CS 452/652 Winter 2022 - Lecture 8

RedBoot

• power cycle: all memory reset, devices reset
• soft reset (red button, reset command, watchdog: low (RedBoot) memory reset, devices reset
  • program/kernel memory state from previous crash still available
• return from program/kernel: nothing reset

Communication: send-receive-reply (SRR)

• sender state: Ready, SendWait, ReplyWait
• receiver state: Ready, ReceiveWait
• sender first
  • receiver tid: look up receiver - not in ReceiveWait
  • sender blocks (Ready -> SendWait)
  • where to store information about senders? per receiver
• receiver arrives to waiting sender
  • finds sender - sanity check for SendWait
  • sender still blocked (SendWait → ReplyWait)
  • kernel copies data
• receiver first
  • no sender found
  • receiver blocks (Ready → ReceiveWait)
  • store receive-blocked tasks in kernel container? only for cleanup/housekeeping
• sender arrives to waiting receiver
  • receiver tid: look up receiver - in ReceiveWait
  • unblock receiver (ReceiveWait → Ready)
  • block sender (Ready → ReplyWait)
  • kernel copies data
• reply
  • non-blocking: sender must be waiting
  • sender tid: look up sender - sanity check for ReplyWait
  • unblock sender (ReplyWait → Ready)
  • kernel copies data
• receive and reply can be decoupled
  • in time: sender parked until later reply (reordering)
  • in space: different task replies than original receiver (delegation)
• kernel provides SRR functionality
  • provide mechanisms for blocking senders and/or receivers
  • message copying for safe asynchronous operation of sender & receiver
    • direct copy from sender to receiver and vice versa
    • no message buffering in kernel
  • kernel must also set return codes appropriately
• messages: structured data
  • no conversion (marshalling) needed as in heterogeneous distributed systems
  • copied as byte (char) array
  • type-checking: verify type of message for type of task?
    ⇒ need at least a global type field per message

Synchronization

• no shared memory; no memory synchronization
• no lock, semaphore, condition variable
• task synchronization via SRR
• resource synchronization via task patterns
  • example: track server mediates access to track
• device synchronization via AwaitEvent() system call

Server

• execution pattern:
  1. receive request
  2. process request
  3. (delayed) reply
• server always ready or working, never blocked!
• server never in SRR-"send" role