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\begin{document}
\lecture{19 --- Apr 25, 2007}{Spring 2007}{Prof.\ Erik Demaine}{Aditya Rathnam}
\section{Overview}
Previously we worked in the RAM or cell probe models, in which the cost of
an algorithm only depended on the total number of accesses to memory locations.
However, these models do not reflect the \emph{memory hierarchy} of real
computers that have more than one layer of memory, with different access
characteristics.
In a modern computer, the CPU operates on values in its registers, and
can fetch values from main memory, usually via several layers of
caches. Large files are stored on disk, and data is transferred from
disk to main memory. The memory closest to the CPU is fastest but
smallest, and the memory farthest from the CPU is largest but has the
highest latency. Moreover, the slower layers of memory have greater
parallelism: it takes about as long to fetch a consecutive kilobyte
from disk as it does to fetch a single byte, since most of the cost is
latency for the disk to find the right location. So we might as well
always transfer a full \emph{block} of data at a time; the slower
layers of memory have larger block sizes. This prompts the need for
algorithms and data structures that exploit locality of reference,
arranging the layout of memory with data that is frequently accessed
together placed in the same blocks, in order to minimize the number of
blocks that need to be accessed.
This lecture covered two models of computation that take this memory
hierarchy into account: the \emph{external-memory model} and its
refinement, the \emph{cache-oblivious model}.
\section{External-Memory Model}
\label{sec:external-memory}
The external-memory model was introduced by Aggarwal and Vitter in
1988~\cite{DBLP:journals/cacm/AggarwalV88}, and is sometimes also
known as the \emph{I/O model} or \emph{disk access model (DAM)}. It
captures a two-layered memory hierarchy. The model is based on a
computer with a CPU connected directly to a fast cache of size $M$,
which is connected to a much larger and slower disk. Both the cache
and disk are divided into blocks of size $B$; the cache thus
holds $\frac{M}{B}$ blocks, while the disk can hold many more. The CPU
can only operate directly on the data stored in cache.
Algorithms can make \emph{memory transfer} operations, which read a
block from disk to cache, or write a block from cache to disk. The
cost of an algorithm is the number of memory transfers required;
operations on cached data are considered free.
Clearly any algorithm that has running time $T(N)$ in the RAM model
can be trivially converted into an external-memory algorithm that
requires no more than $T(N)$ memory transfers, by ignoring
locality. We want to do better than this. Ideally, we would like to
achieve $\frac{T(N)}{B}$, but this optimum is often hard to achieve.
\subsection{Scanning}
\label{sec:external-memory:scanning}
It is easy to see that we can scan $N$ elements in $\bigO{\lceil \frac{N}{B} \rceil}$ memory transfers.
\subsection{Searching}
\label{sec:external-memory:searching}
The ideal search tree structure for the external-memory model is a
B-tree with branching factor $\bigTheta{B}$, such that each node fits
in a block. This allows us to perform $\proc{Insert}$,
$\proc{Delete}$, and $\proc{Search}$ operations using
$\bigO{\log_{B+1} N}$ memory transfers and $\bigO{\log N}$ time in the
comparison model.
This is an optimal bound for the external-memory comparison-model
searching, as an information theoretic argument shows. Suppose that
we wish to discover where some element $x$ is located in an array of
$n$ elements. Expressing the answer (e.g. as an index into the array)
requires at least $\lg(N + 1)$ bits. Each time a block is transferred,
it reads at most $\bigTheta{B}$ elements, learning where $x$ fits into
these $B$ elements. This provides no more than $\bigO{\lg(B+1)}$ bits
of information. Since we need to determine $\lg(N + 1)$ bits of
information, at least $\frac{\lg(N + 1)}{\bigO{\lg(B + 1)}} =
\bigOmega{\lg_{B+1} N}$ memory transfers are required.
\subsection{Sorting}
\label{sec:external-memory:sorting}
The natural companion to the searching problem is sorting. In the RAM
model, we can sort $N$ elements by inserting them into a B-tree and
performing $N$ $\proc{Delete-Min}$ operations. This gives us $\bigO{N
\lg N}$ runtime, which we know to be optimal. We can do the same
in the external-memory model, and perform sorting in $\bigO{\frac{N}{B}
\lg_{B+1} N}$ memory transfers. But this is \emph{not} optimal.
We can do better with \emph{$\frac{M}{B}$-way mergesort}, which
divides a sorting problem into $\frac{M}{B}$ subproblems, recursively
sorts them, and merges them. This gives us a cost of
$\bigO{\frac{N}{B} \log_{\frac{M}{B}} \frac{N}{B}}$ memory transfers.
