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Hardware Support for Dynamic Memory Management. J. Morris Chang Witawas Srisa-an Chia-Tien Dan Lo Illinois Institute of Technology Edward F. Gehringer North Carolina State University. The Problem. O-o applications make frequent requests for dynamic memory.

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Hardware Support for Dynamic Memory Management

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Hardware Support for Dynamic Memory Management

J. Morris Chang

Witawas Srisa-an

Chia-Tien Dan Lo

Illinois Institute of Technology

Edward F. Gehringer

North Carolina State University


The Problem

  • O-o applications make frequent requests for dynamic memory.

    • C++ programs do an order of magnitude more than C programs.

  • Most objects are abandoned quickly.

  • --> Much time used in memory mgt.

    • Up to 30% in C programs ...

  • Garbage collection has been optimized, but still takes time.


Hardware-Implemented Allocation

  • Makes use of an allocation vector (A-vector) and a bit-flipper.

address

0

1

2

3

4

5

6 7

the A-vector

before

the allocation

1 0 1 1

0

0

1 1

(a) Combinational logic (the complete binary tree) determines that

there is enough free memory to fill the request for two blocks

(b) The address of the free block is 100

.

2

(c) The bits at 100

and 101

are flipped.

2

2

the A-vector

after

the allocation

1 0 1 1

1

1

1 1


The Complete Binary Tree

  • A binary tree of bits is used to locate the first free region combinationally.

Level 0

Size 24

1

Level 1

Size 23

1

1

Level 2

Size 22

1

0

1

0

Level 3

Size 21

1

1

0

0

0

1

0

0

Size 20

Level 4

1 1 0 1 0 0 0 0 0 0 1 1 0 0 0 0

A-vector address

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15


Keeping Track of Object Size

  • Meanwhile, the size bit-vector (S-vector) records the boundaries between objects.

Complete Binary Tree

Allocation bit-vector

(A-vector)

S-Unit

(Size encoder)

Size bit-vector

(S-vector)


Five Hardware-Implemented Instructions

  • h_malloc• mark

  • h_free• sweep

  • h_realloc

  • All are implemented in the Dynamic Memory Management Unit.

  • DMMU manages the heap


The DMMU

  • Each entry contains three bit-vectors.

  • X-vector used for reallocation & g.c.

A-vector

S-vector

X-vector

h_malloc / h_free / h_realloc

mark / sweep

gc_ack

O.S.

sbrk/brk

CPU

DMMU

object_size

Kernel

object_pointer


The ALB

  • Each entry in the DMMU tracks the allocation status of a region of memory.

  • Compare with a TLB, which tracks the location of a region of virtual memory.

  • So, these entries make up the Allocation Lookaside Buffer.

  • Entries can be saved and fetched to A-, S-, and X- bitmaps.


Steps in Allocation

  • Compare requested size with largest_available_size in each ALB entry.

  • Select an entry & pass requested size to CBT

  • CBT locates first available chunk.

  • Chunk is allocated using buddy system.

  • Unused words at end are returned to free memory.

  • Address of block is returned, and status changed to allocated.

  • S-vector is updated accordingly.

Size (A1)

Address pointer (A1)

Complete Binary Tree (

CBT )

h_

malloc

Allocation bit-vector

(A bit-vector)

(A2)

(Size encoder)

S-Unit

(A3)

Size bit-vector

(S bit-vector)


Steps in Deallocation

  • Deallocation is very similar to allocation.

Address pointer (D1)

Complete Binary Tree (

CBT )

h_

free

Allocation bit-vector

(A bit-vector)

(D2)

(Size encoder)

S-Unit

(D3)

Size bit-vector

Size boundaries

(S bit-vector)


Steps in Marking

  • Each live-object pointer sent to CBT, one after another.

    • Page # of object pointer selects a bit-vector.

  • Signal generated by CBT is latched in X-vector.

Address pointer

Complete Binary Tree (

CBT )

mark

Auxiliary bit-vector

Live-object pointers

(X-vector)


Steps in Sweeping

  • Bit-sweeper receives the sweep signal.

  • Size info from S-vector and liveness status from X-vector generate new alloc. status and largest_avail_size.

Allocation bit-vector

(A vector)

(E2)

(Size encoder)

S-Unit

(E2)

(E1)

Size bit-vector

(S vector)

(E1)

sweep (E1)

GC_

ack

(E3)

Bit-Sweeper/ X-Unit

(C1)

Auxiliary bit-vector

(X vector)


Putting it All Together

Size (A1)

Address pointer (A1)

Complete Binary Tree (

CBT )

Address pointer (B1, D1)

h_

malloc

, h_ free, mark (A1, B1, D1)

Allocation/

deallocation

output (A1, B1)

(D1)

Allocation bit-vector

(A vector)

(A2,B2,E2)

(Size encoder)

S-Unit

(A2,B2,E2)

(E1)

Size bit-vector

Size boundaries (B1).

(S vector)

(C1, E1)

h_

realloc

(C1) / sweep (E1)

GC_

ack

(E3)

Bit-Sweeper/ X-Unit

(C1)

(E1)

Starting_address (C1)

(C1)

Auxiliary bit-vector

Enable signal

live object pointer

Ending_address(C1)

generator

(X-vector)

(C2)

Reallocation Status (RS-Unit)

A. Steps required for allocation

B. Steps required for

deallocation

C. Steps required for reallocation

D. Steps required for marking

Reallocation Status (C2)

E. Steps required for sweeping


Memory Usage

  • Most schemes encode size information in objects themselves.

    • This is more efficient with large objects.

    • Bit-vector is more efficient with small objects.

  • If object contains 8 bytes for size and1 for marking, bitmap scheme more efficient when avg. size < 384 bytes.

  • Avg. object size for C++ & Java programs:  101 bytes.


Performance Gain

  • ALB miss penalty.

    • Bit-vector length of 500 bits ( 64 bytes) gives 97% hit ratio.

    • This => ALB entry is 192 bytes.

    • 64-bit 100 MHz bus gives 800 MB/s. transfer rate.

    • => miss penalty is 96 cycles (192x400/800)

  • With ALB hit, it takes 2 cycles to allocate memory.

  • => avg. hw. malloc time is 4.82 cycles.

  • Software malloc varies from 51 to 900 cycles, with avg. 192.

  • In an application that spends 30% of time allocating, speedup would be 41%.


Summary

  • O-o applications spend a lot of their time allocating memory.

  • To allocate in hardware, we use a bit-vector based approach.

  • Allocation/deallocation done combinationally using a complete binary tree on top of the bit-vector.

  • Yields speedup of > 40% on memory-intensive programs.


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