RATFAT: ReAl-Time FAT for Cooperative Multitasking Environments in - PowerPoint PPT Presentation

Platzhalter für Bild, Bild auf Titelfolie hinter das Logo einsetzen RATFAT: ReAl-Time FAT for Cooperative Multitasking Environments in WSNs Sebastian Schildt, Wolf-Bastian Pöttner, Felix Büsching, Lars Wolf PWSN, Boston - May 23, 2013

RAT + FAT? + ? http://crunchall.com/wp-content/ http://5plus2.info/images/article/ uploads/2013/03/podgana.jpg article_small_33.jpg Wolf-Bastian Pöttner | RATFAT | Page 2 / 19

Motivation • Many WSN apps need persistent data storage • Some WSN apps want large-scale data storage SD Cards • Often data is stored on nodes and processed on PC • Some WSN apps require real-time guarantees We need a file system for WSN nodes that is “PC-compatible” and real-time capable! Wolf-Bastian Pöttner | RATFAT | Page 3 / 19

State of the Art in WSN file systems COFFEE in Contiki • Unpredictable timing behavior (non real-time) • Bad performance on large (> a few MB) volumes • Not readable on standard PC Operating Systems • But: high memory e ffi ciency FatFs + tinyFAT • Unpredictable timing behavior (non real-time) Wolf-Bastian Pöttner | RATFAT | Page 4 / 19

State of the Art in WSN Operating Systems TinyOS 2 • Cooperative multitasking; no preemptions • Round-robin scheduler Contiki • Cooperative multitasking; no preemptions • Round-robin scheduler • RTimers allow (certain) real-time tasks • Interrupting ongoing SPI operations may lead to undefined states • Concurrent access to (di ff erent) SPI devices is problematic Wolf-Bastian Pöttner | RATFAT | Page 5 / 19

Organization of a FAT Volume File Allocation Tables Cluster #1 #2 Master Clusters Root Boot Directory Record SD Card Wolf-Bastian Pöttner | RATFAT | Page 6 / 19

RATFAT Features Highlights • Fully compliant to FAT specification • Support for FAT16 and FAT32 volumes and 8.3 naming scheme • Runs on any* underlying flash memory (SD, SPI flash, etc.) • Single-Bu ff er design (512 Byte) • Compatible to Contiki File System API • Extended API for additional functionality Limitations • No support for „long filenames“ • Keeping FATs in sync is “expensive” Wolf-Bastian Pöttner | RATFAT | Page 7 / 19

RATFAT Architecture Application RT Management Contiki CFS API FAT Extended API FAT16/32 Implementation MBR Block IO Layer Mgmt SD Card driver SPI Flash Driver Wolf-Bastian Pöttner | RATFAT | Page 8 / 19

RATFAT RT Overview API • Non-blocking API • Enqueue file system requests in constant time File System Process (FSP) • Background process handling all file system requests • Separate Stack, allows to hand control back to scheduler • Splits up FS requests into multiple atomic operations • On INGA, atomic operations take up to 12 ms • Maximum execution time per invocation • Respects system timers, yields before Wolf-Bastian Pöttner | RATFAT | Page 9 / 19

RATFAT RT Concept Sampling interval T RT Process Write request Write request Write request RATFAT non-RT Deadline Miss Deadline Miss RATFAT RT reschedule FSP reschedule FSP Wolf-Bastian Pöttner | RATFAT | Page 10 / 19

Behavior of the file system process (FSP)                          Wolf-Bastian Pöttner | RATFAT | Page 11 / 19

Evaluation Setup Micro SD Card Slot INGA Sensor Node • Atmel ATmega 1284P: 128kb ROM, 16kb RAM 2GB Transcend Micro SD Card Wolf-Bastian Pöttner | RATFAT | Page 12 / 19

Baseline: Writing n times 8 byte with COFFEE 14000 140000 Writing time for 8 byte Cumulative writing time 12000 120000 Average write time : 21.5 ms Median write time : 19.1 ms 10000 100000 Maximum write time : 5573.3 ms time cumulative [ms] time per write [ms] 8000 80000 6000 60000 4000 40000 2000 20000 0 0 0 1000 2000 3000 4000 5000 6000 n'th write Wolf-Bastian Pöttner | RATFAT | Page 13 / 19

Writing n times 8 byte with RATFAT (non-RT) 90 4500 Writing time for 8 byte Cumulative writing time 80 4000 70 Average write time : 0.72 ms 3500 Median write time : 0.38 ms time cumulative [ms] Maximum write time : 71.3ms 60 3000 time per write [ms] 50 2500 40 2000 30 1500 20 1000 10 500 0 0 0 1000 2000 3000 4000 5000 6000 n'th write Wolf-Bastian Pöttner | RATFAT | Page 14 / 19

RATFAT RT Measurement Setup Sampling interval T RT Process Write request Write request Write request RATFAT non-RT Deadline Miss Deadline Miss RATFAT RT reschedule FSP reschedule FSP Wolf-Bastian Pöttner | RATFAT | Page 15 / 19

Maximum Sampling rate with RATFAT RT and non-RT 5 RATFAT-no-rt RATFAT-rt 4 missed deadlines 3 2 1 0 10 20 30 40 50 60 70 80 sampling rate [Hz] Wolf-Bastian Pöttner | RATFAT | Page 16 / 19

Comparison of COFFEE, RATFAT non-RT and RATFAT RT 10000 COFFEE RATFAT-no-rt RATFAT-rt total 8000 RATFAT-rt queueing time cummulative [ms] 6000 4000 2000 0 0 1000 2000 3000 4000 5000 6000 n'th write Wolf-Bastian Pöttner | RATFAT | Page 17 / 19

Memory Footprint Comparision 20000 18000 16000 14000 12000 Bytes 10000 8000 6000 4000 2000 0 COFFEE RATFAT non-RT RATFAT RT ROM (.text) RAM (.data & .bss) INGA: 128kb ROM, 16kb RAM Wolf-Bastian Pöttner | RATFAT | Page 18 / 19

Sebastian Schildt Wolf-Bastian Pöttner schildt@ibr.cs.tu-bs.de poettner@ibr.cs.tu-bs.de Conclusion Motivation • FAT on SD cards allows exchanging data between node and PC • Some applications require real-time operations on WSN nodes RATFAT • FAT implementation for Contiki-based sensor nodes • (Optional) real-time support for cooperative multitasking env. Results • RATFAT shows more predictable timing behavior than COFFEE • Supports real-time tasks up to 64 Hz invocation frequency Code: http://goo.gl/Ky1Ns Wolf-Bastian Pöttner | RATFAT | Page 19 / 19

Maximum Sampling rate with RATFAT RT and non-RT 180 RATFAT-no-rt RATFAT-rt 160 140 120 missed deadlines 100 80 60 40 20 0 0 20 40 60 80 100 120 140 sampling rate [Hz] Wolf-Bastian Pöttner | RATFAT | Page 20 / 19

RATFAT Memory Footprint ROM (.text) RAM (.data & .bss) COFFEE 10,748 Bytes 233 Bytes RATFAT non-RT 14,772 Bytes 907 Bytes RATFAT RT 19,786 Bytes 1,556 Bytes INGA (Atmega 1284p) 128,000 Bytes 16,000 Bytes • Measured on INGA (8 bit MCU) Wolf-Bastian Pöttner | RATFAT | Page 21 / 19