Multi-processing and
Multi-threading
Dr. Soumya Indela
Topics
• Multi-processing
• Amdahl’s Law
• Scaling
• Multi-threading
• Fine-grained
• Coarse-grained
• Simultaneous
Process vs Thread
• Processes
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An instance of a computer program
Shared Memory, different address spaces
Separate Resources - Registers, file storage,
network connections, etc.
Independent of each other
• Threads – subset of process
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Set of tasks within a process
Shared Address space
Shared Registers, file storage, network connections.
Interdependent, can be scheduled independently
Amdahl’s Law
• Improvement in Performance or Speedup is limited by the Serial portion
1
• Theoretical Speed-up =
𝑓
1−𝑓 + 𝑠
• f : Fraction of execution for the parallel portion
• s : Scaling factor or number of processes parallel portion can be split into
Program
Serial Portion
Parallel Portion
Scaling
• Strong Scaling: Speed-up achieved without increasing size of
program
• Weak Scaling: Speed-up achieved by increasing size of program
Program
Serial Portion
Parallel Portion
Program
Serial Portion
Parallel Portion
Context Switch
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Important when switching between threads
Save current state of thread being stopped
If required, retrieve previous state of thread being resumed
Otherwise, start a new thread
Context or State
Thread ID
Registers
Program Counter
Stack Pointer
Etc.
Example Threads
• Every line is an instruction
• Stalls - Blank space between set of instructions
Time
Fine-grained Multi-threading
• Switch between threads after every instruction
• Advantage: less throughput losses due to short and long stalls, as
instructions from other threads can be executed during the stalls
• Disadvantage: more execution time for individual threads, since a
thread that is ready to execute will be delayed
Time
Coarse-grained Multi-threading
• Switch between threads after significant events like cache miss, etc.
• Advantage: Less thread switching, so less likely to slow down the
execution of an individual thread
• Disadvantage: Limited ability to overcome throughput losses from
shorter stalls due to pipeline refill overhead
Time
Simultaneous Multi-threading
• Executes multiple threads simultaneously utilizing multi-issue
resources
• Dynamically scheduled microarchitecture
• Advantage: Least cost of multithreading
• Disadvantage: Additional Resources required
Time
Multi-processing vs Multi-threading
Multi-processing
Multi-threading
Multiple processors
Single Processor
Single or Multiple threads
Multiple threads
Parallel Execution
Concurrent Execution
Only Shared Memory, all other
resources are separate
All shared resources – address
space, registers, file storage,
network connections, etc.
Applicable for Compute
intensive processes, like CPU
bound processes
Applicable for I/O bound
processes
Questions
Thank you!