Parsing JSON Really Quickly: Lessons Learned
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Video and slides synchronized, mp3 and slide download available at URL https://bit.ly/2PXv97g. Daniel Lemire talks about the lessons learned while writing the fast JSON parser, simdjson. One of the most important lessons is the importance of a nearly obsessive focus on performance metrics - to constantly measure the impact of the choices. Filmed at qconsf.com. Daniel Lemire is a computer science professor at the Université du Québec (TELUQ). He has written over 70 peer-reviewed publications, including more than 40 journal articles. He serves on the program committees of leading computer science conferences. During the 2016-2017 NSERC Discovery Grant competition, he received a rating of outstanding for the excellence of the researcher.
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Parsing JSON Really Quickly: Lessons Learned
- 2. InfoQ.com: News & Community Site • Over 1,000,000 software developers, architects and CTOs read the site world- wide every month • 250,000 senior developers subscribe to our weekly newsletter • Published in 4 languages (English, Chinese, Japanese and Brazilian Portuguese) • Post content from our QCon conferences • 2 dedicated podcast channels: The InfoQ Podcast, with a focus on Architecture and The Engineering Culture Podcast, with a focus on building • 96 deep dives on innovative topics packed as downloadable emags and minibooks • Over 40 new content items per week Watch the video with slide synchronization on InfoQ.com! https://www.infoq.com/presentations/ simdjson-parser/
- 3. Purpose of QCon - to empower software development by facilitating the spread of knowledge and innovation Strategy - practitioner-driven conference designed for YOU: influencers of change and innovation in your teams - speakers and topics driving the evolution and innovation - connecting and catalyzing the influencers and innovators Highlights - attended by more than 12,000 delegates since 2007 - held in 9 cities worldwide Presented at QCon San Francisco www.qconsf.com
- 7. Readingtextlines(CPUonly) ~0.6GB/son3.4GHzSkylakeinJava void parseLine(String s) { volume += s.length(); } void readString(StringReader data) { BufferedReader bf = new BufferedReader(data); bf.lines().forEach(s -> parseLine(s)); } Sourceavailable. ImprovedbyJDK-8229022 5
- 8. Readingtextlines(CPUonly) ~1.5GB/son3.4GHzSkylake inC++(GNUGCC8.3) size_t sum_line_lengths(char * data, size_t length) { std::stringstream is; is.rdbuf()->pubsetbuf(data, length); std::string line; size_t sumofalllinelengths{0}; while(getline(is, line)) { sumofalllinelengths += line.size(); } return sumofalllinelengths; } Sourceavailable. 6
- 9. source 7
- 10. JSON SpecifiedbyDouglasCrockford RFC7159byTimBrayin2013 Ubiquitousformattoexchangedata {"Image": {"Width": 800,"Height": 600, "Title": "View from 15th Floor", "Thumbnail": { "Url": "http://www.example.com/81989943", "Height": 125,"Width": 100} } 8
- 13. JacksonJSONspeed(Java) twitter.json:0.35GB/son3.4GHzSkylake Sourcecodeavailable. speed Jackson(Java) 0.35GB/s readLinesC++ 1.5GB/s disk 2.2GB/s 11
- 14. RapidJSONspeed(C++) twitter.json:0.650GB/son3.4GHzSkylake speed RapidJSON(C++) 0.65GB/s Jackson(Java) 0.35GB/s readLinesC++ 1.5GB/s disk 2.2GB/s 12
- 15. simdjsonspeed(C++) twitter.json:2.4GB/son3.4GHzSkylake speed simdjson(C++) 2.4GB/s RapidJSON(C++) 0.65GB/s Jackson(Java) 0.35GB/s readLinesC++ 1.5GB/s disk 2.2GB/s 13
- 18. Writerandomnumbersonanarray. while (howmany != 0) { out[index] = random(); index += 1; howmany--; } e.g.,~3cyclesperiteration 16
- 19. Writeonlyoddrandomnumbers: while (howmany != 0) { val = random(); if( val is odd) { // <=== new out[index] = val; index += 1; } howmany--; } 17
- 21. Gobranchless!while (howmany != 0) { val = random(); out[index] = val; index += (val bitand 1); howmany--; } backtounder4cycles! Detailsandcodeavailable 19
- 25. ISA where max.registerwidth ARMNEON(AArch64) mobilephones,tablets 128-bit SSE2...SSE4.2 legacyx64(Intel,AMD) 128-bit AVX,AVX2 mainstreamx64(Intel,AMD) 256-bit AVX-512 latestx64(Intel) 512-bit 23
- 34. ValidatingUTF-8withif/else/while if (byte1 < 0x80) { return true; // ASCII } if (byte1 < 0xE0) { if (byte1 < 0xC2 || byte2 > 0xBF) { return false; } } else if (byte1 < 0xF0) { // Three-byte form. if (byte2 > 0xBF || (byte1 == 0xE0 && byte2 < 0xA0) || (byte1 == 0xED && 0xA0 <= byte2) blablabla ) blablabla } else { // Four-byte form. .... blabla } 32
