptor

The address associated with a process is stored in memory descriptor with type mm_struct. All the descriptors are stored in a doubly linked list with type mmlist.

Heap Management

malloc(size): c stdlib method to request size of Dynamic memory. On success returns pointer to  first linear address. On failure returns non zero integer.

calloc(n, size):c stdlib method to request size of Dynamic memory. On success returns pointer to  first linear address, set the memory to 0. On failure returns non zero integer.

realloc(size):c stdlib method that changes the size of memory location of previously allocated memory region.

free(): frees the allocated memory.

brk(address): Modifies the heap directly

sbrk(incr): increases/decreases the memory region

brk() System call:

  • brk() is the only function that is a syscall. All the other functions listed above are implemented using brk() and mmap()
  • it allocated and deallocated whole pages since it acts on memory region.

System calls a.k.a syscalls:

syscalls are implemented in Kernel and can be called from a process running in user space with appropriate privileges. There are around 330 number of system calls in Linux. The Linux API is written in c.

Process Termination

Process termination can be normal (finishing intended code execution) or can be abnormal. The Abnormal execution is because of following reasons:

  • Kernel, the process itself or any other process send certain signal.
  • Sigfault: Process tries to access memory location which it does not have privilege of access.
  • Abnormal conditions is exit with non-zero code.

On process exit Kernel releases the memory and other resources held by the process like file descriptors.

When Parent process dies, the children of that process becomes orphan processes and the process init becomes parent of those processes. Sometimes parents do not wait for child processes after fork(). These child processes becomes zombi processes after termination. A process can wait (block) when its children are in the running state using wait or waitpid.

  • The wait function can block the caller until a child process terminates, whereas waitpid has an option that prevents it from blocking.
  • The waitpid function doesn’t wait for the child that terminates first; it has a number of options that control which process it waits for.

Linux Memory Management

Memory Address

For x86 Architecture there are 3 types of addresses:

  • Logical Address:
    • This address is address of an instruction or data in machine language
    • This address consist of a segment and an offset i.e. distance from segment start address.
  • Linear address or Virtual address:
    • This address is a binary number in virtual memory that enables a process to use a location in main memory independently of other processes and to use more space than actually exists in primary storage by temporarily relegating some contents to a hard disk or internal flash drive.
  • Physical Address:
    • Address of the memory cells in RAM of the computer.

Need for Virtual Addressing

  • The main memory (RAM) available for a computer is limited.
  • Many processes use a common code in libraries.
  • Using Virtual addressing, a CPU and Kernel gives an impression to a process that the memory is unlimited.

Address Translation

  • Since 2 out of 3 address are virtual mentioned above, there is a need for address translation from Logical to Linear and Linear to Physical address.
  • For this reason each CPU contains a hardware names as Memory Management Unit (MMU).
  • Segmentation Unit: converts the Logical address to Linear.
  • Paging Unit: converts Linear address to Physical.
  • The address translation from linear address is done using two translation tables
    • Page Directory
    • Page Table


Image Credit:

Address Translation in Intel x86


Linear-Address Translation to a 4-KByte Page using IA-32e Paging


Formats of CR3 and Paging-Structure Entries with PAE Paging



Image Credit: By Mdjango, Andrew S. Tanenbaum (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

Segmentation and Paging in Linux

  • Both segmentation and Paging is redundent and hence Linux use segmentation in limited way.
  • Segmentation can assign a different linear address space to each process
  • Paging can map the same linear address space into different physical address spaces.

Segments and their usage

  • Each segment is described by 8-byte Segment Descriptor.
  • These descriptors are defined in Global Descriptor Table (GDT) or in the Local Descriptor Table(LDT).


Image credit: By John Källén (jkl at commons) (Own work) [Public domain], via Wikimedia Commons

Types of Segment Descriptors

  • Code Segment Descriptor
  • Data Segment Descriptor
  • Task State Segment Descriptor

Segmentation in Linux


image credit

Data structures for segmentation

  • GDT Global Descriptor Table
    • There is one GDT per CPU in Linux.
    • There are 18 descriptors in each GDT for various purposes as follows:
      • Descriptors for Kernel code and User code.
      • Task State Segment (TSS)
      • default Local Descriptor Table (LDT)
      • Thread-Local Storage (TLS) segments
      • Advanced Power Management (APM )
      • Plug and Play (PnP ) BIOS services
      • Special segment used for handling exceptions.


