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An Empirical Evaluation of Semiconductor File Memory as a Disk Cache

An Empirical Evaluation of Semiconductor File Memory as a Disk Cache. John C. Koob Duncan G. Elliott Bruce F. Cockburn VLSI Design Lab ECE Department University of Alberta Edmonton, Alberta Canada. Outline. Motivation Extended Storage File Memory Experimental Platform

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An Empirical Evaluation of Semiconductor File Memory as a Disk Cache

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  1. An Empirical Evaluation of SemiconductorFile Memory as a Disk Cache John C. Koob Duncan G. Elliott Bruce F. Cockburn VLSI Design Lab ECE Department University of Alberta Edmonton, Alberta Canada

  2. Outline • Motivation • Extended Storage • File Memory • Experimental Platform • Cost/Performance Analysis • Conclusions • Future Work

  3. Motivation Source: Computer Architecture, Hennessy & Patterson, 2003

  4. Access Time Gap Problem • Use Extended Storage • Cheaper per bit than main memory • Faster than disk • Slower than main memory • Not necessarily semiconductor media • Potential for power savings • How to fill the access time gap?

  5. Historical Systems • Extended storage first appeared in expensive systems • IBM 3090mainframe • Main memory: 0.5 GB • Extended storage: 4 GB • Terminology: Expanded Storage Image courtesy of www.ibm.com

  6. Historical Systems • Extended storage first appeared in expensive systems • Cray Y-MPsupercomputer • Main memory: 1 GB of 15-ns bipolar SRAM • Extended storage: 4 GB of 50-ns DRAM • Terminology: Solid State Disk Image courtesy of the Charles Babbage Institute

  7. Recent Research • Compressed caching (1999-2003) • Use compression to reduce paging costs • Adaptive sizing of the compressed portion required • Evaluated using a modified Linux 2.4 kernel • Multi-level main memory (WMPI 2004) • 30% of memory must run at DRAM speed • Remaining memory can be slower • Hardware compression • Remote memory box • Portions can be powered down

  8. Extended Storage Today? • Emerging technology may prompt a return to extended storage • Semiconductor file memory • Up to 5 times slower than DRAM • Must be cheaper per bit than DRAM • MEMS probe-based storage • 5 times faster than disk • 10 times more expensive than disk

  9. What is File Memory? • File memory leverages current DRAM technology • DRAM design constraints increase costs per bit • 100% of nominal capacity must be functional • Contiguous address space • Consistently good access time • File memory relaxes DRAM’s design constraints • Bad block marking to improve yield • Address space is not contiguous • Improve density at the expense of performance (e.g. multi-level DRAM or hardware compression)

  10. Contiguous Memory Non-Contiguous Memory Feasibility of File Memory • A precedent for file memory exists in the non-volatile memory market • NOR Flash memory • Limited capacity • Moderate reliability • Random-access supported • NAND Flash memory • High capacity • Low reliability  bad block marking • Restricted to sequential access

  11. Extended Storage Disk Cache • To evaluate file memory as extended storage: • Require an experimental platform • Modify Linux 2.4.18 OS kernel • ESDC Design Summary • High memory support • Page cache containment • Configurable performance • CPU caching issues • Performance metrics • Verification

  12. Configurable Performance • Need configurable file memory properties • Capacity • Access time • How to model different file memory access times? • Use multiple page copies • Gives accurate file memory slowdown ratios • Problem: • Repeated page copies would be cached • Solution: • Use IA-32 memory type range registers (MTRRs) • Disable CPU caches for ESDC

  13. Experimental Setup • Experimental Platform • Processor 2.4 GHz Pentium 4 • Memory 2 GB DDR SDRAM • Hard disk 18-GB Seagate SCSI • Disk buffer 4-MB • Experimental Suite • PostMark – benchmark for many small files • Bonnie – file system benchmark • Kernel compilation – Linux kernel build

  14. Postmark Results for Original Hierarchy

  15. Postmark Results using File Memory

  16. Postmark Results Analysis Need 39% more file memory for equivalent performance

  17. Summary of Postmark Results

  18. Conclusions • Use non-contiguous file memory for extended storage • Leverage DRAM cell technology • Relax DRAM design constraints • Use bad block marking • Preliminary evaluation of ESDC • Use file memory up to 4 times slower than DRAM • Even though ESDC replaces the page cache, system performance can be improved • Ongoing research • Evaluate hierarchies with file memory and page cache

  19. Selected References Bray. Bonnie. www.textuality.com/bonnie, 1996. Castro et al. Adaptive compressed caching. Symp. on Comp. Arch. And High Performance Computing, Nov. 2003. Ekman and Stenstrom. A case for multi-level main memory. WMPI 2004. Hennessy and Patterson. Computer architecture: A quantitative approach. Third Edition, 2003. Katcher. PostMark: A new filesystem benchmark. TR3022, Network Appliance, Oct. 1997. Koob et al. Test and characterization of a variable capacity multilevel DRAM. In Proc. VLSI Test Symp., pp. 189-197, May 2005. Uysal et al. Using MEMS-based storage in disk arrays. FAST 2003, pp. 89-101.

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