SMALL IDENTIFIERS FOR CHARE ARRAY ELEMENTSWith contributions from Akhil Langer, Harshitha Menon, Bilge Acun, Ramprasad Venkataraman, and L.V. KaléPhil Miller

Array Element Location Management

Enables process virtualization Directs messages from sender PE to host

PE Maps element identifier to object pointer on

host PE Processes element instantiation &

deletion Placement at creation On-demand creation Detects duplicate insertion

Array Element Location Management

Hooks for RTS introspection and adaptivity Tracing Load instrumentation Migration Fault tolerance

Array Index Structure

Fixed 16 bytes No less, even for

small/simple arrays No more, even for

sophisticated arrays

Home PE

Track assigned elements Existence Current host PE

Default host PE Tell other PEs as necessary Assigned by Array Map

Static Array Maps

Array Index to Home PE Simple strategies

Block Round-robin (cyclic) File

Application-specific OpenAtom CharmLU

Pushing the Envelope

Array message variant takes 34-38 bytes Next largest only needs 18 bytes

Could save 16-20 bytes on every message!

Goals of a shorter ID

Reduce envelope size Shrink memory footprint Improve fine-grain performance Enable future index evolution

Design parameters

Preserve API: Send messages by index Maps and home PEs

Avoid extra communication Maintain or improve performance

Scheme

64 Bits

Protocol

Home PE generates ID at construction Simple counter in element field Async request if constructing elsewhere

ID requests piggy-back on location requests

Extra messages only for unusual construction

Potential Optimizations

PE-level caching & pointer lookups Index compression instead of lookups

Index Compression

Many arrays fit directly within 48-bit space All 1D with 32-bit ‘int’ 2D < (16M)2

Etc. Specify bounds, RTS will bit-pack if they

fit Could also enable hashing

Collisions would be disastrous Known indices and perfect hashing?

Summary

Current Status Implemented, passing all tests Performance

Comparable in coarse-grain apps Slightly slower in fine-grain

Future Direction: arbitrary index types AMR, tree codes