- The document describes progress on a swap-aware JVM garbage collection policy. - An initial implementation was summarized, and issues with allowing free space between live objects were identified. - A page-level reimplementation is underway, informed by related work. Validation tests using Deeplearning4Java and Spark workloads are planned. - Future work includes optimizing the GC policy, adding journaling to the Lustre file system, and additional validation experiments.

1. Hyojeong Lee
Distributed Computing System Laboratory
Department of Computer Science and Engineering
Seoul National University, Korea
Progress Report

2. Table of Contents
DOING TODO
Swap-aware
JVM GC
1차 구현 내용 정리
- 모듈별 실행시간
- Evaluation 보완 (SPECjvm)
Page level 재구현
- 관련 연구 참고
Page level 완료
- Validation (DL4J, Spark)
- Compare with other policies
Plan for optimized GC
Journaling in
Lustre FS
서베이 및 논문 수정 최적화 및 실험 추가

3. Swap-aware JVM GC: 1차 정리
● Motivation:
● Necessity of OS swap
● JVM GC policy is not aware of OS swap
● Background: GC process

4. Swap-aware JVM GC: 1차 정리
● Scheme: Skip source-destination mapping
● Validation: with test program

5. ● SPECjvm2008 for evaluation (heap)
● scimark.fft.large (4G), crypto.aes (1G), serial (2G),
xml.validation (0.5G)
● derby (2G)
Swap-aware JVM GC: 1차 정리
Swapout Swapin

6. ● Problem
● Current GC doesn’t allow free space between live objects.
● In detail, verifying object before/after GC will fail if object is
free(hole).
● Solution
● Goal
● Avoid source-destination mapping of regions to skip,
without modifying existing metadata and policy.
● Ours
● Set metadata (i.e. dest_addr) properly considering swapness.
● Problem (debugging)
● full_cp and dense prefix are changed.
(cf) first_dead_space_region()
● Related work (APSys’18, VEE’19)
● Insert dummy objects.
Swap-aware JVM GC: 재구현

7. ● Solution (in detail)
● Related work
● summarize_skip_ver()
: insert dummy objects to hole.
Swap-aware JVM GC: 재구현

8. ● 내용 파악/수정
Serialization due to waiting
transaction commit
→ Parallel journal commit
● 최적화 및 실험 추가 (Todo)
i.e. mdtest
Journaling in Lustre FS

9. TODO
DONE/DOING TODO
Swap-aware
JVM GC
1차 구현 내용 정리 ~3/18
- 모듈별 실행시간
- Evaluation 보완 (SPECjvm)
Page level 재구현 ~3/18
- 관련 연구 참고
Page level 완료 ~3/29
- Validation (DL4J, Spark)
- Compare with other policies
Plan for optimized GC Next
Journaling in
Lustre FS
서베이 및 논문 수정 ~3/15(Done) 최적화 및 실험 추가 Next

10. Bkup_Target workloads (~ 3/15)

11. ● Attempts
1. Can’t skip entire memcpy.
2. ‘Validation after GC error’ because of wrong metadata.
3. DOING
● Evaluation
● Simple java program (microbench)
● Make swapped objects target for compaction
● Benchmarks (benchmark)
● DaCapo, SPECjvm2008, JOlden, Hyracks
● Neo4j (real workload)
● embedded, disk-based, fully transactional Java persistence engine that
manages graph data
● Spark (real workload)
● Sparkbench’s Graph-computation & Machine-learning algorithm
● Deeplearning4Java (real workload)
● Deep learning platform for Java
Target list

12. ● SPECjvm2008 for evaluation
● scimark.fft.large (4G)
● derby (2G)
● crypto.aes (1G)
● serial (2G)
● xml.validation (0.5G)
SPECjvm2008

13. ● Kernel(page) level: Swap flag - Attempt (2)
● Evaluation w/ DL4J
● Object Detection: House Number Detection
● Dataset: http://ufldl.stanford.edu/housenumbers/
● 73,257 digit images for training
● 26,032 digit images for testing
● 531,131 additional, somewhat less difficult samples, to use as
extra training data
● Result
● Baseline (DONE)
● Iteration for training: 100 times
● FGC: 2 times
● Memory usage: ~ 9 GB
● Optimized (TODO)
DL4J

