Crossing Guard: Mediating Host-Accelerator Coherence Interactions & An Analysis of Persistent Memory Use with WHISPER

Tuesday, March 21, 2017 -
4:00pm to 5:00pm
CS 1240

Speaker Name: 

Lena Olson & Sanketh Nalli

Speaker Institution: 

UW-Madison / Google




Architecture Seminar Series --

We will be having 2 practice ASPLOS 2017 presentations during this hour session.

Presenter: Lena Olson
Title: "Crossing Guard: Mediating Host-Accelerator Coherence Interactions"
Abstract: Specialized hardware accelerators have performance and energy-efficiency advantages over general-purpose processors. To fully realize these benefits and aid programmability, accelerators may share a physical and virtual address space and full cache coherence with the host system. However, allowing accelerators – particularly those designed by third parties – to directly communicate with host coherence protocols poses several problems. Host coherence protocols are complex, vary between companies, and may be proprietary, increasing burden on accelerator designers. Bugs in the accelerator implementation may cause crashes and other serious consequences to the host system.
We propose Crossing Guard, a coherence interface between the host coherence system and accelerators. The Crossing Guard interface provides the accelerator designer with a standardized set of coherence messages that are simple enough to aid in design of bug-free coherent caches. At the same time, they are sufficiently complex to allow customized and optimized accelerator caches with performance comparable to using the host protocol. The Crossing Guard hardware is implemented as part of the trusted host, and provides complete safety to the host coherence system, even in the presence of a pathologically buggy accelerator cache.

Presenter: Sanketh Nalli
Title: An Analysis of Persistent Memory Use with WHISPER

Abstract: Emerging non-volatile memory (NVM) technologies promise durability with read and write latencies comparable to volatile memory (DRAM). We define Persistent Memory (PM) as NVM accessed with byte addressability at low latency via normal memory instructions. Persistent-memory applications ensure the consistency of persistent data by inserting ordering points between writes to PM allowing the construction of higher-level transaction mechanisms. An epoch is a set of writes to PM between ordering points. To put systems research in PM on a firmer footing, we developed and analyzed a PM benchmark suite called WHISPER that comprises ten PM applications we gathered to cover all current interfaces to PM. A quantitative analysis reveals several insights: (a) only 4% of accesses in PM-aware applications are to PM and the rest are to volatile memory, (b) software transactions are often implemented with 5 to 50 ordering points (c) 75% of epochs update exactly one 64B cache line, (d) 80% of epochs from the same thread depend on previous epochs from the same thread, while few epochs depend on epochs from other threads.