Image Credit: Lars H. Rohwedder (User:RokerHRO – selfmade work


  • paging unit translates linear addresses into physical ones
  • For efficiency the linear addresses are divided in fixed length intervals called as pages. These continuous linear addresses within a page are mapped into continuous physical addresses.
  • Page frames: main memory is divided into fixed lenght page frames. Each page frame contains a page. Page is just block of data in memory or disk.
  • page tables: The data structures that map linear to physical addresses.
  • Page Size: 4 KB
  • Huge Pages: 2 MB and 1 GB.
  • Page address (32 bit) = Directory (10 bits) + Page Table (10 bits) + Offset(12 bit)

Physical Address Extension (PAE)

  • In order to use more than 4 GB memory Intel started using 36 pins for address translation effectively supporting more addresses.

Caching in Hardware

  • There are atleast 3 levels of caches supported in modern microprocessor.
  • The caches work on principle of spatial locality and principle of temporal locality.
  • Cache is devided into cache line generally of 64 bytes.
  • Most of caches are N-way associative.
  • Cache unit resides between the paging unit and the main memory.
  • It includes both a hardware cache memory and a cache controller.
  • Each cache line has a tag and some flags that stores the status of cache line.
  • CPUs first look for address in cache before looking into main memory.
  • Flushing cache is done using write Back mechanism which is more efficient. Only cache entry is updated by CPU and the main memory is updated eventually.

Translation Lookaside Buffers (TLB)

  • This is kind of a cache used for storing recently converted addresses between linear to physical.
  • The Address Translation unit first looks in TLB for physical address for given linear address if not found, the hardware goes through page tables to find the page.


Process page tables

  • Linux stores pages using 4 levels
  • Page Global Directory
  • Page Upper Directory
  • Page Middle Directory
  • Page Table
  • Each process has its own Page Global Directory and its own set of Page Tables.
  • Linear addresses from 0x00000000 to 0xbfffffff can be addressed when the process runs in either User or Kernel Mode.
  • Linear addresses from 0xc0000000 to 0xffffffff can be addressed only when the process runs in Kernel Mode.

Virtual Memory

What is Virtual Memory?

It allows us to run more applications on the system than we have enough physical memory to support. Virtual memory is simulated memory that is written to a file on the hard drive.

Why Virtual Memory?

Most of the modern CPUs has memory management unit built in to generate virtual memory addresses. Using virtual memory has following advantages.

  • Main Memory is limited: especially in small devices like raspberry pi which runs linux can run multiple processes each with 32 bit addressing (4GB/process)
  • Sharing: It makes easy for library programs to be shared across applications example glibc.
  • Memory protection: Memory corruption in one program does not affect memory for another program.
  • Only a part of program needs to be in memory execution.


Virtual Memory is implemented with

  • Demand Paging
  • Demand Segmentation

Physical memory management

Zones are defined using data structure struct zone, which is defined in <linux/mmzone.h>. On X86 the physical memory is divided in zones as follows

  • ZONE_DMA First 16MB of memory
  • This is used by (legacy) hardware because the hardware can address only upto 16 bytes.
  • ZONE_NORMAL 16MiB – 896MB
  • The DMA and NORMAL zone page frames can be accessed directly by Kernel.
  • ZONE_HIGHMEM 896 MB – End
  • In 32 bit systems – the page frames belongs to HIGHMEM can not be accessed directly by Kernel.
  • In 64-bit system this problem does not exists because the linear space available is vastly large than the RAM available on the system.

Each zone has a zone descriptor with following fields:

  • free_pages: Number of free pages
  • pages_min: Number of reserved pages
  • pages_low: Low watermark for reclaiming page frames
  • pages_high: High watermark for reclaiming page frames
  • free_area: Blocks of free pages
  • lru_lock: spinlock for any operation
  • wait_table: wait queues of processes waiting on one of the pages in the zone.
  • zone_mem_map: pointer to first page descriptor
  • name: “DMA”, “Normal” or “HighMem”

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Zones in NUMA System

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Memory Management in Linux

  • Some part of main memory is assigned permanenetly to Kernel.
  • Remaining memory is called as Dynamic Memory
  • The effeciancy of system depends upon how effectively the memory is managed.

