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<P><I>C/C++ Users Journal</I> December, 2005</P>
<H1>Policy-Based Memory Allocation</H1>
<H2>Fine-tuning your memory management</H2>
<H3>By Andrei Alexandrescu and Emery Berger</H3><I>Andrei
Alexandrescu is a graduate student in Computer Science at the
University of Washington and author of Modern C++ Design. He can be
contacted at andrei@metalanguage.com.</I>
<HR>
<P>One of the perks of doing hard work for next to no money, an
activity also known as being a graduate student, is that you get to
rub shoulders with researchers working on interesting problems. I've
had the pleasure of meeting in person both Emery Berger and Kathryn
McKinley, two of the authors (with Benjamin Zorn) of "Composing
High-Performance Memory Allocators" [6], a paper that I found highly
interesting for "I can't believe I didn't think of that" reasons.
But let's start at the beginning.</P>
<P>One chapter of <I>Modern C++ Design</I> [2] is somewhat different
than all the others. Actually, very different. While the book is
dedicated to creating elegant, flexible, extensible designs, one
chapter stands out like a sore thumb by actually describing one
fixed, monolithic, rigid, awkward piece of software: Loki's small
object allocator. Somehow, I thought that memory management was a
low-level operation and, as such, didn't lend itself to the elegant
exploits of policy-based design. At the same time, some of the
compiler-provided <B>new</B>/<B>delete</B> implementations were
pretty bad at dealing with small objects, so I felt there would be a
need for something to fill the void. So I sat down and wrote an
implementation that has portability as its only merit; other than
that, it's not particularly performant, interesting, or original.
The main problem with Loki's allocator, however, does not lie in its
implementation (which, by the way, has been entirely rewritten by
Rich Sposato, to the end of greatly improving its performance), but
in its design, which is not configurable nor extensiblejust a few
monolithic classes, much like what you'd expect in the bad ol'
days.</P>
<P>However, at about the same time, Emery Berger and others were
experimenting with a mixin-based memory allocator for C++ that is at
the same time highly efficient (rivaling the best general-purpose
allocators out there), highly customizable (rivaling the best
special-purpose allocators, too), and highly portable by virtue of
its customizable design! As soon as I became acquainted with that
research, I realized what the chapter on memory allocation in
<I>Modern C++ Design</I> should have looked like.</P>
<P>But before we delve into the fascinating topic of configurable
memory allocation, let's tie a few knots with past comments received
via e-mail.</P>
<H3>Mailcontainer</H3>
<P>I wrote the sidebar "Memory: More Than Just Any Resource" in the
last Generic<Programming> installment with the serene
resignation with which Galileo Galilei must have said his famous
"Eppur si muove." "And yet, garbage is collectable," I mumbled into
my three-days beard when submitting the article for publication. The
expected outcome was that the opponents of garbage collection would
exemplarily mortify me via an endless flow of angry e-mails.
However, they must've chosen instead to boycott me, because I
haven't gotten a single response in protest.</P>
<P>Ivan Godard sent me an e-mail on the subject of Observers [3, 4].
