Download - Experience with Safe Memory Management in Cyclone

Experience with Safe Memory Management in Cyclone

Michael HicksUniversity of Maryland, College Park

Joint work with

Greg Morrisett - Harvard

Dan Grossman - UW

Trevor Jim - AT&T

Cyclone

• Derived from C, having similar goals– Exposes low-level data representations,

provides fine-grained operations

• But safe– Restrictions to C (e.g., (int *)1 not allowed)– Additions and types to regain flexibility

• Today: balancing safety and flexibility when managing memory

Goal: Programmer Control

• Many memory management choices– Garbage collection– Stack allocation

– malloc/free– Reference counting (Linux, COM)– Arenas (bulk free) (Apache, LCC)

• Depends on the application

Unifying Theme: Region types

• Conceptually divide memory into regions– Different kinds of regions (e.g., not just bulk-free)

• Associate every pointer with a region

• Prevent dereferencing pointers into dead regions

int *`r x; // x points into region `r *x = 3; // deref allowed if `r is live

(inference often obviates annotations `r)

Liveness by type & effects system (Tofte&Talpin)

Outline

• Motivation and basic approach

• Regions description– Basics: LIFO arenas, stack and heap regions– Unique and reference-counted pointers– Dynamic arenas

• Programming experience• Experimental measurements• Conclusions

LIFO Arenas

• Dynamic allocation mechanism• Lifetime of entire arena is scoped

– At conclusion of scope, all data allocated in the arena is freed.

LIFO Arena Example

FILE *infile = …

Image *i;

if (tag(infile) == HUFFMAN) {

region<`r> h; // region `r created

struct hnode *`r huff_tree;

huff_tree = read_tree(h,infile); // allocates with h

i = decode_image(infile,huff_tree,…);

// region `r deallocated upon exit of scope

} else …

Stack and Heap Regions

• Stack regions– Degenerate case of LIFO arena which does

not allow dynamic allocation– Essentially activation records

• Heap region– A global region `H that is always live– Like a LIFO arena that never dies; objects

reclaimed by a garbage collector

Scoped Regions Summary

See PLDI `02 paper for more details

Region Variety

Allocation

(objects)

Deallocation

(what) (when)

Aliasing

(objects)

Stack static whole region

exit of scope

free

LIFO Arena

dynamic

Heap single objects

GC

Benefits

• No runtime access checks• Arena/stacks

– costs are constant-time• region allocation• region deallocation• object creation

– useful for• Temporary data (e.g., local variables)• Callee-allocates data (rprintf)• Lots of C-style code

Limitations

• Lack of control over memory usage– Spurious retention of regions and their objects– Fragmentation– Extra space required by the garbage collector

• Lack of control over CPU usage– Garbage collection is “one-size-fits-all”

• Hard to tune

– Cannot avoid GC in some cases: LIFO arenas not expressive enough

• E.g., objects with overlapping lifetimes

Overcoming the Limitations

• Allow greater control over lifetimes– Object lifetimes

• Unique pointers and reference-counted pointers

– Arena lifetimes• Dynamic arenas

• But not for nothing ...– Restrictions on aliasing– Possibility of memory leaks

Unique Region

• Distinguished region name `U

• Individual objects can be freed manually• An intraprocedural, flow-sensitive analysis

– ensures that a unique pointer is not used after it is consumed (i.e. freed)

– treats copies as destructive; i.e. only one usable copy of a pointer to the same memory

– Loosely based on affine type systems

Unique Pointer Example

void foo() {

int *`U x = malloc(sizeof(int));

int *`U y = x; // consumes x

*x = 5; // disallowed

free(y); // consumes y

*y = 7; // disallowed

}

Temporary Aliasing

• Problem: Non-aliasing too restrictive• Partial solution: Allow temporary, lexically-

scoped aliasing under acceptable conditions– Makes unique pointers easier to use– Increases code reuse

Alias construct

extern void f(int *`r x); // `r any scoped region

void foo() {


*x = 3;

{ alias <`r>int *`r y = x; // `r fresh

f(y); // y aliasable, but x consumed

} // x unconsumed

free(x);

}

Alias inference

extern void f(int *`r x); // `r any scoped region

void foo() {


*x = 3;

f(x); // alias inserted here automatically

free(x);

}

Reference-counted Pointers

• Distinguished region `RC • Objects allocated in `RC have hidden

reference-count field

• Aliasing tracked as with unique pointers. Explicit aliasing/freeing via

à *`RC alias_refptr(à *`RC);

void drop_refptr(à *`RC);

Reference-counting Example

struct conn * `RC cmd_pasv(struct conn * `RC c) { struct ftran * `RC f; int sock = socket(...); f = alloc_new_ftran(sock,alias_refptr(c)); c->transfer = alias_refptr(f); listen(f->sock, 1); f->state = 1; drop_refptr(f); return c;}

Regions Summary

Region Variety

Allocation

(objects)

Deallocation

(what) (when)

Aliasing

(objects)

Stack static whole region

exit of scope

free

LIFO dynamic

Dynamic manual

Heap single objects

GC

Unique manual

restricted

Refcounted

Ensuring Uniformity and Reuse

• Many different idioms could be hard to use– Duplicated library functions– Hard-to-change application code

• We have solved this problem by– Using region types as a unifying theme– Region polymorphism with kinds

• E.g., functions independent of arguments’ regions

– All regions can be treated as if lexical• Temporarily, under correct circumstances• Using alias and open constructs

Boa web server

BetaFTPD ftp server

Epic image compression

Kiss-FFT portable fourier transform

MediaNet streaming overlay network

CycWeb web server

CycScheme scheme interpreter

Programming Experience

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Application Characteristics



MediaNet Datastructures

• Platform– Dual 1.6 GHz AMD Athlon MP 2000

• 1 GB RAM• Switched Myrinet

– Linux 2.4.20 (RedHat)

• Software – C code: gcc 3.2.2– Cyclone code: cyclone 0.8– GC: BDW conservative collector 6.24– malloc/free: Lea allocator 2.7.2

Experimental Measurements

• CPU time– Most applications do not benefit from switching

from BDW GC to manual approach– MediaNet is the exception

• Memory usage– Can reduce memory footprint and working set

size by 2 to 10 times by using manual techniques

Bottom Line



Throughput: Webservers



Throughput: MediaNet



Memory Usage: Web (I)

Memory Usage: Web (II)



Memory Usage: Web (III)





Memory Usage: MediaNet

(4 KB packets)

Related Work

• Regions– ML-Kit (foundation for Cyclone’s type system)– RC– Reaps– Walker/Watkins

• Uniqueness– Wadler, Walker/Watkins, Clean– Alias types, Calculus of Capabilities, Vault– Destructive reads (e.g., Boyland)

Future Work

• Tracked pointers sometimes painful; want– Better inference (e.g. for alias)– Richer API (restrict; autorelease)

• Prevent leaks– unique and reference-counted pointers

• Specified aliasing– for doubly-linked lists, etc.

• Concurrency

Conclusions

• High degree of control, safely:

• Sound mechanisms for programmer-controlled memory management– Region-based vs. object-based deallocation– Manual vs. automatic reclamation

• Region-annotated pointers within a simple framework– Scoped regions unifying theme (alias,open)– Region polymorphism, for code reuse

More Information

• Cyclone homepage– http://www.cs.umd.edu/projects/cyclone/

• Has papers, benchmarks from this paper, and free distribution– Read about it, write some code!