Page Frame sizes

  • Linux uses 4 KB and 4MB page size by default.
  • With PAE (Physical Address Extension) enabled it can support 2 MB pages.
  • 32-bit architectures have 4KB pages
  • 64-bit architectures have 8KB pages.
  • This means: 4KB pages and 1GB of memory, physical memory is divided into 262,144 distinct pages.

The reason to support 4 KB page size

  • Data transfer between disk and main memory is effecient when smaller page size is used.

Page Descriptors

  • The page information structure is defined using struct page structure. This structure is defined in <linux/mm_types.h>
  • Information about the state of the page frame is stored in page descriptor data structure.
  • Page descriptor store following information:
    • Flags
      • Various flags store the information about page.
      • The flag values are defined in <linux/page-flags.h>.
    • Frame’s reference counter
      • This is important field. When its value is -1, the page frame free and can be accessed by any process or kernel. A 0 or positive number implies, that the page is assigned to one or more processes.
    • Number of page entries
    • Index
    • pointer to least recently used doubly linked list

Memory Allocation

  • Zoned Page Frame Allocator is used by Kernel to handle memory allocation requests for a group of continuous page frames. The algorithm used is called as “Buddy System”. Kernel provides various macros as follows for allocating page frames.
    • alloc_pages(gfp_mask, order)
    • alloc_page(gfp_mask)
    • _ _get_free_pages(gfp_mask, order)
    • _ _get_free_page(gfp_mask)
    • get_zeroed_page(gfp_mask)
    • _ _get_dma_pages(gfp_mask, order)

    Deallocation of page frames is done using following:

    • _ _free_pages(page, order)
    • free_pages(addr, order)
    • _ _free_page(page)
    • free_page(addr)

    High-Memory Page frame mappings:

    There are 3 mechanisms to map page frames in high memory:

    • Permanent Kernel mapping
    • Temporary Kernel Mapping
    • Non Contiguous Memory Allocation


    • Frequent allocations and deallocations of page frames of different sizes may result in several small blocks of free page frames scattered in the block of page frames. Then it is impossible to allocate large chunk of page frames.
    • To fix this issue, Kernel keeps track of existing blocks of free contiguous page frames and avoid need to split up large blocks if request comes for smaller chunk.

    Buddy System

    • This is one of the algorithms for allocating memory from fixed-size segment consisting of physically-contiguous pages.
    • Memory is allocated from the segment using a power-of-2 allocator
      • This method satisfies requests in units sized as power of 2
      • The requests rounded up to next highest power of 2
      • When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of 2
      • Continue until appropriate sized chunk available
    • For example, assume 256KB chunk available, kernel requests 21KB
      • Split into AL and Ar of 128KB each
        • One further divided into BL and BR of 64KB
          • One further into CL and CR of 32KB each – one used to satisfy request
    • Advantage – quickly coalesce unused chunks into larger chunk
    • Disadvantage – fragmentation


Slab Allocator

  • There is an alternate strategy for Memory allocation Slab Allocator.
  • A slab allocator is a list that contains a block of available, already allocated, data structures.
  • When needed the data structure is allocated from the slab, when not needed, it’s returned to the slab rather than freeing.
  • The slab allocator works as generic data structure caching layer.
  • Slab is one or more physically contiguous pages
  • Cache consists of one or more slabs
  • Single cache for each unique kernel data structure
    • Each cache filled with objects – instantiations of the kernel data structure
  • Slab-allocation algorithm uses caches to store kernel objects
    • When cache created, filled with objects marked as free
    • When a new object for a kernel data structure is needed, the allocator can assign any free object from the cache to satisfy the request.
    • The object assigned from the cache is marked as used.
    • There are 3 states of a slab,  full slab has no free objects, an empty slab has no allocated objects and partial slab has some free and some allocator objects.
  • If slab is full of used objects, next object allocated from empty slab
    • If no empty slabs, a new slab is allocated from contiguous physical pages and assigned to a cache
  • Benefits include no fragmentation, fast memory request satisfaction

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Demand Paging

Software Programs are generally very large in size and can not fit entirely in main memory. Hence only part of the pages are brought in main memory from disk when absolutely required.