This is optimal: it can be shown by a similar
information-theoretic argument to the one above that sorting $N$
elements in the comparison model requires $\bigOmega{\frac{N}{B}
\log_{\frac{M}{B}} \frac{N}{B}}$ memory transfers
\cite{DBLP:journals/cacm/AggarwalV88}.
\def\sortingbound{\frac{N}{B} \log_{\frac{M}{B}} \frac{N}{B}}
\subsection{Permutation}
\label{sec:external-memory:permutation}
The permutation problem is, given $N$ elements and a permutation, to
rearrange the elements according to that permutation. We can do this
in $\bigO{N}$ memory transfers by moving each element to its new position,
ignoring locality. We can also solve the problem with
$\bigO{\sortingbound}$ memory transfers by sorting the elements,
sorting the permutation, and then applying the permutation in
reverse. This gives us a bound of
$\bigO{\min\left(N,\;\sortingbound\right)}$.
In the \emph{indivisible model}, where elements cannot be divided
between blocks, there is a matching lower bound. It remains an open
problem whether one can do better in a general model.
\subsection{Buffer Trees}
\label{sec:buffer-trees}
We noted in Section~\ref{sec:external-memory:sorting} that B-trees are
optimal for searching, but cannot be used as the basis for an optimal
sorting algorithm. The buffer tree \cite{arge-buffer-tree} is a data
structure that provides \proc{Insert}, \proc{Delete},
\proc{Delete-Min}, and \proc{Batched-Search} operations using
$O\left(\frac{1}{B} \log_{\frac{M}{B}} \frac{N}{B}\right)$ amortized
memory transfers per operation. (Notice that this bound is usually
$\littleO{1}$, so it has to be amortized!) \proc{Batched-Search} is a
delayed query: it performs a search for a particular value in the tree
as it appears at a particular time, but does not give the results
immediately; they become available later, after other operations have
been performed.
\section{Cache-Oblivious Model}
\label{sec:cache-oblivious}
The cache-oblivious model is a variation of the external-memory model
introduced by Frigo, Leiserson, Prokop, and Ramachandran in
1999~\cite{flpr-cache-oblivious,prokop-meng}. In this model, the
algorithm does not know the size $M$ of its cache, or the block size
$B$. This means that it cannot perform its own memory management,
explicitly performing memory transfers. Instead, algorithms are RAM
algorithms, and block transfers are performed automatically, triggered
by element accesses, using the offline optimal block replacement
strategy. Though the use of the offline optimal block replacement strategy
sounds like it would pose a problem for practical applications, in
fact there are a number of competitive block replacement strategies
(FIFO, LRU, etc.) that are within a factor of 2 of optimal given a
cache of twice the size.
Though the algorithm does not know the size $M$ of its cache, we will
routinely assume that $M \ge c \cdot B$ for any constant $c$,
i.e. that the cache can hold at least $c$ blocks. (In practice,
however, the algorithms we consider will not require $c$ to be very
large at all.)
From a theoretical standpoint, the cache-oblivious model is appealing
because it is very clean. A cache-oblivious algorithm is simply a RAM
algorithm; it is only the analysis that differs. The cache-oblivious
model also works well for multilevel memory hierarchies, unlike the
external-memory model, which only captures a two-level hierarchy.
Since a cache-oblivious algorithm provides the desired result for any
cache size and block size, it will work with \emph{every} cache and
block size at each level of the hierarchy, thus giving the same bound
overall.
\subsection{Survey of Results}
\label{sec:cache-oblivious:survey}
\paragraph{Searching}
A cache-oblivious variant of the B-tree \cite{btree-focs00,
btree-soda02, btree-soda02-2} provides the \proc{Insert},
\proc{Delete}, and \proc{Search} operations with $\bigO{\log_{B+1} N}$
memory transfers, as in the external-memory model.
\paragraph{Sorting}
As in the external-memory model, sorting $N$ elements can be performed
cache-obliviously using $\bigO{\sortingbound}$ memory transfers
\cite{flpr-cache-oblivious,brodal-fagerberg-icalp}. Note, however,
that this requires the \emph{tall-cache assumption}: that $M =
\bigOmega{B^{1+\epsilon}}$. The external-memory sorting algorithm does
not require this to be the case. In \cite{brodal-fagerberg-stoc}, it
was shown that the tall-cache assumption is necessary.
\paragraph{Priority queue}
A priority queue can be implemented that executes the \proc{Insert},
\proc{Delete}, and \proc{Delete-Min} operations in $O\left(\frac{1}{B}
\log_{\frac{M}{B}} \frac{N}{B}\right)$ memory transfers
\cite{arge-bender-priority-queue,brodal-fagerberg-isaac}.
