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Towards Thousand-Core RISC-V Shared Memory Systems
Quan Nguyen (qmn@mit.edu) Massachusetts Institute of Technology
30 November 2016
Outline
• The Tardis cache coherence protocol – Example – Scalability advantages
• Thousand-core prototype • RISC-V and Tardis
2
Tardis
3
Tardis
• Scalable cache coherence protocol
3
Tardis
• Scalable cache coherence protocol– N-core system: O(log N) storage
3
Tardis
• Scalable cache coherence protocol– N-core system: O(log N) storage
• Enforces consistency through timestamps
3
Tardis
• Scalable cache coherence protocol– N-core system: O(log N) storage
• Enforces consistency through timestamps• Key idea: logical leases
3
Tardis
• Scalable cache coherence protocol– N-core system: O(log N) storage
• Enforces consistency through timestamps• Key idea: logical leases– Can read if have valid lease
3
Tardis
• Scalable cache coherence protocol– N-core system: O(log N) storage
• Enforces consistency through timestamps• Key idea: logical leases– Can read if have valid lease– Can write if lease expires
3Xiangyao Yu and Srinivas Devadas, “Tardis: Time Traveling Coherence Algorithm for Distributed Shared Memory”, PACT 2015.
Block diagram
4
Block diagramCore
D$Core
D$
4
Block diagramCore
D$Core
D$
Last-level cache
Network-on-chip
4
Block diagramCore
D$Core
D$
Last-level cache
Main memory
Network-on-chip
4
New state
5
New state
• Per cache line:
5
New state
• Per cache line:– Client:
tag state wts rts
5
New state
• Per cache line:– Client:
– Manager:tag state owner wts rts
tag state wts rts
5
New state
• Per cache line:– Client:
– Manager:
• Per core:
tag state owner wts rts
tag state wts rts
5
pts
Example
6
Example
6
Core 0: Core 1: store A load B store B load A load B
Example
6
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Example
6
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Example
6
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
6
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
6
• Core 0 stores A
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A– Sets owner, wts, rts
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A– Sets owner, wts, rts
• Core 0:
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A– Sets owner, wts, rts
• Core 0:– Gets A with
[wts, rts] = [0, 0]
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
A state: M wts: 0 rts: 0
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A– Sets owner, wts, rts
• Core 0:– Gets A with
[wts, rts] = [0, 0]– Writes A’, creates new
version at [1, 1]
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
A state: M wts: 0 rts: 0A’ state: M wts: 1 rts: 1
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
Example
6
• Core 0 stores A• Manager leases A– Sets owner, wts, rts
• Core 0:– Gets A with
[wts, rts] = [0, 0]– Writes A’, creates new
version at [1, 1]– Updates its pts to 1
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
pts: 1
A state: M wts: 0 rts: 0A’ state: M wts: 1 rts: 1
Manager
A state: I owner: wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
A state: M owner: 0 wts: 0 rts: 0
7
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
7
• Core 0 loads B
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
7
• Core 0 loads B– Sends pts to manager
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0
Example
7
• Core 0 loads B– Sends pts to manager
• Manager leases B
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0B state: S owner: wts: 1 rts: 11
Example
7
• Core 0 loads B– Sends pts to manager
• Manager leases B– Sets lease based on pts
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0B state: S owner: wts: 1 rts: 11
Example
7
• Core 0 loads B– Sends pts to manager
• Manager leases B– Sets lease based on pts– [wts, rts] = [1, 11]
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0B state: S owner: wts: 1 rts: 11
Example
7
• Core 0 loads B– Sends pts to manager
• Manager leases B– Sets lease based on pts– [wts, rts] = [1, 11]
• Core 0 reads B at pts 1
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: I wts: rts:
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: I owner: wts: 0 rts: 0B state: S owner: wts: 1 rts: 11
B state: S wts: 1 rts: 11
Example
8
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11
Example
8
• Core 1 stores B
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11
Example
8
• Core 1 stores B– Sends pts to manager
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11B state: M owner: 1 wts: 1 rts: 11
Example
8
• Core 1 stores B– Sends pts to manager
• Instantly grant Core 1 exclusive ownership
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11B state: M owner: 1 wts: 1 rts: 11
B’ state: M wts: 12 rts: 12
Example
8
• Core 1 stores B– Sends pts to manager
• Instantly grant Core 1 exclusive ownership
• Core 1 writes B’ at pts 12
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11B state: M owner: 1 wts: 1 rts: 11
B’ state: M wts: 12 rts: 12
pts: 12
Example
8
• Core 1 stores B– Sends pts to manager
• Instantly grant Core 1 exclusive ownership
• Core 1 writes B’ at pts 12
• Different versions of B coexist!