Advantages of demand paging

  • Similar to a paging system with swapping
  • Less I/O needed, no unnecessary I/O
  • Less memory needed. Example: Raspberry pi.
  • Faster response because of shared libraries.
  • More users can be using the operating system at a time

Details of demand paging

  • Extreme case – start a process with no pages in memory
    • OS sets instruction pointer to the first instruction of process, non-memory-resident page fault
    • And for every other process pages on first access faults for the page
    • Pure demand paging never bring a page into memory until it is required.
  • Actually, a given instruction could access multiple pages multiple page faults
    • One page for the instruction and many for data : unacceptable system performance
    • Fortunately, pain decreased because of locality of reference reasonable performance
  • Hardware support needed for demand paging
    • Page table with valid / invalid bit
    • Secondary memory, usually a high-speed disk. (swap device with swap space)
    • Instruction restart capability after a page fault
      • We must be able to restart the process in exactly the same place and state,
      • With the saved state (registers, condition code, instruction counter) of the interrupted process

Stages in Demand Paging

  1. Trap to the operating system
  2. Save the user registers and process state
  3. Determine that the interrupt was a page fault
  4. Check that the page reference was legal and determine the location of the page on the disk
  5. Issue a read from the disk to a free frame:
    1. Wait in a queue for this device until the read request is serviced
    2. Wait for the device seek and/or latency time
    3. Begin the transfer of the page to a free frame
  6. While waiting, allocate the CPU to some other user : context switch
  7. Receive an interrupt from the disk I/O subsystem (I/O completed)
  8. Save the registers and process state for the other user
  9. Determine that the interrupt was from the disk
  10. Correct the page table and other tables to show page is now in memory: set to v
  11. Wait for the CPU to be allocated to this process again : context switch
  12. Restore the user registers, process state, and new page table, and then resume the interrupted instruction: restart

Copy on Write

  • Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory: fork() system call
    • If either process modifies a shared page, only then is the page copied
  • COW allows more efficient process creation as only modified pages are copied
  • In general, free pages are allocated from a pool of zero-fill-on-demand pages
    • Why zero-out a page before allocating it?
  • vfork() as a variation on fork() system call has parent process suspended and child process uses address space of parent.
    • Different from fork() with copy-on-write.
    • Do not use copy-on-write; changes made by the child process on any pages of the parent’s address space the altered pages will be visible to the parent once it resumes
    • Must be used with caution to prevent the child from modifying the parent address space
    • vfork() is intended to be used when the child calls exec()immediately after creation
    • No copy of pages, very efficient method of process creation.

Handling Page Fault

  1. Check an internal table (within PCB) to decide:
    1. Invalid reference ⇒ abort, terminate the process
    2. Valid reference ⇒ page it in when it is not in memory
  2. Find a free frame (taking one from the free-frame list)
  3. Schedule a disk operation to read the desired page into the new frame
  4. Reset tables to indicate the page is now in memory
  5. Restart the instruction that was interrupted by the trap caused by the page fault

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Page Replacement Algorithms

  1. Find the location of the desired page on disk
  2. Find a free frame:
    1. If there is a free frame, use it
    2. If there is no free frame, use a page replacement algorithm to select a victim frame
    3. Write the victim frame to the disk if dirty
  3. Bring  the desired page into the (newly) free frame; update the page and frame tables
  4. Continue the process by restarting the instruction that caused the trap

Note now potentially 2 page transfers (one out and one in) for page fault – increasing EAT

LRU Algorithm

  • This is generally good algorithm and used most widely.
  • Use past knowledge rather than future
  • Replace page that has not been used for the longest period of time
  • Associate time of last use with each page