\subsection{Static Search Trees}
\label{sec:cache-oblivious:static-search-trees}
We will now see how to construct a static search tree that can perform
searches using $\bigO{\log_{B+1} N}$ memory transfers. We do this by
constructing a complete binary search tree with the $N$ elements
stored in sorted order.
In order to achieve the desired memory transfer bound, we will store
the tree on disk using a representation known as the \emph{van Emde
Boas layout} \cite{prokop-meng}. It uses the van Emde Boas idea of
dividing the tree at the middle level of edges, giving a top subtree
of $\sqrt{N}$ elements, and $\sqrt{N}$ subtrees of $\sqrt{N}$ elements
each. We recursively lay out each of the $\sqrt{N}+1$ subtrees, then
concatenate them, ensuring that each subtree is stored consecutively.
\begin{claim}
Performing a search on a search tree in the van Emde Boas layout
requires $\bigO{\log_{B+1} N}$ memory transfers.
\end{claim}
\begin{proof}
We can stop our analysis when we reach a subtree that has size less
than $B$: the algorithm will continue, but since the subtree fits
entirely within one block, it will fit in the cache, and no further
memory transfers will be required. So consider the level of
recursion that ``straddles'' B: the whole structure at that level
has size greater than $B$, but each subtree has size at most
$B$. Each subtree requires at most two memory transfers to access
(it can fit in one block, but it might actually need to be stored in
two blocks if it is ``out of frame'' with the block boundaries.)
Since we are considering the level that straddles $B$, each subtree
must have height at least $\frac{1}{2} \lg B$: otherwise two levels
of these subtrees would have height less than $\lg B$, and thus
contain less than $B$ elements, violating the definition of the
level that straddles $B$. So we will need to access $\frac{\lg
N}{\frac{1}{2} \lg B} = 2 \log_B N$ subtrees. Each subtree access
can be done in 2 memory transfers, so we need no more than $4 \log_B
N$ memory transfers to perform a search.
\end{proof}
Note that this technique can be generalized to trees whose height is
not a power of 2, and non-binary trees of
constant degree (except for degree 1, of course).
\subsection{Ordered File Maintenance}
\label{sec:cache-oblivious:ordered-file-maintenance}
Before we proceed to describe how to make these search trees dynamic,
we will need a result to use as a black box.
The ordered file maintenance (OFM) problem is to store $N$ elements in
a specified order in an array of size $\bigO{N}$. This array can have gaps, but
any two consecutive elements must be separated by at most $O(1)$
gaps. An ordered file maintenance data structure must support two
operations: to \proc{Insert} an element between two other elements,
preserving the order of the array, and to \proc{Delete} an element.
Our black box can accomplish these two operations by rearranging
$\bigO{\lg^2 N}$ consecutive elements, amortized. The next lecture
will describe how to do this.
\subsection{Dynamic Search Trees}
\label{sec:dynamic-search-trees}
Now we can turn to how to build a dynamic search tree. This
description follows that of \cite{btree-soda02}, a simplification of
\cite{btree-focs00}.
We store our elements in an ordered file structure, then build a static
search tree ``on top'' of the ordered file structure: the leaves
correspond to the array slots in the ordered file structure (including
blank spaces where there are gaps in the ordered file array). The
search tree invariant is that each internal node stores the maximum
value of its children (ignoring the blank spaces due to empty slots).
This structure allows us to perform searches with $\bigO{\lg_{B+1} N}$
memory transfers in the standard way. Performing an insertion is a bit
more complicated. We begin by performing a \proc{Search} to find the
predecessor or successor of our new element, and thus finding where
to insert it into the ordered file maintenance structure. Performing
the OFM insert changes $\bigO{\lg^2 N}$ cells. Then, for all of these
cells, we update the corresponding leaves of the search tree, and
propagate the changes upward in a post-order traversal of the changed
leaves and their ancestors.
\begin{claim}
If $k$ cells in the ordered file structure are changed, the cost of
updating the corresponding leaves and ancestors in the search tree is
$\bigO{\lg_{B+1} N + \frac{k}{B}}$ memory transfers.
\end{claim}
\begin{proof}
As before, consider the level of detail straddling $B$. Consider the
bottom two levels of subtrees (each of size at most $B$). We are
performing updates in a post-order traversal of the tree. For the
bottom two levels, this is essentially a scanning over the subtrees,
each of which fits in one block: we perform the updates in one
subtree at the bottom level, then move up to the subtree at the next
higher level to perform some updates before moving on to the next
subtree at the bottom level. So as long as our cache is big enough
to hold six blocks at once (for a block of the ordered file
maintenance structure, the subtree at the bottom level, and the
subtree at the second-from-bottom level --- actually two each, since
we do not know where the block boundaries are and may be ``out of
frame'' as before), we need $\bigO{\frac{k}{B}}$ memory transfers to
update the bottom two levels.
