Like
multiprocessor systems, clustered systems gather together
multiple CPUs to accomplish computational work. Clustered
systems are composed of two or more individual systems
coupled together. Clustered computers share storage
and are closely linked via a local-area network (LAN).
Clustering is usually used to provide high-availability
service; that is, service will continue even if one
or more systems in the cluster fails. A layer of cluster
software runs on the cluster nodes. Each node can
monitor one or more of the others. If the monitored
machine fails, the monitoring machine can take ownership
of its storage and restart the applications that were
running on the failed machine.
Clustering can be structured asymmetrically or symmetrically.
In asymmetric clustering, one machine is in hot-standby
mode while the other is running the applications.
The hot-standby host machine does nothing but monitor
the active server. If that server fails, the hot-standby
host becomes the active server.
In symmetric mode, two or more hosts are running applications,
and are monitoring each other. This mode is more efficient
as it uses all of the available hardware. Other forms
of clusters include parallel clusters and clustering
over a wide-area network (WAN).
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The operating system picks and begins to execute one
of the jobs in memory. Sometimes, the job may have
to wait for some task, such as an I/O operation, to
complete. In such a situation, the operating system
simply switches to and executes another job. When
that job needs to wait, the CPU is switched to another
job, and so on. Eventually, the first job finishes
waiting and gets the CPU back.
Time-sharing is a logical extension of multiprogramming.
In time-sharing system, the CPU executes multiple
job by switching among them, but the switches occur
so frequently that the users can interact with each
program while it is running.
A time-shared operating system allows many users to
share the computer simultaneously. As the system switches
rapidly from one user to the next each user is given
the impression that the entire computer system is
dedicated to his use, even though it is being shared
among many users.
Time-sharing and multiprogramming require several
jobs to be kept simultaneously in memory. Since in
general main memory is too small to accommodate all
jobs, the jobs are kept initially on the disk in the
job pool. This pool consists of all processes residing
on the disk awaiting allocation of main memory. If
several jobs are ready to be brought into memory,
and if there is not enough room for all of them, then
the system must choose among them. This is called
job scheduling.
A proper memory management is required in the operating
system since several programs are residing in the
memory at the same time.
The main memory may be filled with several jobs, then
the system must choose each job among them. This is
called CPU scheduling.
In a time-sharing system, the operating system must
ensure reasonable response time, which is sometimes
accomplished through swapping, where processes are
swapped in and out of main memory to the disk.
The virtual-memory scheme enables
users to run programs that are larger than actual
memory.
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