1. FlashVM: Virtual Memory Management on Flash Mohit Saxena and Michael M. Swift Introduction Flash storage is the largest change to memory and storage systems in decades. In addition to serving a fast, low-power disk replacement or accelerator, flash devices can also be used for virtual memory to handle large workloads cheaply. FlashVM is a system architecture and extension to the Linux VM subsystem pursuing this challenge. PerformanceReliability Flash Benefits for VMFlashVM The FlashVM architecture consists of dedicated flash for paging and the FlashVM manager to control Linux’s use of flash. References [1] Saxena M., and Swift, M. M. FlashVM: Revisiting the Virtual Memory Hierarchy. In HotOS-XII (2009). [2] Agrawal N., Prabhakaran V., Wobber T., Davis J., Manasse M., and Panigraph R. Design Tradeoffs for SSD performance. In USENIX (2008). [3] Park S., Jung D., Kang J., Kim J., and Lee J., CFLRU: A replacement algorithm for flash memory, In CASES (2006). [4] Mogul J. C., Argollo E., Shah M., and Farboschi P., Operating System support for NVM+DRAM hybrid main memory. In HotOS (2009). FlashVM Enhancements PerformanceParameter Tuning, Prefetching ReliabilityPage sampling, Page Sharing Garbage CollectionDummy discard, Merged discard DiskVM FlashVM Memory Size M Execution Time T M  Reduced DRAM  Same performance  Lower system price T  Faster and scalable  No additional DRAM  Similar system price Block Device Driver DRAM Page Swapping FlashVM Manager: Performance, Reliability and Garbage Collection Dedicated Flash Cheaper than disk for VM Reduced FS interference Disk Disk Scheduler Block Layer VM Memory Manager Dedicated MLC NAND Flash Unused memory No locality FlashVM benefits system operating with moderate paging: FlashVM can improve performance with the same amount of DRAM, by speeding page faults or achieve the same performance with a cheaper mix of Flash and less DRAM leading to more page faults. Application Performance and Memory Savings 84% memory savings 94% less execution time Write Reduction Performance 14% reduction 93% reduction Garbage Collection 10X faster 15% slower Write Reduction Dirty? Dirty Clean Inactive LRU Page List free_pages Flash FlashVM Sample dirty pages to elide writes Classify page contents Write-back non-zero dirty pages Free Page List classify Non-Zero Zero sample evict leave in memory Page Prefetching Disk VM assumption -Seek and rotational delays are longer than the transfer cost of extra blocks Linux sequential prefetching -Minimize costly disk seeks by reading only blocks adjacent to request FlashVM prefetching -Exploit fast flash random reads and spatial/temporal locality in reference pattern -Seek over free and bad blocks Current VM systems are optimized for disk performance – Slow random reads – High access and seek costs – Symmetrical read/write performance FlashVM retunes VM parameters for Flash: – Page write back: unit of writing – Page scanning: congestion timeouts – Disk scheduling: work-conserving FIFO FlashVM aggressively prefetches pages Discard swap map FREE BAD FlashVM Request Linux Flash chips lose durability after 10,000— 100,000 writes – Challenge: reduce the number of writes Page sampling: probabilistically leave old, dirty pages in memory instead of writing back – Measure page dirtying rate – Increase sampling probability with low rate, – Decrease with high rate Page sharing: avoid storing multiple copies of same data – Marks zero-filled pages in swap map and avoid writing back – Avoids read/write cost Garbage Collection All writes to flash go to new location – SSD aware of freed page clusters when the blocks are overwritten Discard/Trim: notifies SSD that blocks are unused – More free blocks for writing – Avoids copying data for partial overwrites Observation: Overwriting a block – notifies SSD it is empty – after discarding it, uses the free space made available by discard FlashVM dummy discard – Monitors rate of allocation – Virtualize discard by reusing blocks likely to be overwritten soon Native Linux discard once per page cluster – Result: 55 ms latency for freeing 32 pages FlashVM merged discard – Defer and batch discard until 100MB of free pages available – Pages discarded may be non-contiguous Dummy Discard Merged Discard