"Processes, Threads, System Calls & IPC" — Practice

Question 1

How many new processes are created by four consecutive unconditional fork calls?

Accepted Answer

The population becomes 2 to the power 4, which is 16, so 15 new processes are created.

Question 2

Which of these are private to a thread: stack, heap, program counter, open file descriptors, register set?

Accepted Answer

Stack, program counter and register set are private. Heap and open file descriptors belong to the process and are shared.

Question 3

What does fork return in the parent, in the child, and on failure?

Accepted Answer

The child's PID in the parent, zero in the child, and negative one in the parent on failure, in which case no child exists.

Question 4

Trace the process count for: fork(); if (fork() != 0) fork();

Accepted Answer

After the first fork there are 2. Both run the second fork, giving 4. The condition is true for the two parents of that second fork, so those two fork again, adding 2. Total 6 processes, 5 of them created.

Question 5

A system call costs 1 microsecond per mode switch and a context switch costs 15 microseconds. A program makes 10000 calls per second, 10 percent blocking. Compute the overhead per second.

Accepted Answer

Mode switches cost 10000 times 2 times 1, which is 20000 microseconds. Blocking calls number 1000, each with 2 context switches, giving 1000 times 2 times 15, which is 30000 microseconds. Total 50000 microseconds, or 5 percent of the second.

Question 6

Why does a context switch between two threads of the same process cost less than one between two processes?

Accepted Answer

Threads of one process share an address space, so the page table base register is not reloaded and the translation lookaside buffer need not be flushed. Only the registers, program counter and stack pointer are swapped.

Question 7

Under a many-to-one threading model, one thread issues a blocking read. What happens to the others, and why?

Accepted Answer

All of them stop. The kernel sees only one kernel thread for the whole process, so blocking it blocks every user thread mapped onto it, even though the others are runnable.

Question 8

A parent forks 1000 children in a loop and never calls wait, then sleeps forever. Describe the resulting resource situation and how it resolves.

Accepted Answer

Each terminated child becomes a zombie holding only a PCB entry, so 1000 entries leak while the parent lives. Killing the parent reparents them to init, which waits routinely and releases every entry.

Question 9

A producer writes 512-byte records to a 4 KB pipe as fast as it can; the consumer reads one record every 5 milliseconds. Give the steady-state throughput and the producer's state.

Accepted Answer

The buffer fills after 8 writes and the producer then blocks, waking only when a read frees 512 bytes. Throughput is set by the consumer at 512 bytes every 5 milliseconds, about 100 KB per second, and the producer is blocked almost all the time.

Question 10

Explain why copy-on-write makes fork cheap and identify the case where it does not help.

Accepted Answer

Both processes share the physical frames read-only, so no copying happens at fork time and only pages actually written are duplicated. It does not help when the child immediately writes across most of a large address space, since every touched page is then copied anyway.

Practice — "Processes, Threads, System Calls & IPC"

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