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 0
A state: I wts: rts:
B state: I wts: rts:
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: S owner: wts: 1 rts: 11B state: M owner: 1 wts: 1 rts: 11
B’ state: M wts: 12 rts: 12
pts: 12
Example
9
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
Example
9
• Core 1 loads A
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request• Core 0 downgrades
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
A’ state: S wts: 1 rts: 1
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request• Core 0 downgrades
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
A’ state: S wts: 1 rts: 1
A’ state: S owner: wts: 1 rts: 1
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request• Core 0 downgrades• Core 1 receives new
lease based on its pts
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
A’ state: S wts: 1 rts: 1
A’ state: S owner: wts: 1 rts: 1A’ state: S owner: wts: 12 rts: 22
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request• Core 0 downgrades• Core 1 receives new
lease based on its pts– [wts, rts] = [12, 22]
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
A’ state: S wts: 1 rts: 1
A’ state: S owner: wts: 1 rts: 1A’ state: S owner: wts: 12 rts: 22
Example
9
• Core 1 loads A• Manager sends Core 0
writeback request• Core 0 downgrades• Core 1 receives new
lease based on its pts– [wts, rts] = [12, 22]
• Core 1 reads A’ at pts 12
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: M wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A state: I wts: rts:
B’ state: M wts: 12 rts: 12
Manager
A state: M owner: 0 wts: 0 rts: 0
B state: M owner: 1 wts: 12 rts: 12
A’ state: S wts: 1 rts: 1
A’ state: S owner: wts: 1 rts: 1
A’ state: S wts: 12 rts: 22
A’ state: S owner: wts: 12 rts: 22
Example
10
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
10
• Core 0 loads B
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
10
• Core 0 loads B• Cache hit; simply loads
B from data cache
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
10
• Core 0 loads B• Cache hit; simply loads
B from data cache• Sequential order ( ) ≠ physical order ( )
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
10
• Core 0 loads B• Cache hit; simply loads
B from data cache• Sequential order ( ) ≠ physical order ( )
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
10
• Core 0 loads B• Cache hit; simply loads
B from data cache• Sequential order ( ) ≠ physical order ( )
Core 0: Core 1: store A load B store B load A load B
Core 0 pts: 1
A’ state: S wts: 1 rts: 1
B state: S wts: 1 rts: 11
Core 1 pts: 12
A’ state: S wts: 12 rts: 22
B’ state: M wts: 12 rts: 12
Manager
A’ state: S owner: wts: 12 rts: 22
B state: M owner: 1 wts: 12 rts: 12
Example
A case for scalability
11
A case for scalability
• Track only one node: O(log N) storage
11
A case for scalability
• Track only one node: O(log N) storage• No broadcast invalidations
11
A case for scalability
• Track only one node: O(log N) storage• No broadcast invalidations• Timestamps not tied to core count
11
A case for scalability
• Track only one node: O(log N) storage• No broadcast invalidations• Timestamps not tied to core count– Can be compressed
11
A case for scalability
• Track only one node: O(log N) storage• No broadcast invalidations• Timestamps not tied to core count– Can be compressed– No need for synchronized real-time clocks
11
Outline
• The Tardis cache coherence protocol • Thousand-core prototype • RISC-V and Tardis
12
Thousand-core shared memory systems
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh– Aurora links, six connectors per board
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh– Aurora links, six connectors per board
• Demonstrate shared memory at scale
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh– Aurora links, six connectors per board
• Demonstrate shared memory at scale• Name:
13
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh– Aurora links, six connectors per board
• Demonstrate shared memory at scale• Name:
13
T-1000
Thousand-core shared memory systems
• Fit as many cores will fit on a ZC706• Connect in a 3D mesh– Aurora links, six connectors per board
• Demonstrate shared memory at scale• Name:
13Terminator 2: Judgment Day (1991) Carolco Pictures, Lightstorm Entertainment, Le Studio Canal+, and TriStar Pictures
T-1000
Outline
• The Tardis cache coherence protocol • Thousand-core prototype • RISC-V and Tardis
14
Tardis and RISC-V
15Quan Nguyen, “Synchronization in Timestamp-Based Cache Coherence Protocols”, S.M. thesis, MIT, 2016.
Tardis and RISC-V
• RISC-V: clean, extensible, orthogonal, free
15Quan Nguyen, “Synchronization in Timestamp-Based Cache Coherence Protocols”, S.M. thesis, MIT, 2016.
Tardis and RISC-V
• RISC-V: clean, extensible, orthogonal, free• Chisel simplifies extending hardware
15Quan Nguyen, “Synchronization in Timestamp-Based Cache Coherence Protocols”, S.M. thesis, MIT, 2016.
Tardis and RISC-V
• RISC-V: clean, extensible, orthogonal, free• Chisel simplifies extending hardware• Things to consider: – Release consistency – Atomic instructions – Synchronization (see S.M.)
