Realtime System Fundamentals B. Ramamurthy 1/16/2019

Review of Project Plans Hardware modifications Software to generate xinu.boot Cross compiler (host system to MIPS) Fronzak 206 is ready for use David will provide you instructions to access the NEXOS setup You can use this to compile the software you write for shell and drivers and anything else you add to exinu. Uploading xinu.boot to test your shell and drivers: You can do it from any system with a serial port , hyperterminal and tftp installed. Such a setup will be available in the Franzack lab and is currently available during recitation/lecture on Tuesdays at 340 Bell. This will be available for remote access from next week 1/16/2019

Realtime scheduling We will discuss realtime system scheduling: Earliest deadline scheduling (EDS) Starting deadline Completion deadline Dynamic priority scheduling Rate monotonic scheduling (RMS) Periodic tasks are prioritized by the frequency of repetition (high priority to tasks with shorter periods) Preemptive scheduling Fixed priority scheduling Schedulability according to RMS Σ(Ci/Ti) <= n(21/n-1) Cyclic executives (pre-scheduled) Concepts of cycle, slot and frame Repeated execution times 1/16/2019

Critical sections and Semaphores When multiples tasks are executing there may be sections where only one task could execute at a given time: critical region or critical section There may be resources which can be accessed only be one of the processes: critical resource Semaphores can be used to ensure mutual exclusion to critical sections and critical resources 1/16/2019

Semaphores See semaphore.h of xinu 1/16/2019

Priority Inversion When we allow concurrent task to execute and with semaphore and mailboxes and other synchronization primitives, it is possible that a low priority task may come to block a high priority task. This situation is known as priority inversion. 1/16/2019

Priority inversion (Priority: t1>t2>t3) Critical section task1 time 0 1 2 3 4 5 6 7 8 9 10 blocked task2 task3 1/16/2019

Priority Ceiling Protocol CS Used by Priority Ceiling S1 t1,t2 P(t1) S2 t1,t2,t3 S3 t3 P(t3) Acquire S1 Release S1 task1 Attempt to Acquire S1 Acquire S1 Acquire S2 No way task2 Acquire S2 Release S2 Critical section task3 0 1 2 3 4 5 6 7 8 9 10 time 1/16/2019

Inter-task communication and synchronization Buffering data: For inter-task communication For sending data To handle incompatibilities between tasks typically a buffer is introduced Classical buffer Double buffer Ring buffer Mail boxes 1/16/2019

Mailboxes Mailboxes are intertask communication device available in many commercial operating systems. A mailbox is a memory location that one or more tasks can use to pass data or for synchronization. post and pend are used for mailbox write and read 1/16/2019

Mailbox implementation List of tasks and needed resources (mailboxes, printers etc.) is kept along with a second table of list of resources and their state taskID Resource Status 100 printer Has it 102 Mailbox 1 104 Mailbox 2 pending resource status owner printer busy 100 Mailbox 1 102 Mailbox 2 empty none 1/16/2019

Mailbox implementation (contd.) Task 104 requests mailbox 2 “pend” or “read”; It blocks waiting for read. Operating system checks task table, if “key” is full or available for mailbox 2, then it unblocks Task 104 and lets it read from mailbox 2; Task 104 reads from mailbox 2 and set it “key” back to empty. Accept is another function for mailbox; it is a non-blocking call; it reads data if available or it fails 1/16/2019

Task states Ready, running and blocked Critical regions Semaphores for protection of critical regions Identify the critical region and enclose it with p() and v() operators of the semaphore Lets look at an example of implementation of mailboxes using semaphores 1/16/2019

Mailboxes using semaphores Semaphore mutex, proc_sem; Bool full, empty; void post(int mailbox, int message) { wait(mutex); if (empty_slots){ insert(mailbox, message); update; signal(mutex); signal(proc_sem); } else {// no empty slots; wait for empty slots wait(proc_sem); wait(mutex); insert(mailbox,message); update(); signal(proc_sem); } 1/16/2019

Mailboxes using semaphores void pend(int *mailbox, int *message) { wait(mutex); if (full_slots) { extract(mailbox, message); update(); signal(mutex); } else { .. signal(mutex); wait(proc_sem); extract(mailbox,message); } 1/16/2019