That message was so smart, and had me look up words in the
dictionary so often, that I've decided to quote it almost in
entirety, at the risk of blowing up the size of this article:</P>
<BLOCKQUOTE>Your difficulties with <B>attach</B> and <B>detach</B>
while in the middle of notification are canonical concurrency
problems, and can be dealt with in the usual way. The (only
reasonable?) semantics for Observer is to see <B>subject</B>,
<B>observer</B>, and <B>event</B> as three independent and
asynchronous processes, with a time stream localized at
<B>subject</B>. That is, an <B>observer</B> cannot directly
determine the time relation between its attempt to attach and the
occurrence of a particular event because the event is not within
the observer's horizon. Only the <B>subject</B>, which receives
both events and requests to <B>attach</B>, can time-order
these.<BR><BR>Consequently a putative observer will see some
sequence of events that (in the subject's time frame) begins some
time after the attempt to attach and ends some time after the
attempt to detach. The sequence is guaranteed to be ordered (in
subject time) and dense, but where it starts and where it ends is
not known. Of course, if the observer is itself a source of events
and the channel between observer and subject is order-preserving,
then the observer can enforce somewhat more stringent bounds on
how far the observed sequence extends into the subject's past or
subject's future by injecting an event just before the attach or
just after the detach.<BR><BR>So much for relativity theory. In
practice, this implements naturally by considering the attempt to
attach and the attempt to detach as being themselves events. The
subject enqueues an event (including an <B>attach</B>/
<B>detach</B>) on an action list, and walks the list of observers
applying each event in turn to each observer. Both observers and
events are timestamped; a simple counter will do. Events are
discarded when there are no observers with lesser stamps. The
observer list is preceded by an observer (the subject itself) that
is watching for attach events and that adds the observer to the
observer list. All observers are watching for their own detach
events and remove themselves when the subject applies the
corresponding detach event. [...]<BR><BR>Most of the difficulties
you encountered appear to be a consequence of the assumption that
attach and detach were somehow global and absolute, rather than
just another event in the sequence of events encountered by the
subject. Remove that mistaken idea and your invalidated-iterators
problems disappear.<BR></BLOCKQUOTE>
<P>This view of attachment and detachment as simple events is very
interesting. This generalization would, however, make it harder to
define policies that lead to the simple implementations in the
article. Let's admit, such simple solutions are widely used and have
their place in spite of their risks and limitations. If anyone would
like to embark on defining a policy-based design for such a
"relativistic" Observer, and if the design does allow efficient
simple implementations as well, I'd be highly interested. Contact
Ivan at igodard@pacbell.net with questions.</P>
<P>Brian Wood comments on the "Walking Down Memory Lane"
article:</P>
<BLOCKQUOTE>While you introduce <B>auto_vector</B>, you didn't
mention either of the <B>ptr_vector</B> implementations. Thorsten
Ottosen has written an interesting article about <B>ptr_vector</B>
in the October 2005 issue of <I>Dr. Dobb's Journal</I>.
<B>auto_vector</B> and the <B>ptr_vector</B> by Ottosen are
obviously closely related, but also have differences. Some mention
to <B>ptr_vector</B> and the differences would have been helpful,
I think.<BR><BR>Anyway, from my own experience, I agree with you
that things like <B>auto_vector</B> and <B>ptr_vector</B> are
often preferable to <B>vector<shared_ptr<T>
></B>.<BR></BLOCKQUOTE>
<P>Great, thanks Brianand readership, consider yourself tipped on
where to look for more material related to scoped memory
management.</P>
<H3>Memory Allocation: One Size Doesn't Fit ll</H3>
<P>Today's general-purpose memory allocators have reached reasonable
speed and low fragmentation for a large category of programs.
