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Chapter 26 Concurrency & Thread

Chapter 26 Concurrency & Thread. Chien -Chung Shen CIS, UD cshen@cis.udel.edu. Introduction. Abstractions p rocess - virtualize one CPU for multiprogramming a ddress space – virtual memory

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Chapter 26 Concurrency & Thread

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  1. Chapter 26Concurrency & Thread Chien-Chung Shen CIS, UD cshen@cis.udel.edu

  2. Introduction • Abstractions • process - virtualize one CPU for multiprogramming • address space – virtual memory • Multi-threaded program has more than one point of execution - multiple PCs, each of which is being fetched and executed from • multiple “light-weight” processes sharing the same address space • context switching between threads

  3. Stacks Single-threaded 2-threaded

  4. Thread Creation • Figure 26.2 • Execution traces: Tables 26.1-3

  5. Shared Data • Figure 26.3 - two threads update a global shared variable • Non-deterministic results due to uncontrolled Scheduling • prompt> objdump -d main// Linux • counter = counter + 1; • mov 0x8049a1c, %eax add $0x1, %eax mov %eax, 0x8049a1c • Table 26.4

  6. Race Condition • Results depend on the timing execution of the code- with some bad luck (i.e., context switches that occur at untimely points in the execution), we get the wrong (or different) result • Because multiple threads executing this code can result in a race condition, we call this code a critical section • A critical section is a piece of code that accesses a shared variable (or more generally, a shared resource) and must not be concurrently executed by more than one thread • Mutual exclusion - property guarantees that if one thread is executing within the critical section, the others will be prevented from doing so

  7. Atomicity • One way to solve race condition to have more powerful instructions that, in a single step, did exactly whatever we needed done and thus removed the possibility of an untimely interrupt • Synchronization primitives on top of hardware • Every kernel data structure has to be carefully accessed, with the proper synchronization primitives, to work correctly

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