All quizzes

Question 1: Which of the following operations are likely to require performing at least one system call on an operating system with a monolithic kernel design like xv6?
Select all that apply

Question 2: Suppose an xv6 system is running two processes. Process A starts to read from the keyboard, and while it is waiting for input from the keyboard, xv6 context switches and starts running process B in user mode. Then, a key is pressed and the corresponding interrupt happens. Where will process B's registers be saved while the interrupt is happening?

Question 3: Unix-like operating systems like xv6 provide programs access to I/O devices, like keyboards and monitors, primarily through the abstraction of

Question 1 (0 points): Consider the following POSIX C program:

int main() {
    for (int i = 0; i < 3; i += 1) {
        pid_t p = fork();
        if (p == 0) {
            i += 1;
        }
        printf("%d", i);
        fflush(stdout);
    }
}

Assuming fork(), printf(), and fflush() do not fail, which of the following are possible outputs?
Select all that apply

Question 2: On a typical implementation of POSIX, when an application performs a library call like read(fd, buffer, 16) to read from the keyboard, _____.
Select all that apply

Question 3: Which of the following are true about a call to waitpid like waitpid(pid, &status, 0)?
Select all that apply

Question 4: Consider the following POSIX C program firstkilobyte:

#include <unistd.h>
int main(void) {
    char buffer[1024];
    ssize_t amount_read = read(STDIN_FILENO, buffer, 1024);
    if (amount_read > 0) {
        write(STDOUT_FILENO, buffer, amount_read);
    }
}

This program attempts to read the first up to 1 kilobyte from its input and send it to its output. When run like

    firstkilobyte <input.dat >output.dat`

the program always seems to work correctly, but when run as part of a shell pipeline like

    ./some_other_program | ./firstkilobyte > output.dat

sometimes it outputs less than the first kilobyte. Which of the following is a possible explanation for this?
Select all that apply

Question 1: Which changes to a system are likely to improve a system's throughput?
Select all that apply

Question 2: In lecture, we discussed multi-level feedback queue schedulers that attempts to adjust a process's priority based on the length of its CPU bursts. In such a scheduler, programs at the highest priority
Select all that apply

Question 1: In Linux's Completely Fair Scheduler (CFS), when a process becomes runnable after not having been runnable for some time, if it's virtual time is too low compared to other programs, it is increased. To decide whether the virtual time is "too low", the scheduler has a maximum amount of difference that is allowed between the newly runnable program's virtual time and the "current" virtual time. Increasing this limit (allowed difference) will generally
Select all that apply

Question 2: Some schedulers are susceptible to starvation where, given some mixes of running programs, some of those programs will never be run no matter how long they wait. Which are examples of schedulers with this property?
Select all that apply

Question 3: On a system which implements kernel threads, creating a new thread would require allocating
Select all that apply

Question 4: Consider the following code snippet:

void *bar(void *arg) {
    int *q = (int*) arg;
    ...
}

int foo() {
    pthread_t;
    int value = 42;
    int *p = &value;
    pthread_create(&t, NULL, bar, (void*) p);
    pthread_join(t, NULL);
    return value;
}

Assume that the pthread_create and pthread_join calls is successful. What is true abou the above code?
Select all that apply

Question 1: In lecture, we discussed a way to implement a mutex lock using a spinlock and a queue of waiting threads. In this design, threads is added to queue

Question 4: Suppose we are writing a multithreaded program where some threads are responsible for computing graphs and other threads are responsible for displaying them. We want the graph displaying threads to wait until a graph has been computed before it starts trying to display a graph. To implement this waiting using a counting semaphore, we could initialize the sempahore to _________, use the ______ operation after a thread computes a graph, and use the ________ operation before a thread displays a graph.

Question 1: Suppose an xv6 system has a page table base pointer that is set to point to physical page 0x123, or physical byte address 0x123000. Recall that xv6 runs with a two-level page table, where page tables at each level contain 1024 4-byte page table entries. What is the physical byte address of the first-level page table entry that the processor will access when accessing virtual address 0x555555?

Question 2: xv6 contains a userspace malloc() function which is implemented in terms of the sbrk() system call. When an xv6 program uses malloc(), the kernel tracks
Select all that apply

Question 3: Consider the following POSIX C code:

    int fd = open("example.dat", O_RDONLY);
    char *ptr = (char *) mmap(0, 16 * 1024, PROT_READ | PROT_WRITE, MAP_SHARED);
    unsigned sum  = 0;
    for (int i = 0; i < 16 * 1024; i += 1) {
        sum += ptr[i];
    }
    printf("%d\n", sum);

On an operating system with 4KB pages [edited 2019-03-05: word "pages" previously omitted] that implements mmap by only setting page table entries when required by a page fault, how many page faults will occur when executing the for loop above?

