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Sample Technical Questions

Unix Sample Questions

Questions on memory management in uinx

56. How the Kernel handles the fork() system call in traditional Unix and in the System V Unix, while swapping?

    Answer:

    Kernel in traditional Unix, makes the duplicate copy of the parent's address space and attaches it to the child's process, while swapping. Kernel in System V Unix, manipulates the region tables, page table, and pfdata table entries, by incrementing the reference count of the region table of shared regions.

57. Difference between the fork() and vfork() system call?

    Answer:

    During the fork() system call the Kernel makes a copy of the parent process's address space and attaches it to the child process.
    But the vfork() system call do not makes any copy of the parent's address space, so it is faster than the fork() system call. The child process as a result of the vfork() system call executes exec() system call. The child process from vfork() system call executes in the parent's address space (this can overwrite the parent's data and stack ) which suspends the parent process until the child process exits.

58. What is BSS(Block Started by Symbol)?

    Answer:

    A data representation at the machine level, that has initial values when a program starts and tells about how much space the kernel allocates for the un-initialized data. Kernel initializes it to zero at run-time.

59. What is Page-Stealer process?

    Answer:

    This is the Kernel process that makes rooms for the incoming pages, by swapping the memory pages that are not the part of the working set of a process. Page-Stealer is created by the Kernel at the system initialization and invokes it throughout the lifetime of the system. Kernel locks a region when a process faults on a page in the region, so that page stealer cannot steal the page, which is being faulted in.

60. Name two paging states for a page in memory?

    Answer:

    The two paging states are:
    The page is aging and is not yet eligible for swapping,
    The page is eligible for swapping but not yet eligible for reassignment to other virtual address space.

61. What are the phases of swapping a page from the memory?

    Answer:

    Page stealer finds the page eligible for swapping and places the page number in the list of pages to be swapped.

    Kernel copies the page to a swap device when necessary and clears the valid bit in the page table entry, decrements the pfdata reference count, and places the pfdata table entry at the end of the free list if its reference count is 0.