% cpu/cpu.tex
% mainfile: ../perfbook.tex
% SPDX-License-Identifier: CC-BY-SA-3.0

\QuickQuizChapter{chp:Hardware and its Habits}{Hardware and its Habits}{qqzcpu}
%
\Epigraph{Premature abstraction is the root of all evil.}
	 {A cast of thousands}

Most people intuitively understand that passing messages between systems
is more expensive than performing simple calculations within the confines
of a single system.
But it is also the case that communicating among threads within the
confines of a single shared-memory system can be quite expensive.
This chapter therefore looks at the cost of synchronization and communication
within a shared-memory system.
These few pages can do no more than scratch the surface of shared-memory
parallel hardware design; readers desiring more detail would do well
to start with a recent edition of \pplsur{John L.}{Hennessy}'s and
\pplsur{David A.}{Patterson}'s classic
text~\cite{Hennessy2017}.

\QuickQuiz{
	Why should parallel programmers bother learning low-level
	properties of the hardware?
	Wouldn't it be easier, better, more elegant, and more productive
	to remain at a higher level of abstraction?
}\QuickQuizAnswer{
	It is often easier to ignore the detailed properties of the
	hardware, and higher levels of abstraction can greatly improve
	productivity.
	But those of us working with low-level concurrent software would
	be quite foolish to ignore those properties.

	After all, if you accept that the only purpose of parallelism
	is to increase performance, and if you further accept that
	performance depends on detailed properties of the hardware,
	then it logically follows that parallel programmers are going
	to need to know at least a few hardware properties.

	This is also the case in most engineering disciplines.
	Would \emph{you} want to use a bridge designed by an
	engineer who did not understand the properties of
	the concrete and steel making up that bridge?
	If not, why would you expect a parallel programmer to be
	able to develop competent parallel software without at least
	\emph{some} understanding of the underlying hardware?

	In short, you might not care about the laws of physics, but the
	laws of physics cares deeply about your code!
}\QuickQuizEnd

\input{cpu/overview}
\input{cpu/overheads}
\input{cpu/hwfreelunch}
\input{cpu/swdesign}

So, to sum up:

\begin{enumerate}
\item	The good news is that multicore systems are inexpensive and
	readily available.
\item	More good news:
	The overhead of many synchronization operations is much lower
	than it was on parallel systems from the early 2000s.
\item	The bad news is that the overhead of cache misses is still high,
	especially on large systems.
\end{enumerate}

The remainder of this book describes ways of handling this bad news.

In particular,
\cref{chp:Tools of the Trade} will cover some of the low-level
tools used for parallel programming,
\cref{chp:Counting} will investigate problems and solutions to
parallel counting, and
\cref{chp:Partitioning and Synchronization Design}
will discuss design disciplines that promote performance and scalability.

\QuickQuizAnswersChp{qqzcpu}