Question 4: Consider the following C program:

    int fd = open("example.dat", O_RDONLY);
    char *ptr = (char *) mmap(0, 16 * 1024, PROT_READ | PROT_WRITE, MAP_PRIVATE);
    ptr[1] = 'X';
    printf("%c %d\n", ptr[1], sum);
    exit(0);

On an operating system that uses 4KB pages, 32-bit pointers, and copy-on-write as part of its mmap implementation, how many bytes must be copied by the operating system in order to implement the assignment to ptr above?

Question 1: Consider a system which uses 4 physical pages for its page cache and is running exactly one program that uses it (and nothing else uses the page cache). The page cache initially contains virtual pages A, B, C, and D (each assigned to a physical page). Then the program access virtual pages in the following sequence:

A, B, C, B, C, D, C, D, E, A

If the system uses a least recently used replacement policy, what is the contents of the page cache after the access pattern completes?

Question 4: Which are examples of access patterns for which a least-recently used policy page replacement performs poorly?
Select all that apply

Question 1: Suppose a keyboard controller is connected to a processor via its memory bus. This keyboard controller uses programmed I/O rather than direct memory access. The operating system would most likely determine what key was pressed occurs by

Question 2: Suppose an OS performs the following operations on an SSD:

• writes X to sector 10
• writes Y to sector 11
• writes new data to sectors 500-10000
• writes Z to sector 10

and makes sure that all the operations result in changes to the flash chips of the SSD rather than just modifying any DRAM cache the SSD controller may have.

Which of the following are true about this process?
Select all that apply

Question 3: In the FAT filesystem with 1KB clusters, truncating a file foo/bar/baz.txt from 2.5KB to 1.5KB requires modifying which of the following? Ignore any updates to creation, modification, and access times.
Select all that apply

Question 4: Suppose a inode-based filesystem has 5 direct blocks, 1 indirect block, and 1 double-indirect block in its inode. If the filesystem uses 1KB (1024 byte)blocks with 2 byte block numbers, then how many blocks are allocated solely to store block numbers for a 1MB (1024 * 1024 byte) file?

Question 1: Suppose a system uses RAID 4 across 5 disks holding a total of 12 sectors of data, where the data on the disks is laid out as follows:

Suppose all Disk 3 is inaccessible to a hardware failure. How many sectors must be read in order to read sectors 3, 4, 5, and 6 into memory, recovering using the parity Disk 5 if necessary?

Question 2: Compared to a inode-based filesystem that does not use block groups, a filesystem that uses block groups lowers distance on disk between _____.
Select all that apply

Question 3: Suppose a filesystem uses redo logging and supports renaming a file. As part of a renaming the file, the filesystem simulatenously updates the modification time of the directory. Using redo logging, The filesystem attempts to ensure that the renaming operation either updates both the file name and the directory's modification time or updates neither. What would be most useful for the filesystem to store in its log for this operation?

Question 4: In a copy-on-write filesystem that supports snapshots, removing a file X in a directory A will most likely create new copies of ______. Ignore any changes required to update modification times, access times, etc.
Select all that apply

Question 1: To make a server that handles multiple clients at the same time in the POSIX socket API, which of the following strategies would work?
Select all that apply

Question 2: In POSIX internet sockets, a successful to bind() typically
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Question 3: Which of the following are consistent with a stateless protocols a network file server?
Select all that apply

Question 4: Using remote procedure calls is typically unlike normal (local) procedure calls because _____.
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Question 1: Suppose two systems use NFSv3, with stateless servers and its close-to-open consistency model, and programs on the two machine access the shared file as follows, where (1) and (2) represent unknown values:

If system A and B both do caching of files on the shared filesystem if they have available memory as allowed by the NFS protocol and its consistency model, which are possible values of (1) and (2)? (Consider both cases where A and B have available memory for their caches and where they do not.)
Select all that apply

Question 2: Consider a system performing two-phase commit with two workers A and B and one coordinator. Suppose the coordinator sends a message to both workers to prepare to do an operation. Then, both worker A and worker B send their 'agree/vote to commit' message to the coordinator, but worker A's message is lost due to a temporary network failure. Then, worker A loses power. How can the system recover from this failure?
Select all that apply

Question 3: If we want to allow programs controlled by certain users to play audio on a POSIX-like machine but not allow others to play audio, which of the following would be suitable approaches?
Select all that apply

Question 4: Comparing a capability-based system with an access-control-list-based system, which of the following operations are substantially more difficult in the capability-based system?
Select all that apply

Question 1 (0 points): Suppose a guest operating system is running in a virtual machine implemented using trap-and-emulate. A user program running in the guest operating system tries to access memory that is not allocated but will be allocated on demand by the guest operating system. What will happen?

Question 4: Many operating systems avoid microkernel-based designs for performance reasons. Which of the following are performance issues that microkernel-based designs like seL4 are likely to experience compared to a monolithic kernel design like Linux?
Select all that apply