This paper presents a hybrid SCM-DRAM persistent tree, which stores leaf nodes in SCM (Storage Class Memory) and keeps inner nodes in DRAM for better performance. Its design ideas are worth taking further for SCM devices. Paper: https://wwwdb.inf.tu-dresden.de/misc/papers/2016/Oukid_FPTree.pdf

1. FPTree: A Hybrid SCM-DRAM
Persistent and Concurrency B-Tree
for Storage Class Memory
Presented by Peng Qu

2. Agenda
● What’s Storage Class Memory
● What’s Hardware Transactional Memory
● FPTree design details and algorithms
● Evaluation

3. Storage Class Memory
● As known as persistent memory
● connects to memory slots in a motherboard
● 100 times slower than DRAM
● 10 times faster than flash
● Examples: 3D XPoint

4. SCM Programming Model
● How to manage data consistency
○ Primitives like CLFLUSH/MFENCE/SFENCE
● How to recovery data
○ Persistent pointers (file-id, offset)
○ Persistent Allocator does the map
● How to deal with memory leak
○ BAD: fn alloc(mem) -> PersistentPointer
○ GOOD: fn alloc(mem, &mut PersistentPointer)

5. ● Example: Transactional Synchronization Extensions
○ Hardware Lock Elision
■ XACQUIRE/XRELEASE
○ Restricted Transactional Memory
■ XBEGIN/XEND/XABORT
def transfer_money(from_account, to_account, amount):
with transaction():
from_account -= amount
to_account += amount
Hardware Transactional Memory

6. FPTree design details - selective persistence
● Inner nodes are kept in DRAM
● Leaf nodes are kept in SCM
● No start-key and end-key in leaf node

7. FPTree design details - selective persistence
● Inner nodes will be restored from leaf nodes when recovery
● Persistent Allocator will translate Persistent Pointers to normal

8. FPTree design details - Fingerprints
● Fingerprints are one-byte hashes of leaf keys
● Sorted stored in leaf nodes
● Probability of a given fingerprint appears i time in m different keys
● Expected number of in-leaf key probes

9. FPTree design details - Fingerprints
● If the fingerprints are not sorted, we can’t get the benchmark result

10. FPTree design details - amortized memory alloc
● Allocating persistent memory from SCM is expensive
● Allocating blocks of multiple leaf nodes at once

11. FPTree design details - selective concurrency
● With HTM to update inner nodes in DRAM
● Use primitives to persistent leaf nodes in SCM, outside from the HTM transaction
● XBEGIN/XEND: encapsulated in speculative spin tmux

12. FPTree algorithm - read operations

13. FPTree algorithm - insert operations

14. FPTree algorithm - delete operations

15. FPTree algorithm - update operations

16. Evaluation

17. Thank You!