Implementation of the LRU aalgorithm

  • Need hardware assistance to determine an order for the frames defined by the time of last use.
  • Counter implementation
    • Every page entry has a time-of-use field and a logical clock (counter) is added to the CPU;
    • Every time a page is referenced, the clock is incremented and the clock register value is copied into the time-of-use field in its page-table entry.
    • When a page needs to be changed, look at the counters to find smallest value
      • Search through table needed
  • Stack implementation
    • Keep a stack of page numbers in a doubly linked list
    • When a page referenced:
      • Removed from the stack and put on the top
      • Most recently used page is always at the top of the stack.
      • requires 6 pointers to be changed at worst
    • But each update more expensive
    • No search for replacement; the tail points to the bottom of the stack, the least recently used (LRU) page.
  • LRU and OPT are cases of stack algorithms that don’t have Belady’s Anomaly

Interrupts, Signals and Exceptions

What is an Interrupt

Interrupt is an event that changes the program flow i.e. the instruction stream being executed by the CPU. Interrupts are also generated by various devices connected to the CPU or they caused by bugs within the software. Interrupts are way for hardware to signal to the processor.

Interrupts and Exceptions

  1. Exceptions Synchronous Interrupts:
    Caused by software and produced by control unit of CPU.
    Example: A bug in software or a page fault. Kernel handles these exceptions by following the steps defined (in kernel code) to recover from such a condition.
  2.  Interrupts Asynchronous Interrupts:
    Caused by hardware devices.
    Example: Keypress/mouse movement by user.

Interesting Points about Interrupts

  • Interrupts are asynchronous and they are nested.
  • An interrupt can occur while kernel is handling another interrupt. When kernel is executing some critical region, interrupts are disabled and kept the critical region as small as possible.
  • By disabling interrupts, Kernel guarantees that an interrupt handler will not preempt the critical code.
  • The interrupts and exceptions are identified by a number between 0 to 255.
  • The code executed by interrupt handler is not a process switch, rather its ran at an expense of the process that was running when interrupt was received.
  • Interrupt handling is critical for Kernel but since handling can take long time in the case of slow I/O devices. Hence the interrupt handling is divided into two parts
    • Urgent: Kernel executes this right away.
    • Bottom Halves: Deferred to execute later. (using various techniques like soft irqs, Tasklets, Task/ Work Queues)

Classification of Interrupts

  • Maskable
    • These are interrupt requests issued by I/O devices.
    • There are two states for a maskable interrupt.
      • Masked
      • Unmasked
    • The vectors of maskable interrupts are altered by programming the interrupt control.
  • Nonmaskable
    • They are always recognized by CPU.
    • The vectors of non-maskable interrupts are fixed.

Classification of Exceptions

  • Processor Detected
    • CPU detects an anomalous condition while executing an instruction
  • Faults:
    • Example: Page faults
    • Faults can be corrected and once corrected, program can be resumed.
  • Traps:
    • They can be reported immediately at the next instruction.
    • Used mainly for debugging.
  • Aborts:
    • These are severe errors like hardware failure
    • The process is terminated on receiving this signal.
  • Programmable Exceptions:
    • Often called as software interrupts.
    • Used to implement system calls and debugging.
Exception Number Explanation Type
0 divide by zero error Fault
1 Debug Trap or Fault
2 Not Used
3 Breakpoint Trap
4 Overflow Trap
5 Bounds check Fault
6 Invalid opcode Fault
7 Device not available Fault
8 Double Fault Fault
9 Coprocessor segment overrun Abort
10 Invalid TSS Fault
11 Segment not present Fault
12 Stack segment fault Fault
13 General protection Fault
14 Page Fault Fault
15 Reserved by Intel
16 Floating-point error Fault
17 Alignment check Fault
18 Machine check abort
19 SIMD floating point exception Fault

_Page Fault occurs when the process try to address a page in its address space but is not currently in RAM. When Kernel is handling this exception, it may suspend current process and switch to another process until the page is available in the RAM. The process switch is done because of high latency of RAM (200 ns or serveral hundred CPU cycles). _

Hardware for Interrupt Handling

  • Each hardware device connected to a computer has a single output line named as Interrupt Request (IRQ) line.
  • There is a hardware circuit called Programmable Interrupt Controller (PIC) to which all the IRQ lines are connected.
  • Interrupt Controller (PIC) monitors IRQ lines for raised signals.
  • In case of multiple signals raised simultenously, the signal with lower pine number is selected.
  • When there is a signal raised, its stored in signal vector, then the vector is sent to CPU and signal is raised to CPUs INTR pin to wait until the CPU acknowledges the signal.



