tianzheng wang ryan johnson alan fekete ippokratis pandis the serial safety net: efficient...
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Tianzheng Wang Ryan JohnsonAlan FeketeIppokratis Pandis
The Serial Safety Net: Efficient concurrency control on modern hardware
Modern HW: CPU on critical path
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Low parallelism Focus: hide I/O stalls
Classic DBMS Modern DBMS
High parallelism Focus: minimize cycles
Main-memory OLTP: more pressure on CC
Locks Disks Threads Memory Disk Threads
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CC: current state of the art
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Throughput (ktps)
Abo
rts/
com
mit OCC (fast)
SI (non-serializable)
SSI (slow)
OCC looks good. What’s the problem?
Contentious TPC-C variant
24166
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CC: Robustness matters!
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OCC (unfair) SI (non-serializable)
SSI (slow)
Need all three: fast, fair, serializable
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Contentious TPC-C variantwith retries
The Serial Safety Net (SSN)
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SomeCC >= RC SSN
Serializable execution
Might admit anomalies Cheap certifier
Works even if CC is buggy
Aborts offenders
Dictates access pattern
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Throughput (ktps)
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SSN: fast, fair, serializable
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OCC SI
SSI
SSN
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Database model
R1 R2 R3 R4...
R3
Uncommitted write
Invisible to other readers
Multi-versiondatabase
R1 R1
CC decides whichversion to read
Captures most CC schemes, incl. 2PL
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Serialization graphs and cycles
T1 r:y:w T2 w:x:r T3 r:x:w T1
T1
T2
T3
x
y
xT3
T2
T1
(T3)
com
mit
time
dependency order
T1 r(x,0) r(y,0) w(x,-11) C!!
T2 r(y,0) w(y,10) C!!
T3 r(x,0) r(y,10) C!!
SI read-onlyanomaly
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T3
T2
T1
(T3)
com
mit
time
dependency order
Serialization graphs and cycles
T1 commits last & “closes”
the cycle “Hard” to detectT1 … T3
“Easy” to detectT3 T1
SSN: efficientlydetect T1 … T3
10“exclusion window” of T
SSN in a nutshell
Define π(T) = min {c(S) : T … S}
Define c(T) = “commit time of T”
Forbid $P T : π(T) ≤ c(P) ≤ c(T)
Track T’s earliest
successor
P might bea successor
of T
T might close a cycle
Visualizing SSN
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Exclusion window satisfied
T4T1
π(T4)T2
T3
com
mit
time
dependency order
π(T2)T5
T1
T4
T3T2
(T1)
T3
T4
T1
T2
π(T2)
T2
T1??
π(T2)
Exclusion window violation
Evaluation• System– 4 x 6-core Xeon @ 1.8GHz, 64GB DRAM– A variant of Silo* that supports SI and RC– TPC-C w/ random warehouse selection– RC, SI, OCC (Silo), SSI, {SI, RC} + SSN
– What to test– General performance/scalability– Fairness for updates vs. reads– Robustness under retries
12* Tu et al, “Speedy transactions in multicore in-memory databases”, SOSP`13
SSN performs well
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Higher is better
SSI
OCC
Low implementation overhead
SSN has low abort rate
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Lower is better
OCC
SI (higher) andRC/SI+SSN
SSI
SSN allows more valid schedules than SSI or OCC
SSN provides “safe retry”
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OCC
SI (highest) andRC/SI+SSN
SSI
Higher is better
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SSN is fair
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Ideal OCC SSI SI+SSN SI RC+SSN
% of
total
comm
its
DeliveryNew-Order
Order-StatusPayment
Stock-Level
SSI starveswriters
OCC starvesreaders
SSN fair to both R & W
Ideal OCC SSI SI SI+SSN RC+SSN
Conclusion
• Modern HW puts more pressure on CC– No tolerance for CC with “heavy touch”– Serializability becomes even harder
• Serial safety net – a cheap certifier– Serializable– Compatible with various CC schemes– Lightweight and balanced
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Formal proofs (+ more details) in our paper
Thank you!