Session Two of Many Cores with Java, looking at parallelism within a single modern core. Discusses the links with

1. An End to Order
many cores with java
session two
parallelism in a single core
copyright 2013 Robert Burrell Donkin robertburrelldonkin.name
this work is licensed under a Creative Commons Attribution 3.0 Unported License

2. Pre-emptive multi-tasking operating
systems use involuntary context
switching to provide the illusion of
parallel processes even when the
hardware supports only a single thread
of execution.
Take Away from Session One

3. Reflecting on the Exercises
● Pre-emptive
○ not time sharing
● Code can be interrupted...
○ at almost any time
● Concurrent Java is platform independent...
○ only when the rules are followed

4. If Only Single Cores Were So Simple
● Moore's Law
○ Number of transistors doubles every 2 years
● Pipelines
○ Small steps
○ Improves throughput
○ Facilitates higher clock
● How Long?
○ IBM Stretch - Fetch, Decode, and Execute
○ Classic RISC - 5 steps
○ Pentium D - 31 steps

5. An End to Order
● out of order execution
○ execution order
■ governed by efficiency
■ not program order
● serial execution is an illusion created by
a conspiracy of
○ core
○ compiler and
○ platform
● enter the Java Memory Model

6. Parallel Pipelines
● Superscalar
○ instruction level parallelism
● Siumultaneous MultiThreading
○ aka HyperThreading
○ hardware support for multiple threads of execution
within the same core
○ shared caches

7. What makes Parallelism Hard...?
● High-dimensional problem space
○ lots of potential solutions
● Unpredicatable
○ non-linearity
● Analysis is expensive
○ potentially entire system must be known

8. Concurrent Qualities
Recall:
● Responsiveness
○ to human (or other external) actors
● Liveliness
○ an intrinsic quality
○ all tasks continue to make progress

9. Performance Qualities
(following Doug Lea)
● Throughput
○ operations per unit time
● Latency
○ time taken to service a message
● Efficiency
○ computational resources required to achieve a level
of throughput

10. Performance Qualities
(following Doug Lea)
● Capacity
○ number of simultaneous tasks
■ for target throughput or latency
● Scalability
○ rate at which throughput or latency improves
■ when resources are added
● Degradation
○ rate at which throughput or latency improves
■ when activities are added

11. Even on a single core,
there's no escaping parallelism.
Take Away