- 36. Example:Verifythatallbytevaluesarenolargerthan244 Saturatedsubtraction: x - 244 isnon-zeroifanonlyif x > 244 . _mm256_subs_epu8(current_bytes, 244 ); Oneinstruction,checks32bytesatonce! 34
- 38. Example2.Classifyingcharacters comma(0x2c) , colon(0x3a) : brackets(0x5b,0x5d,0x7b,0x7d): [, ], {, } white-space(0x09,0x0a,0x0d,0x20) others Classify16,32or64charactersatonce! 36
- 41. Startwithanarrayof4-bitvalues [1,1,0,2,0,5,10,15,7,8,13,9,0,13,5,1] Createalookuptable [200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215] 0 200,1 201,2 202 Result: [201,201,200,202,200,205,210,215,207,208,213,209,200,213,205,201] 39
- 42. Findtwotables H1 and H2 suchasthebitwiseANDofthelookclassifythecharacters. H1(low(c)) & H2(high(c)) comma(0x2c):1 colon(0x3a):2 brackets(0x5b,0x5d,0x7b,0x7d):4 mostwhite-space(0x09,0x0a,0x0d):8 whitespace(0x20):16 others:0 40
- 43. const uint8x16_t low_nibble_mask = (uint8x16_t){16, 0, 0, 0, 0, 0, 0, 0, 0, 8, 12, 1, 2, 9, 0, 0}; const uint8x16_t high_nibble_mask = (uint8x16_t){8, 0, 18, 4, 0, 1, 0, 1, 0, 0, 0, 3, 2, 1, 0, 0}; const uint8x16_t low_nib_and_mask = vmovq_n_u8(0xf); Fiveinstructions: uint8x16_t nib_lo = vandq_u8(chunk, low_nib_and_mask); uint8x16_t nib_hi = vshrq_n_u8(chunk, 4); uint8x16_t shuf_lo = vqtbl1q_u8(low_nibble_mask, nib_lo); uint8x16_t shuf_hi = vqtbl1q_u8(high_nibble_mask, nib_hi); return vandq_u8(shuf_lo, shuf_hi); 41
- 45. Canyoutellwherethestringsstartandend? { ""Nam[{": [ 116,"" ... Withoutbranching? 43
- 47. Identifybackslashes: { ""Nam[{": [ 116,"" ___111________________1111_ :B Oddandevenpositions 1_1_1_1_1_1_1_1_1_1_1_1_1_1 :E(constant) _1_1_1_1_1_1_1_1_1_1_1_1_1_ :O(constant) 45
- 48. Doabunchofarithmeticandlogicaloperations... (((B + (B &~(B << 1)& E))& ~B)& ~E) | (((B + ((B &~(B << 1))& O))& ~B)& E) Result: { ""Nam[{": [ 116,"" ... ______1____________________ Nobranch! 46
- 50. { ""Nam[{": [ 116,"" __1___1_____1________1____1 :allquotes ______1____________________ :escapedquotes __1_________1________1____1 :string-delimiterquotes 48
- 51. Findthespanofthestring mask = quote xor (quote << 1); mask = mask xor (mask << 2); mask = mask xor (mask << 4); mask = mask xor (mask << 8); mask = mask xor (mask << 16); ... __1_________1________1____1 (quotes) becomes __1111111111_________11111_ (stringregion) 49
- 53. Example4.DecodebitindexesGiventhebitset 1000100010001 ,wewantthelocationofthe1s(e.g.,0,4,812) 51
- 54. while (word != 0) { result[i] = trailingzeroes(word); word = word & (word - 1); i++; } Ifnumberof1sper64-bitishardtopredict:lotsofmispredictions!!! 52
- 56. Reducethenumberofmispredictionbydoingmoreworkperiteration: while (word != 0) { result[i] = trailingzeroes(word); word = word & (word - 1); result[i+1] = trailingzeroes(word); word = word & (word - 1); result[i+2] = trailingzeroes(word); word = word & (word - 1); result[i+3] = trailingzeroes(word); word = word & (word - 1); i+=4; } Discardbogusindexesbycountingthenumberof1sintheworddirectly(e.g., bitCount ) 54
- 58. Checkwhetherwehave8consecutivedigits bool is_made_of_eight_digits_fast(const char *chars) { uint64_t val; memcpy(&val, chars, 8); return (((val & 0xF0F0F0F0F0F0F0F0) | (((val + 0x0606060606060606) & 0xF0F0F0F0F0F0F0F0) >> 4)) == 0x3333333333333333); } 56
- 59. Thenconstructthecorrespondinginteger Usingonlythreemultiplications(insteadof7): uint32_t parse_eight_digits_unrolled(const char *chars) { uint64_t val; memcpy(&val, chars, sizeof(uint64_t)); val = (val & 0x0F0F0F0F0F0F0F0F) * 2561 >> 8; val = (val & 0x00FF00FF00FF00FF) * 6553601 >> 16; return (val & 0x0000FFFF0000FFFF) * 42949672960001 >> 32; } CandoevenbetterwithSIMD 57
- 61. int json_parse_dispatch(...) { Architecture best_implementation = find_best_supported_implementation(); // Selecting the best implementation switch (best_implementation) { case Architecture::HASWELL: json_parse_ptr = &json_parse_implementation<Architecture::HASWELL>; break; case Architecture::WESTMERE: json_parse_ptr= &json_parse_implementation<Architecture::WESTMERE>; break; default: return UNEXPECTED_ERROR; } return json_parse_ptr(....); } 59
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- 66. Watch the video with slide synchronization on InfoQ.com! https://www.infoq.com/presentations/ simdjson-parser/