1. Hassium: Hardware Assisted Database Synchronization
Hillel Avni
Aharon Avitzur

2. Contents
Hassium
Execution Example
Performance
Page 1

3. Hassium
Single version, shared everything concurrency control algorithm
The following are wrapped with separate HTM transactions:
Row access, for read or write
Commit
Deadlock detection
More characteristics:
Monotonic increasing version counter V per database thread (i.e. per connection)
Unique database thread ID
Each database transaction Has a unique set of ID and version, <ID, V>
At commit, a database transaction marks <ID, V> as committed
More details in the running example…
Page 2

4. Hassium Scalability Characteristics
Compared, single version, shared everything algorithm types:
OCC: Lock write-set at commit time and then and validate read-set
ETL: Lock write-set at encounter time and validate read-set at commit
2PL: Lock both read and write-sets at encounter time.
HTM: Check conflicts by exploiting cache coherence invalidations
T1 and T2, both access row R. Scalability characteristics:
Writer Allows Reader: T1 wrote R  T2 can read R (not ETL and 2PL)
Writer See Writer: T1 wrote R  T2 sees R is locked (not OCC)
Reader Allows Writer: T1 read R  T2 can write R (not HTM and 2PL)
At Least One Survives: T1 and T2 will not mutually abort (not OCC) and ETL)
Page 3

5. Contents
Hassium
Execution Example
Performance
Page 4

6. Example Threads ID = 0 V = 12 Data ID = 0 V = 3 Row 0 ID = 0 V = 3
Safe Versions Array
Page 5

7. Example Threads Public ID = 0 V = 12 Data ID = 0 V = 3 Row 0 ID = 0
Private
Safe Versions Array
Page 6

8. Example ID (0) and V (3) of last writer of Row 1. User data not shown
Backup of last committed version of the row
Threads
ID = 0
V = 12
Data
ID = 0
V = 3
Row 0
Highest committed version of thread 1
ID = 0
V = 3
ID = 1
V = 183
Row 1
Highest committed version of thread 0
ID = 1
V = 170
Row 2
V = 11
V = 182
Safe Versions Array
Page 7

9. Example Threads ID = 0 V = 12 Data ID = 0 V = 3 Row 0 ID = 0 V = 3
Safe Versions Array
Page 8

10. Example: Threads 0 and 1 Write
ID = 0
V = 12
2 Concurrent HTM TX
Data
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
Row 2
V = 11
V = 182
Safe Versions Array
Page 9

11. Example: Threads Read from each other
ID = 0
V = 12
Another 2 Concurrent HTM TX
Data
ID = 0
V = 3
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 11
V = 182
Safe Versions Array
Page 10

12. Example: Thread 0 Validates
Threads
ID = 0
V = 12
HTM TX Start
Data
ID = 0
V = 3
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 11
V = 182
Safe Versions Array
Page 11

13. Example: Thread 0 Validation Passed
Threads
ID = 0
V = 12
HTM TX Continues
Data
ID = 0
V = 3
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 11
V = 182
Safe Versions Array
Page 12

14. Example: Thread 0 Commits
Threads
ID = 0
V = 12
HTM TX Commits
Data
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 12
V = 182
Safe Versions Array
Page 13

15. Example: Thread 0 Commits
Threads
ID = 0
V = 13
NO HTM
Data
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 12
V = 182
Safe Versions Array
Page 14

16. Example: Thread 1 Validates
Threads
ID = 0
V = 13
HTM TX Start
Data
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 12
V = 182
Safe Versions Array
Page 15

17. Example: Threads 1 Validation Fails
HTM TX Commits
Data
ID = 0
V = 3
Row 0
ID = 1
V = 183
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
ID = 1
V = 170
ID = 1
V = 170
Row 2
V = 12
V = 182
Safe Versions Array
Page 16

18. Example: Thread 1 Aborts and rolls back
Threads
ID = 0
V = 13
NO HTM
Data
ID = 0
V = 3
Row 0
ID = 0
V = 3
ID = 1
V = 183
Row 1
ID = 0
V = 12
Row 2
V = 12
V = 182
Safe Versions Array
Page 17

19. Contents
Hassium
Execution Example
Performance
Page 18

20. Possibility Test We execute the following test:
N rows, r1,,,rN
N threads, t1…tN
A thread tk executes:
Start Transaction
Write rk
Read r1…rN
Commit Transaction
All the algorithms we tested stopped at 3 threads:
OCC and ETL: 100% aborts
2PL: Slowed to zero
Page 19

21. Contention Test – TPCC New Order
Mix of reads and writes.
Hassium has Lower DB abort rate than OCC.
When contention rises, Hassium has lower overhead than 2PL
Page 20

22. Contention Test – TPCC Payment
Mostly writes
Hassium has Lower DB abort than ETL due to deadlock detection
OCC has lower part of aborted DB transactions but, higher part of aborted work, because OCC always aborts at commit and waists more work
Page 21