Now we consider the updates above the bottom two levels. Note that
the larger subtrees composed of the bottom two levels have size $J >
B$ since we are considering the level of detail that straddles $B$.
This means that after the bottom two levels, $J$ leaves are reduced
to one node, so there are $\bigO{\frac{K}{J}} = \bigO{\frac{K}{B}}$
elements to be traversed until the least common ancestor has been
reached. We can afford one memory transfer per element. Then, after
the least common ancestor has been reached, there are
$\bigO{\lg_{B+1} N}$ elements on the path to the root. So the total
cost is $\bigO{\lg_{B+1} N + \frac{K}{B}}$ memory transfers.
\end{proof}
We now have a tree that can perform \proc{Insert} operations in
$\bigO{\lg_{B+1} N + \frac{\lg^2 N}{B}}$ amortized memory transfers
(and \proc{Delete} operations with the same cost, in a similar way),
and searches in $\bigO{\lg_{B+1} N}$ memory transfers.
To eliminate the $\bigO{\frac{\lg^2 N}{B}}$ factor, we can use
indirection. We cluster elements into $\Theta(\frac{N}{\lg N})$ groups of $\Theta(\lg N)$
elements, and store the minimum of the group in the ordered file
maintenance structure and search tree as before. Now performing a
\proc{Insert} operation requires rewriting an entire group, but this
costs $\bigO{\frac{\lg N}{B}} = \bigO{\lg_B N}$ memory transfers. If
the size of a group grows too large after $\bigO{\lg N}$
\proc{Insert}s, we may need to split the group, but we can amortize
away the cost of this in the standard way. Now the cost of a update
operation is $\bigO{\lg_{B+1} N + \frac{\lg N}{B}} = \bigO{\lg_{B+1}
N}$ amortized memory transfers.
%\bibliographystyle{abbrv}
%\bibliography{lec23}
\begin{thebibliography}{10}
\bibitem{DBLP:journals/cacm/AggarwalV88}
A.~Aggarwal and J.~S. Vitter.
\newblock The input/output complexity of sorting and related problems.
\newblock {\em Commun. ACM}, 31(9):1116--1127, 1988.
\bibitem{arge-buffer-tree}
L.~Arge.
\newblock The buffer tree: A technique for designing batched external data
structures.
\newblock {\em Algorithmica}, 37(1):1--24, June 2003.
\bibitem{arge-bender-priority-queue}
L.~Arge, M.~A. Bender, E.~D. Demaine, B.~Holland-Minkley, and J.~I. Munro.
\newblock Cache-oblivious priority queue and graph algorithm applications.
\newblock In {\em Proc. {STOC '02}}, pages 268--276, May 2002.
\bibitem{btree-focs00}
M.~A. Bender, E.~D. Demaine, and M.~Farach-Colton.
\newblock Cache-oblivious {B}-trees.
\newblock In {\em Proc. {FOCS} '00}, pages 399--409, Nov. 2000.
\bibitem{btree-soda02}
M.~A. Bender, Z.~Duan, J.~Iacono, and J.~Wu.
\newblock A locality-preserving cache-oblivious dynamic dictionary.
\newblock In {\em Proc. {SODA '02}}, pages 29--38, 2002.
\bibitem{brodal-fagerberg-isaac}
G.~S. Brodal and R.~Fagerberg.
\newblock Funnel heap --- a cache oblivious priority queue.
\newblock In {\em Proc. {ISAAC '02}}, pages 219--228, 2002.
\bibitem{brodal-fagerberg-icalp}
G.~S. Brodal and R.~Fagerberg.
\newblock Cache oblivious distribution sweeping.
\newblock In {\em Proc. {ICALP '03}}, page 426, 2003.
\bibitem{brodal-fagerberg-stoc}
G.~S. Brodal and R.~Fagerberg.
\newblock On the limits of cache-obliviousness.
\newblock In {\em Proc. {STOC '03}}, pages 307--315, 2003.
\bibitem{btree-soda02-2}
G.~S. Brodal, R.~Fagerberg, and R.~Jacob.
\newblock Cache oblivious search trees via binary trees of small height.
\newblock In {\em Proc. {SODA '02}}, pages 39--48, 2002.
\bibitem{flpr-cache-oblivious}
M.~Frigo, C.~E. Leiserson, H.~Prokop, and S.~Ramachandran.
\newblock Cache-oblivious algorithms.
\newblock In {\em Proc. {FOCS '99}}, pages 285--298, 1999.
\bibitem{prokop-meng}
H.~Prokop.
\newblock Cache-oblivious algorithms.
\newblock Master's thesis, Massachusetts Institute of Technology, June 1999.
\end{thebibliography}
\end{document}