Working with the Unix OS

Process Scheduling
Figure 47. Range of Process Priorities
The kernel does not allow processes
with user level priority to cross the
threshold and attain kernel level
priority, unless they make a system
call & goto sleep.
! priority dependent on reason for
sleeping
- A process sleeping and waiting
for completion of disk I/O has a
higher priority than a process
waiting for a free buffer.
- Process waiting for I/O already
has a buffer, when it wakes up it
may release the buffer. The more
resources free, the better chance
processes will not block waiting
for resources.
- Fewer context switches, thus
process response time and system
throughput are better.
- Process waiting for a free buffer
may be waiting for buffer held by
process waiting on I/O.
The kernel adjusts the priority of a process that returns from kernel mode to user mode. The kernel recomputes the
priority of all active processes once
a second. At every clock interrupt,
the clock handler increments the
recent CPU usage.
decay (CPU) = CPU_usage /2
priority = decay (CPU) +
base level priority
Figure 48. Process Scheduling
Example
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