However, in the memory-allocation realm, a little information can go
a long way. Application-specific information about allocation
patterns helps to implement specialized memory allocators that
heavily improve the bottom line of many high-performance
applications. Sometimes, as little intel as "Blocks of size 80 are
allocated more often than blocks of all other sizes," when properly
used (as we'll discuss soon), can incredibly improve the bottom-line
runtime of a program. (Caveat: Sometimes customized allocators are
no better, and can be worse, than general-purpose allocatorssee the
paper "Reconsidering Custom Memory Allocation" [7] for more
details.) While general-purpose allocators have average overheads in
the hundreds of cycles, a good customized memory allocator can
require as few as half a dozen cycles.</P>
<P>That's why many high-profile, high-performance applications (GCC,
Apache, and Microsoft's SQL Server to name just a few) implement
their own memory allocator. A good idea, then, is to generalize such
good specialized allocators and put them in a library. But
"generalization" and "specialization" are in tension: Your
application might have different allocation patterns that would
require yet another behavior from your allocator. What to do?</P>
<P>But wait, there's more. If we do devise a method to easily create
special-purpose memory allocators, we can go full-circle and define
a general-purpose allocator as a combination of wisely chosen
special-purpose allocators. If the resulting general-purpose
allocator compares favorably with the existing monolithic
general-purpose allocators, then the design is valid and useful.</P>
<P>Emery's team worked towards that idea, leading to their library
HeapLayers (http://heaplayers.org/). To define configurable
allocators, they used mixins (also known as Coplien's curiously
recurring pattern in the C++ community): defining classes with a
parameterized base. Each layer defines only two member functions,
<B>malloc</B> and <B>free</B>:</P><PRE>template <class T>
struct Allocator : public T {
void * malloc(size_t sz);
void free(void* p);
// system-dependent value
enum { Alignment = sizeof(double) };
// optional interface
size_t getSize(const void* p);
};
</PRE>
<P>Each layer implementation would get a crack on allocation and
deallocation, possibly (and likely) requesting memory from its base
class. A self-contained allocator sits at the very top of the
hierarchyone that forwards requests straight to the system's
<B>new</B> and <B>delete</B> operators, <B>malloc</B> and
<B>free</B> functions, and the such. In HeapLayers terminology,
these are the top heaps. To exemplify (careful with the
qualifications so we don't enter infinite recursion):</P><PRE>struct MallocHeap {
void * malloc(size_t sz) {
return std::malloc(sz);
}
void free(void* p) {
return std::free(p);
}
};
</PRE>
<P>Top heaps can also be implemented around system calls for getting
memory, such as UNIX's <B>sbrk</B> or <B>mmap</B>. This is necessary
when you're completely replacing <B>malloc</B> and friends.</P>
<P>The <B>getSize</B> function has a special status. Not everybody
is going to need it, so defining it is optional (if you combine the
wrong layers, the compiler will tell you). If it does, all you have
to do is insert a layer that stores the block size and offers the
<B>getSize</B> primitive; see <A
href="http://devnet.developerpipeline.com/documents/s=9843/q=1/cuj0512alexandrescu/0512alexandrescul1.html"
target=_BLANK>Listing 1</A>.</P>
<P><B>SizeHeap</B> is the perfect illustration of how to implement a
useful layer that hooks into its base's <B>malloc</B> and
<B>free</B> functions, does something extra, and returns "doctored"
results to the client. <B>SizeHeap</B> does its work by allocating
extra memory to store the block size, with the appropriate cautions
(the union) to stay as immune as possible to the alignment issue.
The client gets access to the memory right next to that extra data.
It's not hard to imagine building a debug heap that pads the memory
block before and after with some bytes filled with a particular
pattern, and then verify for overruns by checking whether the
pattern has been preserved. In fact, that's exactly what HeapLayers'
<B>DebugHeap</B> layer does. Pretty neat.</P>
<P>But wait, something's suboptimal here. Some systems already offer
a primitive to compute the size of a <B>malloc</B>ated block. On
those systems, <B>SizeHeap</B> would actually waste space. In that
case (for example, on Microsoft Visual C++), you wouldn't need
<B>SizeHeap</B> in conjunction with <B>MallocHeap</B>, because
<B>MallocHeap</B> would implement <B>getSize</B> out of the box:</P><PRE>struct MallocHeap {
... as above ...
size_t getSize(void* p) {
return _msize(p);
}
};
</PRE>
<P>But wait, something's still suboptimal here. Remember, we're
counting cycles. What if a system's <B>malloc</B> documentation
states that the block size is stored in a word prior to the actual
block? In that case, <B>SizeHeap</B> would still waste memory by
storing yet another word next to the one already planted by the
system. What's needed is a layer that implements <B>getSize</B> just
the way <B>SizeHeap</B> does, but doesn't hook <B>malloc</B> and
<B>free</B>. That's why <B>HeapLayers</B> segregates the previously
shown <B>SizeHeap</B> in two; see <A
href="http://devnet.developerpipeline.com/documents/s=9843/q=1/cuj0512alexandrescu/0512alexandrescul2.html"
target=_BLANK>Listing 2</A>.</P>
<P>Now <B>SizeHeap</B> always (correctly) adds the
<B>UseSizeHeap</B> layer and exploits its <B>getSize</B>
implementation, while <B>UseSizeHeap</B> can also be used in other
configurationsa very elegant design.</P>
<H3>A Useful Example: FreelistHeap</H3>
<P>Let's face it, so far we have kind of set up the stage. We do
have an architecture, but no clue on how to write an efficient
specialized allocator using layers. A popular and "most bang for the
buck" strategy is the following:</P>
<OL>
<LI>Collect stats about your application's allocation counts for
each size.
