A Robust Main-Memory Compression Scheme Magnus Ekman and Per Stenström Chalmers University of Technology Göteborg, Sweden

Motivation Memory resources are wasted to compensate for the increasing processor/memory/disk speedgap >50% of die size occupied by caches >50% of cost of a server is DRAM (and increasing) Lossless data compression techniques have the potential to free up more than 50% of memory resources. Unfortunately, compression introduces several challenging design and performance issues

core L1-cache L2-cache Main memory space Request Data

core L1-cache L2-cache Compressed main memory space Translation table Address Translation Request Decompressor Data Fragmented compressed main memory space

Contributions A low-overhead main-memory compression scheme: Low decompression latency by using simple and fast algorithm (zero aware) Fast address translation by a proposed small translation structure that fits on the processor die Reduction of fragmentation through occassional relocation of data when compressibility varies Overall, our compression scheme frees up 30% of the memory at a marginal performance loss of 0.2%!

Outline Motivation Issues Contributions Effectiveness of Zero-Aware Compressors Our Compression Scheme Performance Results Related Work Conclusions

Frequency of zero-valued locations 12% of all 8KB pages only contain zeros 30% of all 64B blocks only contain zeros 42% of all 4B words only contain zeros 55% of all bytes are zero! Zero-aware compression schemes have a great potential!

Evaluated Algorithms Zero aware algorithms: FPC (Alameldeen and Wood) + 3 simplified versions For comparison, we also consider: X-Match Pro (efficient hardware implementations exist) LZSS (popular algorithm, previously used by IBM for memory compression) Deflate (upper bound on compressibility)

Resulting Compressed Sizes Main observations: FPC and all its variations can free up about 45% of memory LZSS and X-MatchPro only marginally better in spite of complexity Deflate can free up about 80% of memory but not clear how to exploit it Fast and efficient compression algorithms exist! SpecInt SpecFP Server

Outline Motivation Issues Contributions Effectiveness of Zero-Aware Compressors Our Compression Scheme Performance Results Related Work Conclusions

Uncompressed data Compressed data Compressed fragmented data Block size vector Address translation A block is assigned one out of n predefined sizes. In this example n= Block Size Table (BST)TLB Address Calculator OS changes –Block size vector is kept in page table –Each page is assigned one out of k predefined sizes. Physical address grows with log 2 k bits. The Block Size Table enables fast translation!

Size changes and compaction Sub-page 0 Sub-page 1 sub-page slack Terminology: block overflow/underflow, sub-page overflow/underflow, page overflow/underflow Block overflow slack Block underflow page slack

Handling of overflows/underflows Block and sub-page overflows/underflows implies moving data within a page On a page overflow/underflow the entire page needs to be moved to avoid having to move several pages Block and sub-page overflows/underflows are handled in hardware by an off-chip DMA-engine On a page overflow/underflow a trap is taken and the mapping for the page is changed Processor has to stall if it accesses data that is being moved!

core L1-cache L2-cache BST Calc. Sub-page 0Sub-page 1 Page 0 CompDec. DMA- engine Putting it all together

Experimental Methodology Key issues to experimentally evaluate: Compressibility and impact of fragmentation Performance losses for our approach Simulation approaches (both using Simics): Fast functional simulator (in-order, 1 instr/cycle) allowing entire benchmarks to be run Detailed microarchitecture simulator driven by a single sim-point per application

Architectural Parameters Instr. Issue4-w ooo Exec units4 int, 2 int mul/div Branch pred.16-k entr. gshare, 2k BTB L1 I-cache32 k, 2-w, 2-cycle L1 D-cache32 k, 4-w, 2-cycle L2 cache512k/thread, 8-w, 16 cycles Memory latency150 cycles 1 Block lock-out4000 cycles 2 Subpage lock-out23000 cycles 2 Page lock-out23000 cycles 2 2 Conservatively assumes 200 MHz DDR2 DRAM; leads to long lock-out time 1 Aggressive for future processsors to not give advantage to our compr. scheme Predefined sizes Block Subpage Page Loads to a block only containing zeros can retire without accessing memory!

Benchmarks SPEC2000 ran with reference set. SAP and SpecJBB ran 4 billion instructions per thread SpecInt2000 Bzip Gap Gcc Gzip Mcf Parser Perlbmk Twolf Vpr SpecFP2000 Ammp Art Equake Mesa Server SAP S&D SpecJBB

Overflows/Underflows Main observations: About 1 out of every thousand instruction causes a block overflow/underflow The use of subpages cuts down the number of page-level relocations by one order of magnitude Memory bandwidth goes up by 58% with subpages and 212% without. Note that this is not the bandwidth to the processor chip Fragmentation with the use of a hierarchy of pages, sub- pages and blocks, reduce memory savings to 30% Note: y axis is logarithmic

Detailed Performance Results We used a single simpoint per application according to [Sherwood et al. 2002] Main observations: Decompression latency reduces performance by 0.5% on average Misses to zero-valued blocks increases performance by 0.5% on average Factoring in also relocation/compaction losses, performance losses are only 0.2%!

Related Work Early work on main memory compression: –Douglis [1993], Kjelso et al. [1999], and Wilson et al. [1999] –These works aimed at reducing paging overhead so the significant decompression and address translation latencies were offset by the wins More recently –IBM MXT [Abali et al. 2001] –Compresses entire memory with LZSS (64 cycle decompression latency) –Translation through memory resident translation table –Shields latency by huge (at that point in time) 32-MByte cache. Sensitive to working set size Compression algorithm –Inspired by frequent-value locality work by Zhang, Yang and Gupta 2000 –Compression algorithm from Alameldeen and Wood, 2004

Concluding Remarks It is possible to free up significant amounts of memory resources with virtually zero performance overhead This was achieved by exploiting zero-valued bytes which account for as much as and 55% of the memory contents leveraging a fast compression/decompression scheme a fast translation mechanism a hierarchical memory layout which offers some slack at the block, subpage, and page level Overall, 30% of memory could be freed up at a loss of 0.2% on average

Backup Slides

Fragmentation - Results

% misses to zero-valued blocks bzigapgccgzimcfparpertwovorvpramartequmessapjbb 512k MB For gap, gcc, gzip, mesa more than 20% of the misses request zero- valued blocks; for the rest the percentage is quite small

Frequent Pattern Compression (Alameldeen and Wood) PrefixPattern encodedData size 00Zero word0 01One byte sign-extended8 bits 10halfword sign-extended16 bits 11Uncompressed32 bits 3 bits (for runs up to 8 ”0”) Zero run Each 32-bit word is coded using a prefix plus data: PrefixPattern encodedData size 000Zero run3 bits (for runs up to 8 ”0”) 0014-bit sign-extended4 bits 010One byte sign-extended8 bits 011Half word sign-extended16 bits 100Half word padded with a zero halfword16 bits 101Two halfwords, each a byte sign-ext.16 bits 110Word consisting of repeated bytes8 bits 111Uncompressed32 bits