15Quan Nguyen, “Synchronization in Timestamp-Based Cache Coherence Protocols”, S.M. thesis, MIT, 2016.
Type Ordering rule Tardis rule
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
RC: ordinary memory ops
respect dependencies
respect dependencies
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
RC: ordinary memory ops
respect dependencies
respect dependencies
RC: acquires acq <p X ⟹ acq <s X acq <p X ⟹ acq <ts X
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
RC: ordinary memory ops
respect dependencies
respect dependencies
RC: acquires acq <p X ⟹ acq <s X acq <p X ⟹ acq <ts X
RC: releases X <p rel ⟹ X <s rel X <p rel ⟹ X <ts rel
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Type Ordering rule Tardis rule
SC X <p Y ⟹ X <s Y X <p Y ⟹ X <ts Y
RC: ordinary memory ops
respect dependencies
respect dependencies
RC: acquires acq <p X ⟹ acq <s X acq <p X ⟹ acq <ts X
RC: releases X <p rel ⟹ X <s rel X <p rel ⟹ X <ts rel
RC: sync S ∈ {acq, rel}
SX <p SY ⟹ SX <s SY SX <p SY ⟹ SX <ts SY
Consistency model comparison
16<p : program order <s : global memory order <ts : timestamp order
Release consistency and Tardis
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
• tsmax: maximal timestamp of preceding ops (in timestamp order)
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
• tsmax: maximal timestamp of preceding ops (in timestamp order)
• Fences: tsmin ← tsmax
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
• tsmax: maximal timestamp of preceding ops (in timestamp order)
• Fences: tsmin ← tsmax
• Track acquires/releases with tsrel
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
• tsmax: maximal timestamp of preceding ops (in timestamp order)
• Fences: tsmin ← tsmax
• Track acquires/releases with tsrel
– Release: tsrel ← tsmax
17
Release consistency and Tardis
• tsmin: minimum timestamp for future ops
• tsmax: maximal timestamp of preceding ops (in timestamp order)
• Fences: tsmin ← tsmax
• Track acquires/releases with tsrel
– Release: tsrel ← tsmax
– Acquire: tsmin ← tsrel
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Load-reserved and store-conditional
18
Load-reserved and store-conditional
• Tardis gives neat solution to LR/SC livelock
18
Load-reserved and store-conditional
• Tardis gives neat solution to LR/SC livelock• wts tracks cache line version
18
Load-reserved and store-conditional
• Tardis gives neat solution to LR/SC livelock• wts tracks cache line version• SC success condition: wtslr == wtsbefore sc
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19
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
19
• Core 0 performs lr on C
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
19
• Core 0 performs lr on C– exclusive ownership
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
19
• Core 0 performs lr on C– exclusive ownership
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
C state: S wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: S wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
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• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
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• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded– succeeds; wtslr == wtsC
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
19
• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded– succeeds; wtslr == wtsC
– writes C’ at pts 1
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
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• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded– succeeds; wtslr == wtsC
– writes C’ at pts 1
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
C’ state: M wts: 1 rts: 1
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• Core 0 performs lr on C– exclusive ownership– wtslr = 0
• Core 1 performs lr on C– core 0 downgraded
• Core 0 performs sc– core 1 downgraded– succeeds; wtslr == wtsC
– writes C’ at pts 1
loop: lr.d x1, 0(C) <do stuff to x1> sc.d x2, x1, 0(C) bnez x2, loop
Core 0 pts: 0
C state: I wts: 0 rts: 0
Core 1 pts: 0
C state: I wts: 0 rts: 0
Manager
C state: I owner: wts: 0 rts: 0
LR/SC example
C state: M wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0C state: M owner: 1 wts: 0 rts: 0
C state: M wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: S wts: 0 rts: 0
C state: M owner: 0 wts: 0 rts: 0
C’ state: M wts: 1 rts: 1
pts: 1
Block diagramRocket Core
HellaCache D$Rocket Core
HellaCache D$
Last-level cache
Main memory
TileLink NoC
20
Block diagramRocket Core
HellaCache D$Rocket Core
HellaCache D$
Last-level cache
Main memory
TileLink NoC
20
timestamps
Block diagramRocket Core
HellaCache D$Rocket Core
HellaCache D$
Last-level cache
Main memory
TileLink NoC
20
timestamps
metadata, hit/miss logic
Block diagramRocket Core
HellaCache D$Rocket Core
HellaCache D$
Last-level cache
Main memory
TileLink NoC
20
timestamps
metadata, hit/miss logic
message timestamps
Block diagramRocket Core
HellaCache D$Rocket Core
HellaCache D$
Last-level cache
Main memory
TileLink NoC
20
timestamps
metadata, hit/miss logic
message timestamps
new coherence logic
Thanks!
• Special thanks to Xiangyao Yu and Srini Devadas for their extensive advice and input
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