Hardware Reference
In-Depth Information
Pitfall A Single Point Of Failure
The calculations of reliability improvement using Amdahl's law on page 48 show that depend-
ability is no stronger than the weakest link in a chain. No mater how much more dependable
we make the power supplies, as we did in our example, the single fan will limit the reliability
of the disk subsystem. This Amdahl's law observation led to a rule of thumb for fault-tolerant
systems to make sure that every component was redundant so that no single component fail-
rule could bring down the whole system. Chapter 6 shows how a software layer avoids single
points of failure inside warehouse-scale computers.
Fallacy Hardware Enhancements That Increase Performance Improve Energy
Efficiency Or Are At Worst Energy Neutral
Esmaeilzadeh et al. [2011] measured SPEC2006 on just one core of a 2.67 GHz Intel Core i7
i7using Turbo mode ( Section 1.5 ) . Performance increased by a factor of 1.07 when the clock rate
increased to 2.94 GHz (or a factor of 1.10), but the i7 used a factor of 1.37 more joules and a
factor of 1.47 more wat-hours!
Fallacy Benchmarks Remain Valid Indefinitely
Several factors influence the usefulness of a benchmark as a predictor of real performance, and
some change over time. A big factor influencing the usefulness of a benchmark is its ability to
resist “benchmark engineering” or “benchmarketing.” Once a benchmark becomes standard-
ized and popular, there is tremendous pressure to improve performance by targeted optimiz-
ations or by aggressive interpretation of the rules for running the benchmark. Small kernels or
programs that spend their time in a small amount of code are particularly vulnerable.
For example, despite the best intentions, the initial SPEC89 benchmark suite included a
small kernel, called matrix300, which consisted of eight different 300 × 300 matrix multiplica-
tions. In this kernel, 99% of the execution time was in a single line (see SPEC [1989] ) . When an
IBM compiler optimized this inner loop (using an idea called blocking , discussed in Chapters 2
and 4 ) , performance improved by a factor of 9 over a prior version of the compiler! This bench-
mark tested compiler tuning and was not, of course, a good indication of overall performance,
nor of the typical value of this particular optimization.
Over a long period, these changes may make even a well-chosen benchmark obsolete; Gcc
is the lone survivor from SPEC89. Figure 1.16 on page 39 lists the status of all 70 benchmarks
from the various SPEC releases. Amazingly, almost 70% of all programs from SPEC2000 or
earlier were dropped from the next release.
Fallacy The Rated Mean Time To Failure Of Disks Is 1,200,000 Hours Or Almost 140
Years, So Disks Practically Never Fail
The current marketing practices of disk manufacturers can mislead users. How is such an
MTTF calculated? Early in the process, manufacturers will put thousands of disks in a room,
run them for a few months, and count the number that fail. They compute MTTF as the total
number of hours that the disks worked cumulatively divided by the number that failed.
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