<LI>For the most often asked size (call it <B>S</B>), maintain a
private, singly linked list.
<LI>Memory allocations for <B>S</B> return memory from that list
if possible, or else from the default allocator (in a layered
architecture, from the superior layer).
<LI>Memory deallocations for blocks of size <B>S</B> push the
block into the list. </LI></OL>
<P>A refinement of the strategy would be to use the same free list
for a range of sizes <B>S1</B> to <B>S2</B>, at the cost of some
slack memory. The needed singly linked list operations are
<B>O(1)</B> and actually only a few instructions. In addition, the
pointer to the next item can be stored in the actual block (the
block stores no useful datait's always a freed block!) so there's
no extra memory required per block. Given that most applications'
allocation size distribution is highly skewed, free lists are any
allocator implementer's indispensable utensil.</P>
<P>Let's implement a layer that implements a free list for a range
of statically known sizes <B>S1</B> to <B>S2</B>; see <A
href="http://devnet.developerpipeline.com/documents/s=9843/q=1/cuj0512alexandrescu/0512alexandrescul3.html"
target=_BLANK>Listing 3</A>.</P>
<P>Now you can define a custom heap like this:</P><PRE>typedef FLHeap<
SizeHeap<MallocHeap>,
24,
32>
SmartoHeapo;
</PRE>
<P><B>SmartoHeapo</B> will be superfast for allocation sizes between
24 and 32, and pretty much the same for all other sizes.</P>
<H3>In-Place Resizing</H3>
<P>Many a C++ programmer has been dreaming for a standard primitive
to reallocate memory in place. You see, C has <B>realloc</B>, which
can do in-place reallocation if possible, or use <B>memcpy</B> when
it comes about copying data around. But <B>memcpy</B> doesn't work
for C++ objects, so <B>realloc</B> doesn't work for C++ objects,;
therefore, any sort of renew primitive can't be implemented using
the Standard C allocator. So C++ doesn't have renew [1].</P>
<P>To give you an idea of the improvements that in-place
reallocation can bring to C++ code, consider:</P><PRE>const int n = 10000;
Vec v;
for (int i = 0; i < n; ++i)
v.push_back(0);
</PRE>
<P>Howard Hinnant of Metrowerks has been working on implementing
in-place expansion for CodeWarrior's Standard Library
implementation. In Howard's own words:</P>
<BLOCKQUOTE>I currently have a <B>vector<T,
malloc_allocator<T> ></B> that does [...] expand-in-place
based on N1085 [8]. It blows the doors off a built-in C-like
array! :-) When <B>Vec</B> is a <B>vector<int></B> without
expand-in-place: <PRE>0.00095674 seconds</PRE>When <B>Vec</B> is a
<B>vector<int></B> with expand-in-place: <PRE>0.000416943 seconds</PRE>The timings should only be trusted
to two significant digits. But rest assured, I've done the work to
secure those two digits. We are not looking at an anomaly here.
