Dependencies

• Sometimes threads are not fully independent
• One thread may generate data that another consumes
• These threads need to interact via synchronization

What if thread A is waiting for thread B and thread B is waiting for thread A?

Deadlock

• Cyclic waiting
• Must be addressed to prevent freezes
• Difficult to solve generally

Transaction

• Unit of computation with no externally visible internal state
• Computation succeeds for fails as a unit
• Useful concurrency primitive

Accidental Interaction

• Threads share memory, so they may interact unintentionally

• This can be mitigated on some systems using virtual memory

  1.7 Supporting Interactions Across Time

Storage

• OS provides long-term storage for computation results
• Results can be retrieved for future computations

File system

• Named persistent storage
• Typically organized in a hierarchy

File access

• Accessed via OS APIs (read, write)
• May be mapped to memory space and accessed directly with load and store

Databases

• Provide persistent storage
• Allows related data to be access easily

What if the system goes down during a write?

Crashes

• Any operation should be immediately interruptable without leaving persistent storage in a degraded state
• Atomic transactions and journaling may be a solution to this