14. ● Kernel(page) level: Swap flag - Attempt (2)
● Evaluation w/ Spark
● Proper target
● SVDPlusPlus
● LogisticRegression
● How to execute?
● 환경 구축, https://docs.google.com/document/d/1seI-
ZzjKvcJeMOJFLLWX0hys7bJVla5TsTobtgrguAE/edit?usp=sh
aring
● 실험 방법,
https://docs.google.com/document/d/1eL5sGQrzUMM3SffME
yp0gQ1qeh7oJ3dbOD2TW6LPy9M/edit?usp=sharing
● Sparkbench 분석 및 베이스라인 실험 결과,
https://docs.google.com/presentation/d/1VYHK2550iJjVg4BeL
zavviCvdDQrehRZ_1oXN27CDKg/edit?usp=sharing
Spark (sparkbench)

15. (cf) Other JVM GC policies
# Policy
Copy swapped
obj
Traverse for
allocation
Other issues etc
1
Concurrent Mark
and Sweep (CMS)
X O
Floating garbage /
More logics to deal with fragmentation /
More space for list
long-lived obj 비율 높고 pause
time 제약 있는 앱에 적합
2 Parallel Compact O X X
pause time 제약 있는 앱에 적합
default in java 8
3 Garbage first (G1) O X X default in java 9
4
SAGP (Swap-
aware Parallel
Compact)
X X
Floating garbage /
Check pagemap (fopen, close) /
More fragmentation
기존 allocation 정책 그대로 사용
가능
커널의 swap 정보를 역으로 읽어
올 수 있으면 오버헤드 제거 가능
https://docs.google.com/presentation/d/1rLyJyny7NMmSLpd9f_z_arjzzngL4bxjAbgo0d5Lnik/edit?usp=sharing

16. Backup slides

17. ● Solutions
● Swap-aware GC policy
● (Fine-grained) Checking pagemap → Inserting DS maintaining
swap info
● (Coarse-grained) Adding reference counter + LRU list
● Optimized GC policy (TODO)
Solutions
dense_prefix
Virtual (heap)
Physical (kernel)
Actually, no need to compact
→ Just remapping virtual space!

18. ● Implementation scheme
Swap-aware JVM GC Policy
dense_prefix
Swap space Swapped live data
Process of Full GC
1. Mark
2. Summarize
3. Compact
Assume that,
- Swapped region =
LRU → Do not need to
swapin.
source_reg
dest_addr
live_size
…
512k
Metadata of Region
Bitmap
1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1
Add variable
‘_swpness’ &
check it using
pagemap
1. Mark
: Set bit for live object by
tracking root set
2. Summarize
: Fill metadata & set
dense prefix
3. Compact
: push_region to stack → do sliding
compaction (memcpy)
…
Region stack list
Region stacks for each GC thread
When src’s _swpness > 0,
skip pushing/copying
draining and
stealing over
stacks

19. ● Kernel(page) level: Swap flag - Attempt (1)
● Evaluation w/ Simple java program
● Simple java program
● Make swapped objects target for compaction
● Result
Swap-aware JVM GC Policy
Allocate 30
objects
Allocate 10 objects
(objects 0 to 9 are
swapped out)
Access 0 to 9 (objects 10 to
19 are swapped out)
&
Make null objects 0 to 9 (be
GC targets)
GC triggered, do mark and
summarize (checking swpness
consumes about 24 sec)
Do compact
Allocate 30
objects
Allocate 10 objects
(objects 0 to 9 are
swapped out)
Access 0 to 9 (objects 10 to
19 are swapped out)
&
Make null objects 0 to 9 (be
GC targets)
GC triggered, do mark and
summarize & Do compact

20. ● Kernel(page) level: Swap flag - Attempt (2)
● Evaluation w/ Simple java program
Swap-aware JVM GC Policy

21. ● Kernel(page) level: Swap flag - Attempt (2)
● Evaluation w/ Simple java program
Swap-aware JVM GC Policy