Single-Chip Multiprocessor
Nirmal Andrews
Case for single chip multiprocessors

Advances in the field of integrated chip processing.
- Gate density
(More transistors per chip)
- Cost of wires

Many studies done in Stanford University during late
90’s and proved CMP (single-chip multiprocessor) is
better than competing technology .
Parallelism

Parallelism becomes a necessity for improving performance.

Parallelism made possible using dynamic scheduling, multiple
instruction issue, speculative execution, non-blocking caches etc., (late
90’s)

Parallelism classifications:
Instruction level
Loop level
Thread level - Future trend
Process level - Future trend
Loop level parallelism

To increase amount of parallelism – exploit
parallelism among iterations of a loop.

ILP that results from data independent loop iterations
is LLP.

No circular dependencies. This could be avoided too
using loop unrolling (beyond the scope of this
lecture).
LLP (Loop Level Parallelism)

15% ILP extracted from a basic block in an integer programs – 7 instructions.

for (i=1; i<=100; i= i+1)
{
a[i] = a[i] + b[i];
//s1
b[i+1] = c[i] + d[i]; //s2
}

s1 depends on s2. So to extract LLP rearrange:
a[1] = a[1] + b[1];
for (i=1; i<=99; i= i+1)
{
b[i+1] = c[i] + d[i];
a[i+1] = a[i+1] + b[i+1];
}
b[101] = c[100] + d[100];
No dependencies.
Competing technology - Superscalar

Executing multiple instruction in the same clock cycle.

Dynamic scheduling

Single processor

Redundant functional units on processor

Mixture between a scalar and vector processor
Competing technology - Superscalar
Wide issue superscalar
Fetch Phase

3 phase: Fetch, Issue, Execution

Bottlenecks: Issue and Execution phase.

*
**
Fetch phase: Provide large and accurate window of decoded instructions - 3 issues:
instruction misalignment, cache miss, mispredicted branch.
- misprediction reduced to under 5% using branch predictor designed by McFarling*.
- instruction misalignement reduced to under 3% by dividing cache into banks (Conte).
- Roesnblum et al. shows that the 60% of latency by cache miss can be hidden**.
S. McFarling, “Combining branch predictors,” WRL Technical Note TN-36, Digital
Equipment Corporation, 1993.
M. Rosenblum, E. Bugnion, S. Herrod, E. Witchel, and A. Gupta, “The impact of
architectural trends on operating system performance,” Proceedings of 15th ACM
symposium on Operating Systems Principles, Colorado,December, 1995.
Issue phase

Issue phase: Register renaming.
2 techniques for register renaming:
- Use a table to map architectural registers and physical registers. Ports
required: operands per instruction* Instruction window size
- Use reorder buffer. Comparators required to find which physical register
should provide data to which packet of instruction. Large number of
comparators required.

In HP – PA 8000 20% of die space occupied by comparators.

Quadratic increase in instruction queue register with increase in issue width.

Queue register uses broadcast to connect to registers which increases the
wires used – increased delay and cost
Execute phase

Execution phase also has similar issues.

Increase in issue width causes increase in renamed registers
leading to quadratic increase in register file complexity.

Increase in execution unit causes quadratic increase in the
complexity of bypass logic.
Bottleneck: Interconnect delay between execution units.
Architecture - Superscalar
Competing technologies –
Simultaneous Multi Threading

Simultaneous Multi threading architecture is similar to that of
the superscalar.

SMT processors support wide superscalar processors with
hardware, to execute instructions from multiple thread
concurrently.

Provides latency tolerance.

Reduces to conventional wide-issue superscalar when no
multiple threads possible.
Competing technologies Simultaneous Multi Threading

SMT (Simultaneous Multi-threading)
Centralized architecture

Disadvantages of centralized architectures such as SMT and
Superscalars are:
- Area increases quadratically with core’s complexity.
- Increase in cycle time – interconnect delays. Delay with wires
dominate delay of critical path of CPU. Possible to make
simpler clusters, but results in deeper pipeline and increase in
branch misprediction penalty.
- Design verification cost high, due to complexity and single
processor
- Large demand on memory system.
Single Chip multiprocessor




Motivation for a decentralized architecture
due to the disadvantages of competing
technologies.
Simple individual processors and high clock
rate.
Low interconnect latency
Exploits thread level and processor level
parallelism.
Single chip Multiprocessor
architecture
Performance comparison



Example: 8 core Cell
processor in the PS3
and the 3 core Xenon
processor in the Xbox
360)
Performance chart
Run for different
benchmark programs.
Summary (CMP)



CMP (Chip level multiprocessor) provides superior
performance with simpler hardware.
No parallelism – Superscalar performance is 30%
better than CMP
Fine grained thread-level parallelism – Superscalar
is 10% better in performance
Coarse grained thread-level parallelism – CMP is 50100% better than superscalar.
Disadvantage – Slow when no multithreading, equal
development of software required.
Reference




K Olukotun, BA Nayfeh, L Hammond, K Wilson, K, “The case
for a single-chip multiprocessor,”
ACM SIGPLAN Notices, 1996.
L Hammond, BA Nayfeh, K Olukotun, “A Single-Chip
Multiprocessor,” IEEE, Sept 1997.
Wikipedia, “Superscalar,” April 2008.
http://en.wikipedia.org/wiki/Superscalar.
Wikipedia, “Multi-core,” April 2008.
http://en.wikipedia.org/wiki/Multi-core_computing.