multi-processor systems

Question 1

in which ways is it difficult to implement multiprocessor systems

Answer 1

when using different types of cpus they may have different layouts / limited io interfaces and may need special libraries
memory to process access costs may be different leading to slower access times

Question 2

how does symmetric multiprocessing work

Answer 2

the processor are considered equal and can run their own schedulers

Question 3

in symmetric multiprocessing in which tow ways can we implement the run queues

Answer 3

shared between different processors
a queue per processor

Question 4

what is a negative of having a run queue per processor in symmetric multiprocessing

Answer 4

adds complexity: need load balancing across different processors

Question 5

how do we load balance between different processors in symmetric multiprocessing

Answer 5

pull: a processor thats free can pull processes from another queue on the system
push: redistribute periodic tasks to different processors via a management processor

Question 6

how does asymmetric multiprocessing work

Answer 6

one processor coordinates all the others in the system

Question 7

what is a positive of asymmetric multiprocessing work

Answer 7

simpler system: there’s only one processor looking at the data structures for example

Question 8

what are two negatives of asymmetric multiprocessing work

Answer 8

high cost of moving one process between processors
- may need to invalidate caches
- io and interrupt handling may be bound to
a specific cpu/ have to be directed
- need the state and context
some processors may not be suited to a specific task

Question 9

how does processor affinity work to distribute processes

Answer 9

each process has a cpu affinity bit mask and a preferred and next cpu
when a cpu becomes free the scheduler looks for a ready process for it based on:
- if the process last ran on that cpu
- if it lists it as its preferred cpu
- if its high priority

Question 10

cpu affinity bit mask

Answer 10

defines the cpus that the process can run on

Question 11

how do interrupts and multiprocessor systems work

Answer 11

interrupts from io operations need to be directed to the correct cpu and if not then the cpu must forward the request or even move code or data

Question 12

negative of interrupts and multiprocessor systems

Answer 12

extra overhead and latency
high cost in non uniform memory allocation (numa) systems as you have to go through different processes (not too bad for symmetric)