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Dynamic Object Sampling for Pretenuring. Maria Jump Department of Computer Sciences The University of Texas at Austin [email protected]. Stephen M. Blackburn Department of Computer Science Australia National University [email protected]. Kathryn S. McKinley - PowerPoint PPT Presentation
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Dynamic Object Sampling for Pretenuring
Maria JumpDepartment of Computer SciencesThe University of Texas at Austin
Stephen M. BlackburnDepartment of Computer Science
Australia National [email protected]
Kathryn S. McKinleyDepartment of Computer SciencesThe University of Texas at Austin
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 2
Current Trends
Object-oriented languages Lots of small objects Runtime system treats them uniformly
Objects have distinctive properties Lifetime, access frequency, locality,
calling context, thread usage, etc. PROBLEM: How can we discover and
exploit object properties?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 3
Gathering Object Statistics
Offline statistics Inconsistent with Java Obscure phase behavior
Online statistics Profiling all objects Dynamic Object Sampling
Statistics only for selected objects Use to estimate behavior of all objects
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 4
Example: Using Object Lifetimes for Pretenuring
PROBLEM: Long-lived objects are allocated into the nursery
SOLUTION: Allocate long-lived objects directly into mature space
CHALLENGE: Correlating lifetime behavior with allocation site
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 5
Outline Motivation Dynamic Object Sampling
Mechanism Dynamic Lifetime Prediction
Accuracy vs. Cost Dynamic Pretenuring
Allocation Survival Phases Coverage, Accuracy, and Performance
Conclusions
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 6
384256128
Dynamic Object Sampling Modify a bump-pointer allocator Sample one object every n bytes
Memory chunk:
Bump Pointer
Sample Tag(for n=128)
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 7
VM_Address alloc(int bytes)
throws VM_PragmaInline {
VM_Addresss oldCursor = cursor;
VM_Address newCursor = oldCursor.add(bytes);
if (newCursor.GT(sampleLimit))
return sample(bytes);
cursor = newCursor;
return oldCursor;
}
Sample Path(Intermediate) :
1) adds sample tag2) collects statistics
Slow Path
Bump-Pointer AllocatorVM_Address alloc(int bytes)
throws VM_PragmaInline {
VM_Addresss oldCursor = cursor;
VM_Address newCursor = oldCursor.add(bytes);
if (newCursor.GT(limit))
return allocSlow(bytes);
cursor = newCursor;
return oldCursor;
}
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 8
During Garbage Collection
Use GC to update statistics Trace live objects Update transient and total statistics
SAMPLE TAG FOUND!1. decode tag word2. collect statistics
survivor
What’s in the Sample Tag?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 9
Dynamic Lifetime Prediction
Encode allocation site identifier into the object sampling sample tag Allocation site is a good lifetime predictor
[Blackburn et al.]
Compiler maps identifiers to sites Accuracy vs. Cost tradeoff
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 10
Implementation and Experimental Methodology
Jikes RVM 2.3.0.1 with MMTk SPEC JVM benchmarks, SPEC JBB 2000 with
constant workload 3.2 GHz Intel Pentium 4 w/ hyperthreading,
1GB main memory, Linux 2.6.0 Generational/Mark-Sweep (GenMS)
4MB bounded nursery Pseudoadaptive compilation Report application time only [Eeckhout et al.]
2nd run methodology
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 11
Accuracy of Sampling
javac
Accuracy92.5%
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 12
Cost of Sampling
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 13
Object Sampling forDynamic Pretenuring
Accurately and efficiently estimate allocation-site lifetimes
At garbage collection time Calculate survival rate per allocation site Compute transient and total statistics
if (survival rate >= threshold) then change the allocation target
Pretenure:
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 14
if (shortLived(site))
region = nursery.alloc(…);
else
region = matureAlloc(…);
break;
Dynamic Allocation Target
case NURSERY_SPACE:region = nursery.alloc(…);
break;
table lookup
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 15
Dynamic Allocation Target Overhead
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 16
Dynamic Pretenuring Overhead
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 17
Allocation Phases
Current survival rate is not a guarantee of future survival
Sampling in older space is ineffective Collections are infrequent Small amount of allocation
Periodically allocate site in nurseryBACKSAMPLING
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 18
Backsampling
Protects the system from bad choices Reduces effectiveness of good
choicesWhen to Backsample?
BacksampleTrigger = n * f
Number of allocations for
pretenuring decision
Growth function:
ConstantLinear
Exponential
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 19
Dynamic Pretenuring Policies
Parameters
Configurations
cfg1 cfg2 cfg3sampling interval 256 256 256
minimum samples 8 4 10
pretenuring threshold 80% 80% 85%
backsampling policy linear exponential linear
backsampling shift 1 4 1
decay shift 0 0 1
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 20
Coverage: How much?
0
5
10
15
20
25
30
35
40
45
jbb javac db mtrt jack raytrace jess compress
Lon
g-L
ived O
bje
cts
(% o
f Tota
l A
llocati
on
s)
None100%95%90%85%80%75%
PretenuringThreshold
Long-LivedPretenured
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 21
0
5
10
15
20
25
30
35
40
45
jbb javac db mtrt jack raytrace jess compress
Pre
dic
ted
Lo
ng
-Liv
ed
Ob
jects
(% o
f To
tal A
llo
cati
on
s)
Accuracy: Are we right?
None100%95%90%85%80%75%
PretenuringThreshold
IncorrectCorrect
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 22
GC Time: javac
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 23
Total Time: javac
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 24
Conclusions Lifetime estimation for dynamic pretenuring
Accurately and efficiently estimates allocation-site lifetimes
Pretenuring long-lived allocation-sites can improve runtime
Few opportunities in our programs
Dynamic Object Sampling Low-overhead High accuracy Generic mechanism
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 25
Questions?
Thank You
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 26
GC Time: jess
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 27
Total Time: jess
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 28
Coverage: How much?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 29
Accuracy: Are we right?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 30
Coverage: How much?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 31
0
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jbb javac db mtrt jack raytrace jess compress
Lon
g-L
ived O
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s)Coverage: How much?
ISMM, Oct. 25, 2004 Jump, Blackburn, McKinley 32
0
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jbb javac db mtrt jack raytrace jess compress
Lon
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(% o
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