Image credit: By Jfmantis – Own work, CC BY-SA 3.0,

APIC – Advanced Programmable Interrupt Controller

In modern multiprocessor systems, there is a local APIC chip per CPU. The APIC has following components:

  1. 32 bit registers
  2. Internal clocks
  3. Local timer device
  4. Two additional lines LINT 0 and LINT 1


Image credit: [Intel Software Developer manual vol 3.](

Categories of Interrupts

  • I/O Interrupts
  • Timer Interrupts
  • Interprocessor interrupts

Types of Actions taken by Linux on interrupts

  • Critical
    • Critical actions are executed within the interrupt handler immediately.
  • Noncritical
    • These are quick to finish and hence executed by interrupt handler immediately.
  • Noncritical deferrable
    • These may be delayed for a long time interval without affecting the kernel operations

IRQ Distribution in Multiprocessor System

The kernel tries to distribute the IRQ signals coming from the hardware devices in a round-robin fashion among all the CPUs.

The interrupts coming from external hardware can be distributed within CPUs in following ways

  • Static Distribution
  • Dynamic Distribution

IRQ affinity

The kernel provides a functionality to redirect all the interrupts to a particular CPU. This is achieved by modifying Interrupt Redirection Table entries of the I/O APIC. IRQ affinity of particular interrupts can also be changed by writing a new CPU bitmap mask into the /proc/irq/n/smp_affinity file.

Interrupt Handling

When an interrupt is received, kernel runs inyerrupt handler or interrupt service routine code. These are ‘C’ functions. A data structure named Interrupt Descriptor Table (IDT) stores each interrupt or exception vector with the address of the corresponding interrupt or exception handler. That Table must be properly initialized before the kernel enables interrupts.

Top Halves vs Bottom Halves

  • Interrupt handling should be fast but there may be large amount of work involved, hence the handling is divided into two parts:
  • Top Half: Executed immediately and perform time critical work like acknoledging the interrupt.
  • Bottom Half: This part can be deferred like communicating with I/O.

Deferred Part of Interrupt Handling

  • As stated above the interrupt handling has two parts: critial and non critical (deferred handling).
  • SoftIRQs, Tasklets, WorkQueues etc are ways to process deferred part of interrupt handling which is also called as bottom halves.
SoftIRQs Tasklets
They are statically allocated. can also be allocated and initialized at runtime
softirqs are reentrant functions and must explicitly protect their data structures with spin lock Do not need synchronization because Kernel handles that for them.
provide the least serialization Tasklets of the same type are always serialized: in other words, the same type of tasklet cannot be executed by two CPUs at the same time
Easy to code

Work Queues

  • They defer work into Kernel Queue.
  • Functions in work queues run in process context and hence the function can be blocking functions or can sleep.
  • Function in a work queue is executed by a kernel thread, so there is no User Mode address space to access.

Exception Handling

  • The exceptions raised by CPU are handled by linux as error conditions.
  • Kernel sends a signal to the process about the erroneous condition.
  • Steps taken to handle exception:
    • Save registers to Kernel Stack
    • Invoke C-level function to handle exception.
    • call ret_from_exception() function and exit!


Signals are software generated interrupts. A signal is generated for a process (or sent to a process) when the event that causes the signal occurs. When the signal is generated, the kernel usually sets a flag of some form in the process table. A signal is delivered to a process when the action for a signal is taken. Between the time of generation and delivery, the signal is pending.

When a process receives a signal, it can do either of following.

  1. Ignore: except for SIGKILL and SIGSTOP all signals can ignored.
  2. catch the signal: call some callback on receiving this signal. Again,  SIGKILL and SIGSTOP can not be caught or blocked.
  3. Apply some default action.

On Termination of the process the memory image of the file is stored in the pwd of the process.

Reentrant functions: functions that are guaranteed to be safe to call from within a signal handler. These functions are async-safe functions meaning they block the signals before entering into a critical region