A process is an execution stream in the context of a particular
process state.
An execution stream is a sequence of instructions.
Process state determines the effect of the instructions. It
usually includes (but is not restricted to):
Registers
Stack
Memory (global variables and dynamically allocated memory)
Open file tables
Signal management information
Key concept: processes are separated: no process can directly affect the state
of another process.
Process is a key OS abstraction that users see - the environment
you interact with when you use a computer is built up out of processes.
The shell you type stuff into is a process.
When you execute a program you have just compiled, the OS
generates a process to run the program.
Your WWW browser is a process.
Organizing system activities around processes has proved to be
a useful way of separating out different activities into coherent
units.
Two concepts: uniprogramming and multiprogramming.
Uniprogramming: only one process at a time. Typical example:
DOS. Problem: users often wish to perform more than one activity
at a time (load a remote file while editing a program, for example),
and uniprogramming does not allow this. So DOS and other uniprogrammed
systems put in things like memory-resident programs that invoked
asynchronously, but still have separation problems. One key problem with
DOS is that there is no memory protection - one program may write
the memory of another program, causing weird bugs.
Multiprogramming: multiple processes at a time. Typical of Unix
plus all currently envisioned new operating systems. Allows system
to separate out activities cleanly.
Multiprogramming introduces the
resource sharing problem - which processes get to use the physical
resources of the machine when? One crucial resource: CPU. Standard
solution is to use preemptive multitasking - OS runs one process
for a while, then takes the CPU away from that process and lets
another process run. Must save and restore process state. Key issue:
fairness. Must ensure that all processes get their fair share of the
CPU.
How does the OS implement the process abstraction? Uses a
context switch to switch from running one process to running
another process.
How does machine implement context switch? A processor has a
limited amount of physical resources. For example, it has only
one register set. But every process on the machine has its own
set of registers. Solution: save and restore hardware state on
a context switch. Save the state in Process Control Block (PCB).
What is in PCB? Depends on the hardware.
Registers - almost all machines save registers in PCB.
Processor Status Word.
What about memory? Most machines allow memory from multiple
processes to coexist in the physical memory of the machine. Some
may require Memory Management Unit (MMU) changes on a context switch.
But, some early personal computers switched all of
process's memory out to disk (!!!).
Operating Systems are fundamentally event-driven systems - they
wait for an event to happen, respond appropriately to the event, then
wait for the next event. Examples:
User hits a key. The keystroke is echoed on the screen.
A user program issues a system call to read a file. The operating system
figures out which disk blocks to bring in, and generates a request to the
disk controller to read the disk blocks into memory.
The disk controller finishes reading in the disk block and
generates and interrupt. The OS moves the read data into the user program
and restarts the user program.
A Mosaic or Netscape user asks for a URL to be retrieved.
This eventually generates requests to the OS to send request packets
out over the network to a remote WWW server. The OS sends the packets.
The response packets come back from the WWW server, interrupting the
processor. The OS figures out which process should get the packets,
then routes the packets to that process.
Time-slice timer goes off. The OS must save the state of the
current process, choose another process to run, the give the CPU to
that process.
When build an event-driven system with several distinct
serial activities, threads are a key structuring mechanism of the
OS.
A thread is again an execution stream in the context of a
thread state. Key difference between processes and threads is that
multiple threads share parts of their state. Typically, allow
multiple threads to read and write same memory. (Recall that
no processes could directly access memory of another process).
But, each thread still has its own registers. Also has its own stack,
but other threads can read and write the stack memory.
What is in a thread control block? Typically just registers.
Don't need to do anything to the MMU when switch threads, because
all threads can access same memory.
Typically, an OS will have a separate thread for each distinct
activity. In particular, the OS will have a separate thread for each process,
and that thread will perform OS activities on behalf of the process.
In this case we say that each user process is backed by a kernel
thread.
When process issues a system call to read a file, the process's
thread will take over, figure out which disk accesses to
generate, and issue the low level instructions required to start
the transfer. It then suspends until the disk finishes reading
in the data.
When process starts up a remote TCP connection, its
thread handles the low-level details of sending out network packets.
Having a separate thread for each activity allows the programmer
to program the actions associated with that activity as a single
serial stream of actions and events. Programmer does not have to
deal with the complexity of interleaving multiple activities on the
same thread.
Why allow threads to access same memory? Because inside OS,
threads must coordinate their activities very closely.
If two
processes issue read file system calls at close to the same time,
must make sure that the OS serializes the disk requests
appropriately.
When one process allocates memory, its thread must find some free
memory and give it to the process. Must ensure that multiple
threads allocate disjoint pieces of memory.
Having threads share the same address space makes it much easier to
coordinate activities - can build data structures that represent
system state and have threads read and write data structures to
figure out what to do when they need to process a request.
One complication that threads must deal with: asynchrony.
Asynchronous events happen arbitrarily as the thread is executing,
and may interfere with the thread's activities unless the programmer
does something to limit the asynchrony. Examples:
An interrupt occurs, transferring control away from one
thread to an interrupt handler.
A time-slice switch occurs, transferring control from
one thread to another.
Two threads running on different processors read and write
the same memory.
Asynchronous events, if not properly controlled, can lead to
incorrect behavior. Examples:
Two threads need to issue disk requests. First thread starts to
program disk controller (assume it is memory-mapped, and must issue
multiple writes to specify a disk operation). In the meantime, the
second thread runs on a different processor and also issues the
memory-mapped writes to program the disk controller. The disk
controller gets horribly confused and reads the wrong disk block.
Two threads need to write to the display. The first thread
starts to build its request, but before it
finishes a time-slice switch occurs and the
second thread starts its request. The combination of the two
threads issues a forbidden request sequence, and smoke starts pouring
out of the display.
For accounting reasons the operating system keeps track of how much
time is spent in each user program. It also keeps a running sum of
the total amount of time spent in all user programs. Two threads
increment their local counters for their processes, then
concurrently increment the global counter. Their increments interfere,
and the recorded total time spent in all user processes is less than
the sum of the local times.
So, programmers need to coordinate the activities of the
multiple threads so that these bad things don't happen. Key mechanism:
synchronization operations. These operations allow threads to
control the timing of their events relative to events in other
threads. Appropriate use allows programmers to avoid problems like
the ones outlined above.
