Reconsidering
Custom Memory Allocation
Emery Berger, Ben Zorn, Kathryn McKinley
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE

Custom Memory Allocation
Programmers replace Very common practice
 
new/delete, bypassing Apache, gcc, lcc, STL,

system allocator database servers…
Language-level
Reduce runtime – often 

support in C++
Expand functionality – sometimes

Widely recommended
Reduce space – rarely 

“Use custom
allocators”
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 2

Drawbacks of Custom Allocators
Avoiding system allocator:

More code to maintain & debug

Can’t use memory debuggers

Not modular or robust:

Mix memory from custom

and general-purpose allocators → crash!
Increased burden on programmers

Are custom allocators really a win?
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 3

Overview
Introduction

Perceived benefits and drawbacks

Three main kinds of custom allocators

Comparison with general-purpose allocators

Advantages and drawbacks of regions

Reaps – generalization of regions & heaps

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 4

(1) Per-Class Allocators
Recycle freed objects from a free list

a = new Class1; Class1
Fast
free list
b = new Class1; +
c = new Class1; Linked list operations
+
a
delete a; Simple
+
delete b;
Identical semantics
b +
delete c;
C++ language support
+
a = new Class1;
c
Possibly space-inefficient
b = new Class1; -
c = new Class1;
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 5

(II) Custom Patterns
Tailor-made to fit allocation patterns

Example: 197.parser (natural language parser)

db
a c
char[MEMORY_LIMIT]
end_of_array
end_of_array
end_of_array
end_of_array
end_of_array
a = xalloc(8); Fast
+
b = xalloc(16); Pointer-bumping allocation
+
c = xalloc(8);
- Brittle
xfree(b);
- Fixed memory size
xfree(c);
- Requires stack-like lifetimes
d = xalloc(8);
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 6

(III) Regions
Separate areas, deletion only en masse

regioncreate(r) r
regionmalloc(r, sz)
regiondelete(r)
- Risky
Fast
+
- Dangling
Pointer-bumping allocation
+
references
Deletion of chunks
+
- Too much space
Convenient
+
One call frees all memory
+
Increasingly popular custom allocator

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 7

Overview
Introduction

Perceived benefits and drawbacks

Three main kinds of custom allocators

Comparison with general-purpose allocators

Advantages and drawbacks of regions

Reaps – generalization of regions & heaps

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 8

Custom Allocators Are Faster…
Runtime - Custom Allocator Benchmarks
Custom Win32
1.75
Normalized Runtime
non-regions regions
1.5
1.25
1
0.75
0.5
0.25
0
r
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As good as and sometimes much faster than Win32

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 9

Not So Fast…
Runtime - Custom Allocator Benchmarks
Custom Win32 DLmalloc
1.75
non-regions regions
Normalized Runtime
1.5
1.25
1
0.75
0.5
0.25
0
lle
e
r
he
c
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DLmalloc: as fast or faster for most benchmarks

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 10

The Lea Allocator (DLmalloc 2.7.0)
Mature public-domain general-purpose

allocator
Optimized for common allocation patterns

Per-size quicklists ≈ per-class allocation

Deferred coalescing

(combining adjacent free objects)
Highly-optimized fastpath

Space-efficient

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 11

Space Consumption: Mixed Results
Space - Custom Allocator Benchmarks
Custom DLmalloc
1.75
non-regions regions
Normalized Space
1.5
1.25
1
0.75
0.5
0.25
0
lle
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UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 12

Overview
Introduction

Perceived benefits and drawbacks

Three main kinds of custom allocators

Comparison with general-purpose allocators

Advantages and drawbacks of regions

Reaps – generalization of regions & heaps

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 13

Regions – Pros and Cons
Fast, convenient, etc.
+
Avoid resource leaks (e.g., Apache)
+
Tear down memory for terminated connections

No individual object deletion
-
Unbounded memory consumption

(producer-consumer, long-running computations,
off-the-shelf programs)
 Apache: vulnerable to DoS, memory leaks
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 14

Reap Hybrid Allocator
Reap = region + heap

Adds individual object deletion & heap

reapcreate(r)
r
reapmalloc(r, sz)
reapfree(r,p)
reapdelete(r)
Can reduce memory consumption
+
Fast
+
Adapts to use (region or heap style)
+
Cheap deletion
+
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 15

Reap Runtime
Runtime - Custom Allocation Benchmarks
Custom Win32 DLmalloc Reap
1.75
Normalized runtime
non-regions regions
1.5
1.25
1
0.75
0.5
0.25
0
lle
e
r
he
c
r
im
c
vp
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lc
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gc
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5.
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UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 16

Reap Space
Space - Custom Allocator Benchmarks
Custom DLmalloc Reap
1.75
non-regions regions
Normalized Space
1.5
1.25
1
0.75
0.5
0.25
0
lle
e
r
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c
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sim
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5.
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UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 17

Reap: Best of Both Worlds
Allows mixing of regions and new/delete

Case study:

 New Apache module “mod_bc”
bc: C-based arbitrary-precision calculator

Changed 20 lines out of 8000

Benchmark: compute 1000th prime

With Reap: 240K

Without Reap: 7.4MB

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 18

Conclusions and Future Work
Empirical study of custom allocators

Lea allocator often as fast or faster

Non-region custom allocation ineffective

Reap: region performance without drawbacks

Future work:

Reduce space with per-page bitmaps

Combine with scalable general-purpose

allocator (e.g., Hoard)
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 19

Software
http://www.cs.umass.edu/~emery
(Reap: part of Heap Layers distribution)
http://g.oswego.edu
(DLmalloc 2.7.0)
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 20

If You Can Read This,
I Went Too Far
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 21

Backup Slides
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 22

Experimental Methodology
Comparing to general-purpose

allocators
Same semantics: no problem

E.g., disable per-class allocators

Different semantics: use emulator

Uses general-purpose allocator

 Adds bookkeeping to support
region semantics
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 23

Why Did They Do That?
Recommended practice

Premature optimization

Microbenchmarks vs. actual performance

Drift

Not bottleneck anymore

Improved competition

Modern allocators are better

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 24

Reaps as Regions: Runtime
Runtime - Region-Based Benchmarks
Custom Win32 DLmalloc Reap
1.75
1.5
Normalized Runtime
1.25
1
0.75
0.5
0.25
0
lcc mudlle
Reap performance nearly matches regions

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 25

Using Reap as Regions
Runtime - Region-Based Benchmarks
Original Win32 DLmalloc WinHeap Vmalloc Reap
4.08
2.5
2
Normalized Runtime
1.5
1
0.5
0
lcc mudlle
Reap performance nearly matches regions
UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 26

Drawbacks of Regions
Can’t reclaim memory within regions

Bad for long-running computations,

producer-consumer patterns,
“malloc/free” programs
unbounded memory consumption

Current situation for Apache:

vulnerable to denial-of-service

limits runtime of connections

limits module programming

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 27

Use Custom Allocators?
Strongly recommended by practitioners

Little hard data on performance/space

improvements
Only one previous study [Zorn 1992]

Focused on just one type of allocator

Custom allocators: waste of time

Small gains, bad allocators

Different allocators better? Trade-offs?

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 28

Kinds of Custom Allocators
Three basic types of custom allocators

Per-class

Fast

Custom patterns

Fast, but very special-purpose

Regions

Fast, possibly more space-efficient

Convenient

Variants: nested, obstacks

UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 29

Optimization Opportunity
Time Spent in Memory Operations
Memory Operations Other
100
% of runtime
80
60
40
20
0
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UNIVERSITY OF MASSACHUSETTS • DEPARTMENT OF COMPUTER SCIENCE 30