“ Real-time system ” Some computer systems have to do certain outputs before particular actual times they have to “ meet deadlines ”
Forms of deadline fixed lead time after some stimulus event; or variable lead time (from a stimulus) before some outside world event happens; or clocked deadlines (usually at regular outside-world intervals); or continuous (e.g. keeping something up- to-date)
Kinds of Stimuli One classification: periodic – e.g. “ will come between 4 and 6 seconds after the last one ” uniform – e.g. “ averaging 10 per minute ” = probability of 1 / 6 th in any second sporadic – a burst of stimuli, then a lull
Severity of deadlines The consequences of missing a deadline vary from systems to system or even between different sorts of deadline in a single system Common classifications: hard – missing the deadline is some sort of catastrophe soft – missing the deadline is embarrassing or irritating optimum – we want to minimize the average response time
Challenge: meeting deadlines How long will a “ task ” take? Will/can it complete between stimulus and deadline? Should concurrent tasks run “ in parallel ” ? or run one at a time “ to completion ” ? when should each task be started?
Challenge: how much memory? Peak memory demand? is there enough RAM? Preventing “ memory leakage ”
Challenge: testing How to generate test stimuli? How to test for having met a deadline? How to test for memory issues? How to test without perturbation – i.e. without the test mechanism affecting speed or size
Challenge: Human issues How to find staff who can visualize the timing issues?
Real-Time System Characteristics
Real-time systems are often also … safety-critical or security-critical; embedded (including control systems)
Real time system size … … varies enormously: from tiny embedded systems (e.g. in a washing machine) to the world-wide “ cashcard ” system
Real-time system “ modes ” Many real-time systems have a number of “ modes ” – and will be in one at a time. E.g. for a washing machine: idle; washing; rinsing; draining; spinning.
Clashing deadlines Suppose another stimulus comes in while the task for another kind of stimulus is processing should the new task wait for the other to end? should the two tasks run “ simultaneously ” ? I.e. “ queued ” or “ concurrent ” ?
Priority of a deadline A concurrent deadline might have a priority will depend on how soon the deadline it is, how severe the deadline is, etc; used in deciding which deadline ’ s process gets the CPU Mode-change => change of priority?
Jitter vs stability A system might be prone to timing races between tasks may cause long-term behaviour changes – jitter otherwise the system is stable
Concurrency – Concepts
A non-concurrent program … … is a sequence of instruction ( “ the code ” ), and a set of data. There is a single continuous “ thread of control ” through the instructions.
Two generalizations: 1) non-continuous; 2) more than one “ thread of control ” These lead to two different forms of concurrency (but we can also have the two forms together)
Non-continuous programming The program runs to complete a “ task ”, which then sleeps or dies; Later the program re-awakens, and runs another task. The program consists of code for the various tasks – the tasks might run in any order; the tasks usually share their data.
Interrupt-driven programming Non-continuous programming where the tasks are started by interrupts the interrupts may be hardware- generated; the interrupts may be user- generated.
Uses of i/r-driven programming include … inside an Operating System for the various OS processes controlled by a scheduler in the OS ’ s kernel in embedded systems (kernel without full OS) in GUI programming for user events.
Concurrency (=parallelism) = multiple continuous “ threads of control ” (possibly scattered across >1 CPU)
Hybrids 1) Multiple non-continuous tasks at the same time 2) Continuous threads and also non- continuous tasks at the same time
Concurrency – processes Process = a concurrent thread of control the code it executes though the data it operates on
States of a process … … typically include running – the/a CPU is executing the process ’ s thread of control ready – it could run, but hasn ’ t currently been scheduled blocked – it has done something that stops it running for now suspended – another process has stopped it
Inter-process communication When processes share resources they need to collaborate not conflict.
An example of conflict (1) Suppose 2 processes share an integer variable V, & both try to add one to it … Initially the variable contains (say) 27 … V=27
An example of conflict (2) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process A takes a copy of V ’ s value … V=27 A ’ s copy = 27
An example of conflict (3) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process B takes a copy of V ’ s value … V=27 A ’ s copy = 27 B ’ s copy = 27
An example of conflict (4) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process A adds one to it ’ s copy of V ’ s value … V=27 A ’ s copy = 28 B ’ s copy = 27
An example of conflict (5) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process B adds one to it ’ s copy of V ’ s value … V=27 A ’ s copy = 28 B ’ s copy = 28
An example of conflict (6) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process A writes it ’ s copy back to V itself … V=28 A ’ s copy = 28 B ’ s copy = 28
An example of conflict (7) Suppose 2 processes share an integer variable V, & both try to add one to it … .. then process B also writes it ’ s copy back to V itself … V=28 (still) A ’ s copy = 28 B ’ s copy = 28
An example of conflict (8) Suppose 2 processes share an integer variable V, & both try to add one to it … .. we have added one to 27, twice … V=28 and the result is 28, not 29!!!
Preventing conflict (1) Put each of the “ add-one ” instructions into a mutual exclusion zone: wait till V is unlocked, then lock it V = V+1 unlock V
Preventing conflict (2) Implementing that wait+lock/ unlock capability needs either interrupts off or that the wait+lock and unlock operations be operating system calls
Cooperation So the “ mutual exclusion ” technique will give us co-operation rather than conflict But mutual exclusion can lead to “ deadly embrace ” avoiding that calls for careful design, rather than indiscriminate use of mutual exclusion
Challenge: Human issues How to find staff who can visualize the conflict/coperation/deadly embrace issues?
Real-Time scheduling of Concurrency If a system is both real-time and concurrent then concurrent => we need scheduling real-time => the scheduling needs to enable meeting deadlines
Techniques of r/t scheduling Very dependant on the application The notes describe assorted detailed techniques
After this double lecture There are a lot of real-time jobs out there are you right for them?