Hard vs Soft Real Time Systems
Here, we have discussed the common diffrences between hard and soft real time systems.
Summary
Here, we have discussed the common diffrences between hard and soft real time systems.
Things to Remember
- A processor is an active component on which the jobs are scheduled. E.g, the data scheduled on a transmission link.
- A resource is a passive component upon which jobs may depend on. For example, memory.
- Hard Real Time System - job never misses deadline.
- Soft Real Time System - job occasionally misses deadlines.
- A timing constraint is hard if the failure meet it is considered fatal error.
- A timing constraint is soft if the usefulness of the result falls off abruptly or may even become negative at the deadline.
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Subjective Questions
Q1:
What are the causes and risk factors of tuberculosis ?
Type: Short Difficulty: Easy
<li>Only people who have active TB infections can spread the TB bacteria. Coughing, sneezing, even talking can release the bacteria into the surrounding air, and people breathing this air can then become infected. This is more likely to happen if you're living in close quarters with someone who has TB or if a room isn't well ventilated.Once a person is infected, the bacteria will settle in the air sacs and passages of the lungs and, in most cases, will be contained by the immune system.</li>
<li>Your chances of becoming infected are higher if you come from – or travel to – certain countries where TB is common. People who are at greater risk for TB infection include the elderly, homeless people, people with substance use problems, individuals who have spent time in a correctional facility, and people with weakened immune systems from HIV or AIDS (acquired immune deficiency syndrome). Of course, the odds increase if you have close or frequent contact with someone who has active TB symptoms. This is especially true for health care workers who may be exposed to patients with active TB.</li>
<li>The following factors may play a role in promoting active disease in someone who has an inactive TB infection:</li>
</ul>
<p> </p>
<ul>
<li>Diabetes</li>
<li>head or neck cancer</li>
<li>illnesses that suppress the immune system, such as HIV or AIDS</li>
<li>kidney disease</li>
<li>Long-term steroid use</li>
<li>Malnutrition</li>
<li>Medications that suppress the immune system, such as anticancer medications (e.g., cyclosporine, tacrolimus)*</li>
<li>Pregnancy</li>
<li>Radiotherapy</li>
</ul>
<p>Predisposing factors:</p>
<ol>
<li>Age: Fetus may be infected from the infected placentas. Similarly, rare cases may be congenital. However , a frequency of infection with tubercle bacilli increases progressively as the child grows on age.</li>
<li>Sex: The adolescent girls are at risk to develop active tuberculosis during puberty.</li>
<li>Malnutrition</li>
<li>Environment: Children living in overcrowded apartment, hostel, camp with inadequate ventilation, lights damp unsanitary and unhygienic living condition are at high risk to have tuberculosis</li>
<li>Low socio-economic status</li>
<li>Low immunity</li>
</ol>
Q2:
Define tuberculosis .
Type: Very_short Difficulty: Easy
Q3:
Explain the pathophysiology of tuberculosis ?
Type: Long Difficulty: Easy
<p> </p>
<p>TB infection begins when the mycobacteria reach the pulmonary alveoli, where they invade and replicate within endosomes of alveolar macrophages.[10][55] Macrophages identify the bacterium as foreign and attempt to eliminate it by phagocytosis. During this process, the bacterium is enveloped by the macrophage and stored temporarily in a membrane-bound vesicle called a phagosome. The phagosome then combines with a lysosome to create a phagolysosome. In the phagolysosome, the cell attempts to use reactive oxygen species and acid to kill the bacterium. However, M. tuberculosis has a thick, waxy mycolic acid capsule that protects it from these toxic substances. M. tuberculosis is able to reproduce inside the macrophage and will eventually kill the immune cell.</p>
<p> </p>
<p>The primary site of infection in the lungs, known as the "Ghon focus", is generally located in either the upper part of the lower lobe or the lower part of the upper lobe. Tuberculosis of the lungs may also occur via infection from the blood stream. This is known as a Simon focus and is typically found in the top of the lung.[56] This hematogenous transmission can also spread an infection to more distant sites, such as peripheral lymph nodes, the kidneys, the brain, and the bones.All parts of the body can be affected by the disease, though for unknown reasons it rarely affects the heart, skeletal muscles, pancreas, or thyroid.</p>
<p>Tuberculosis is classified as one of the granulomatous inflammatory diseases. Macrophages, T lymphocytes, B lymphocytes, and fibroblasts aggregate to form granulomas, with lymphocytes surrounding the infected macrophages. When other macrophages attack the infected macrophage, they fuse together to form a giant multinucleated cell in the alveolar lumen. The granuloma may prevent dissemination of the mycobacteria and provide a local environment for interaction of cells of the immune system.However, more recent evidence suggests that the bacteria use the granulomas to avoid destruction by the host's immune system. Macrophages and dendritic cells in the granulomas are unable to present antigen to lymphocytes; thus, the immune response is suppressed. Bacteria inside the granuloma can become dormant, resulting in latent infection. Another feature of the granulomas is the development of abnormal cell death (necrosis) in the center of tubercles. To the naked eye, this has the texture of soft, white cheese and is termed caseous necrosis.</p>
<p> </p>
<p>If TB bacteria gain entry to the blood stream from an area of damaged tissue, they can spread throughout the body and set up many foci of infection, all appearing as tiny, white tubercles in the tissues.This severe form of TB disease, most common in young children and those with HIV, is called miliary tuberculosis. People with this disseminated TB have a high fatality rate even with treatment (about 30%).</p>
<p> </p>
<p>In many people, the infection waxes and wanes. Tissue destruction and necrosis are often balanced by healing and fibrosis. Affected tissue is replaced by scarring and cavities filled with caseous necrotic material. During active disease, some of these cavities are joined to the air passages bronchi and this material can be coughed up. It contains living bacteria, so can spread the infection. Treatment with appropriate antibiotics kills bacteria and allows healing to take place. Upon cure, affected areas are eventually replaced by scar tissue.</p>
<p> </p>
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Hard vs Soft Real Time Systems
Processor and Resources
A job executes or is executed by the operating system on a processor and may depend on some resources. A processor is an active component on which the jobs are scheduled. For example, the data scheduled on a transmission link, read-write request scheduled on a disk, etc. Each processor has a speed attribute which determines the rate of progress a job makes toward completion. Two processors are of same type if they are functionally identical and can be used interchangeably.