</BLOCKQUOTE><PRE></PRE>
<P>Given the benefits of in-place resizing and that each heap layer
has control over its own allocation algorithms and data structures,
let's augment the heap-layer interface:</P><PRE>template <class T>
struct Allocator : public T {
void * malloc(size_t sz);
void free(void* p);
size_t expand(void* p, size_t min, size_t max);
};
</PRE>
<P>The semantics of expand is, try to expand the block pointed to by
<B>p</B> to the largest size possible between <B>min</B> and
<B>max</B>. Then return whatever size you could expand the memory
to, or zero if no expansion was possible. Fortunately, things can be
arranged such that a layer doesn't have to fuss about the expand
routine unless it wants to. That works if all top allocators inherit
the following little class:</P><PRE>struct TopHeap {
size_t expand(void*, size_t, size_t) {
return 0;
}
// not intended for standalone usage
protected:
~TopHeap() {}
};
</PRE>
<H3>Conclusion</H3>
<P>Configurable memory allocation is, as Emery's research has shown,
a practical, all-in-one alternative to both specialized and
general-purpose allocators. Emery's numbers (refer to the paper [6]
for details) consistently show that allocators created with
HeapLayers perform just as well as, or better than, monolithic
allocators, be they general-purpose or specialized. Moreover,
HeapLayers' layered architecture encourages easier experimentation,
simpler debugging, and unparalleled extensibility. Instead of being
an oddball chapter of <I>Modern C++ Design</I>, memory allocation
should have been one of the best success stories of policy-based
design.</P>
<P><A
href="http://devnet.developerpipeline.com/documents/s=9843/q=1/cuj0512alexandrescu/0512alexandrescut1.html"
target=_BLANK>Table 1</A> shows a relevant subset of the layers
implemented in HeapLayers. There would be a lot of goodies to
discuss, such as the locked heaps for multithreaded operations, the
STL adapter, the various utility heaps, or how the layers can be
combined to create a general-purpose allocator. But conventional
wisdom warns us that "The mind can only enjoy what the butt can
endure," so it's about time to shut downwithout forgetting to
release the memory in destructors. Happy coding.</P>
<H3>References</H3>
<OL>
<LI>Alexandrescu, Andrei. "Generic<Programming>: Typed
Buffers (III)," <I>C++ Experts Online</I>, December 2001.
Available at http://erdani.org/ publications/cuj-12-2001.html.
<LI>Alexandrescu, Andrei. <I>Modern C++ Design</I>, Addison-Wesley
Longman, 2001.
<LI>Alexandrescu, Andrei. "Generic<Programming>: Prying
Eyes: A Policy-Based Observer (I)," <I>C++ Users Journal</I>,
April 2005.
<LI>Alexandrescu, Andrei. "Generic<Programming>: Prying
Eyes: A Policy-Based Observer (II)," <I>C++ Users Journal</I>,
June 2005.
<LI>Berger, Emery D., Kathryn S. McKinley, Robert D. Blumofe, and
Paul R. Wilson. "Hoard: A scalable memory allocator for
multithreaded applications. In International Conference on
Architectural Support for Programming Languages and Operating
Systems (ASPLOS-IX), pp. 117-128. Cambridge, MA, November 2000;
http://citeseer.ist.psu.edu/berger00hoard.html.
<LI>Berger, Emery D., Benjamin G. Zorn, and Kathryn S. McKinley.
"Composing High-Performance Memory Allocators," in SIGPLAN
Conference on Programming Language Design and Implementation, pp.
114-124, 2001; http://citeseer.ist.psu.edu/berger01composing.html.
<LI>Berger, Emery D., Benjamin G. Zorn, and Kathryn S. McKinley.
"Reconsidering custom memory allocation," in Proceedings of the
Conference on Object-Oriented Programming Systems, Languages, and
Applications (OOPSLA) 2002. Seattle, Washington, November 2002;
http://citeseer.ist.psu.edu/berger02reconsidering.html.
<LI>Hinnant, Howard. Proposal to augment the interface of
malloc/free/realloc/calloc. See
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1085.htm.</LI></OL>
<H5>CUJ</H5>
<P> </P>
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distrib
>
Mandriva
>
2010.0
>
i586
>
media
>
contrib-release
>
by-pkgid
>
56882686c7ad11f4bd595dacb1f22fe7
>
files
>
12
