What is meant by arm-stickiness?If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track. This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc). High-density
multisurface disks are more likely to be affected by this than low density ones.
What are the stipulations of C2 level security?
C2 level security provides for:
Discretionary Access Control
Identification and Authentication
Auditing
Resource reuse
What is busy waiting?
The repeated execution of a loop of code while waiting for an event to occur is called busy-waiting.
The CPU is not engaged in any real productive activity during this period, and the process does not
progress toward completion.
Explain the popular multiprocessor thread-scheduling strategies.
Load Sharing: Processes are not assigned to a particular processor. A global queue of threads is
maintained. Each processor, when idle, selects a thread from this queue. Note that load balancing
refers to a scheme where work is allocated to processors on a more permanent basis.
Gang Scheduling: A set of related threads is scheduled to run on a set of processors at the same
time, on a 1-to-1 basis. Closely related threads / processes may be scheduled this way to reduce
synchronization blocking, and minimize process switching. Group scheduling predated this strategy.
Dedicated processor assignment: Provides implicit scheduling defined by assignment of threads to
processors. For the duration of program execution, each program is allocated a set of processors
equal in number to the number of threads in the program. Processors are chosen from the available pool.
Dynamic scheduling: The number of thread in a program can be altered during the course of execution.
What is a trap and trapdoor?
Trapdoor is a secret undocumented entry point into a program used to grant access without normal methods
of access authentication. A trap is a software interrupt, usually the result of an error condition.
What are local and global page replacements?
Local replacement means that an incoming page is brought in only to the relevant process address space.
Global replacement policy allows any page frame from any process to be replaced. The latter is applicable
to variable partitions model only.
Define latency, transfer and seek time with respect to disk I/O.
Seek time is the time required to move the disk arm to the required track. Rotational delay or latency
is the time it takes for the beginning of the required sector to reach the head. Sum of seek time (if any)
and latency is the access time. Time taken to actually transfer a span of data is transfer time.
Describe the Buddy system of memory allocation.
Free memory is maintained in linked lists, each of equal sized blocks. Any such block is of size 2^k.
When some memory is required by a process, the block size of next higher order is chosen, and broken
into two. Note that the two such pieces differ in address only in their kth bit. Such pieces are called
buddies. When any used block is freed, the OS checks to see if its buddy is also free. If so, it is rejoined,
and put into the original free-block linked-list.
What is time-stamping?
It is a technique proposed by Lamport, used to order events in a distributed system without the use of
clocks. This scheme is intended to order events consisting of the transmission of messages. Each system 'i'
in the network maintains a counter Ci. Every time a system transmits a message, it increments its counter
by 1 and attaches the time-stamp Ti to the message. When a message is received, the receiving system 'j'
sets its counter Cj to 1 more than the maximum of its current value and the incoming time-stamp Ti. At each
site, the ordering of messages is determined by the following rules: For messages x from site i and y from
site j, x precedes y if one of the following conditions holds....(a) if Ti<Tj or (b) if Ti=Tj and i<j.
How are the wait/signal operations for monitor different from those for semaphores?
If a process in a monitor signal and no task is waiting on the condition variable, the signal is lost. So
this allows easier program design. Whereas in semaphores, every operation affects the value of the semaphore, so the wait and signal operations should be perfectly balanced in the program.
In the context of memory management, what are placement and replacement algorithms?
Placement algorithms determine where in available real-memory to load a program. Common methods are first-fit, next-fit, best-fit. Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate a new program. The replacement algorithm determines which are the partitions to be swapped out.
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