A resource is a passive component upon which jobs may depend on. For example, memory, database lock, etc. The resources have different types and sizes but do not have a speed attribute. They are usually reusable and are not consumed by use.
If the system contains p different types of resources, then it means
- There are p different types of serially reusable resources.
- There are one or more units of each type of resource and only one job can use each unit at once.
- A job must obtain a unit of needed resource, use it and then, release it.
A resource is plentiful if no job is ever prevented from executing by the unavailability of the resource. The job never block when attempting to obtain a unit of a plentiful resource.
Hard timing constraints and soft timing constraints
Timing constraints can be divided into two types: Hard and Soft. There are numerous definitions of hard and soft timing constraints.
A timing constraint is hard if the failure to meet it is considered a fatal error and the late completion becomes disastrous. This definition is based on the functional criticality of the job. A timing constraint is soft if the usefulness of the result falls off abruptly or may even become negative at the deadline. In this case, we may need validation of the system to confirm that the system always needs timing constraints. A hard deadline is imposed on a job because a late result produced by the job after the deadline may have disastrous consequences. (For example, a late command to stop a train may cause a collision, and a bomb dropped too late may hit a civilian population instead of the intended military target.)
If some deadlines can be missed occasionally with low probability then the system is called soft real-time system. In this case, the deadline is called soft deadline and few misses of soft deadline to know serious damage, but the performance becomes poorer and poorer when more and more jobs with soft deadline complete late.
Hard Timing Constraints and Temporal Quality-of-Service Guarantees
The timing constraint of a job is hard timing constraint, and the job is a hard real-time job, if the user requires the validation that the system always meet the timing constraint. By validation, we mean a demonstration by a provably efficient and correct procedure or by exhaustive simulation and testing.
On the other hand, if no validation is required, or only a demonstration that the job meet some statistical constraint (that is, a timing constraint speciï¬ÂÂÂÂÂÂÂed in terms of statistical averages) is enough, then the timing constraint of the job is soft. The satisfaction of statistical constraints (for example, the average number of missed deadlines per minute is two or less) can usually be demonstrated with a performance proï¬ÂÂÂÂÂÂÂle somewhat more thorough than those that are used to demonstrate the performance of general interactive systems.
It can be stated in another way as, if the user wants the temporal quality (for example, response time and jitter) of the service provided by a task are guaranteed and the satisfaction of the timing constraints deï¬ÂÂÂÂÂÂÂning the temporal(Liu 29) quality are validated, then the timing constraints are hard. On the other hand, if the user wants the best quality of service that the system can provide but allows the system to deliver qualities only below what is deï¬ÂÂÂÂÂÂÂned by the timing constraints, then the timing constraints are soft.
Hard Real-Time System
If a job must never misses its deadline then the system is called hard real-time system. For a hard real-time system, every deadline must be hit. In a real hard real-time system, if the system fails to hit the deadline even once the system is said to have failed.
A hard real-time system is also known as an immediate real-time system. It is a hardware or software that must operate within the confines of a stringent deadline. The application is considered to have failed if it does not complete its function within the given allocated time span. Some examples of hard real-time systems are medical application like pacemakers, aircraft control systems and anti-lock brakes.
- The hard real-time system is called guaranteed services.
- Response time is hard
- Often safety critical
Soft Real Time system
If some deadlines can be missed occasionally acceptably with low probability then the system is called soft real time system. In a soft real time system, even if the system fails to meet the deadline one or more than once, the system is still not considered to have failed. For example, streaming audio-video.
- The soft real-time system is called best effort service.
- Response time is soft
- Non-critical
| Item | Hard | Soft |
| size of data files | small and medium | large |
| peal load performance | predictable | degraded |
| data integrity | short term | long term |
| error detection | autonomous | user supported |
| control of pace | environment | copmuter based |
References
Liu, Jane W. S. Real Time System. Integre Technical Publishing Co., Inc, Jan 12, 2000. print.
slideplayer. "Real Time Scheduling." Real Time Scheduling -Terminologies define -Fixed Priority Scheduler -Ref: Real Time System Chapter 2 & 3. (n.d.). web. <http://slideplayer.com/slide/3416165/>.
Lesson
Hard versus Soft Real-Time Systems
Subject
Real Time System
Grade
IT
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