From c4705381422ead4ad99f4b7a3bc11f059c460401 Mon Sep 17 00:00:00 2001 From: Pieter Noordhuis Date: Fri, 13 Aug 2010 19:28:49 +0200 Subject: [PATCH] Make ziplist schema more efficient for strings with length > 15 --- src/ziplist.c | 227 ++++++++++++++++++++++++++++++++------------------ 1 file changed, 144 insertions(+), 83 deletions(-) diff --git a/src/ziplist.c b/src/ziplist.c index 7a3a8b01..a6383517 100644 --- a/src/ziplist.c +++ b/src/ziplist.c @@ -1,17 +1,63 @@ -/* Memory layout of a ziplist, containing "foo", "bar", "quux": - * "foo""bar""quux" +/* The ziplist is a specially encoded dually linked list that is designed + * to be very memory efficient. It stores both strings and integer values, + * where integers are encoded as actual integers instead of a series of + * characters. It allows push and pop operations on either side of the list + * in O(1) time. However, because every operation requires a reallocation of + * the memory used by the ziplist, the actual complexity is related to the + * amount of memory used by the ziplist. * - * is an unsigned integer to hold the number of bytes that - * the ziplist occupies. This is stored to not have to traverse the ziplist - * to know the new length when pushing. + * ---------------------------------------------------------------------------- * - * is the number of items in the ziplist. When this value is - * greater than 254, we need to traverse the entire list to know - * how many items it holds. + * ZIPLIST OVERALL LAYOUT: + * The general layout of the ziplist is as follows: + * * - * is the number of bytes occupied by a single entry. When this - * number is greater than 253, the length will occupy 5 bytes, where - * the extra bytes contain an unsigned integer to hold the length. + * is an unsigned integer to hold the number of bytes that the + * ziplist occupies. This value needs to be stored to be able to resize the + * entire structure without the need to traverse it first. + * + * is the offset to the last entry in the list. This allows a pop + * operation on the far side of the list without the need for full traversal. + * + * is the number of entries.When this value is larger than 2**16-2, + * we need to traverse the entire list to know how many items it holds. + * + * is a single byte special value, equal to 255, which indicates the + * end of the list. + * + * ZIPLIST ENTRIES: + * Every entry in the ziplist is prefixed by a header that contains two pieces + * of information. First, the length of the previous entry is stored to be + * able to traverse the list from back to front. Second, the encoding with an + * optional string length of the entry itself is stored. + * + * The length of the previous entry is encoded in the following way: + * If this length is smaller than 254 bytes, it will only consume a single + * byte that takes the length as value. When the length is greater than or + * equal to 254, it will consume 5 bytes. The first byte is set to 254 to + * indicate a larger value is following. The remaining 4 bytes take the + * length of the previous entry as value. + * + * The other header field of the entry itself depends on the contents of the + * entry. When the entry is a string, the first 2 bits of this header will hold + * the type of encoding used to store the length of the string, followed by the + * actual length of the string. When the entry is an integer the first 2 bits + * are both set to 1. The following 2 bits are used to specify what kind of + * integer will be stored after this header. An overview of the different + * types and encodings is as follows: + * + * |00pppppp| - 1 byte + * String value with length less than or equal to 63 bytes (6 bits). + * |01pppppp|qqqqqqqq| - 2 bytes + * String value with length less than or equal to 16383 bytes (14 bits). + * |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes + * String value with length greater than or equal to 16384 bytes. + * |1100____| - 1 byte + * Integer encoded as int16_t (2 bytes). + * |1101____| - 1 byte + * Integer encoded as int32_t (4 bytes). + * |1110____| - 1 byte + * Integer encoded as int64_t (8 bytes). */ #include @@ -25,25 +71,20 @@ int ll2string(char *s, size_t len, long long value); -/* Important note: the ZIP_END value is used to depict the end of the - * ziplist structure. When a pointer contains an entry, the first couple - * of bytes contain the encoded length of the previous entry. This length - * is encoded as ZIP_ENC_RAW length, so the first two bits will contain 00 - * and the byte will therefore never have a value of 255. */ #define ZIP_END 255 #define ZIP_BIGLEN 254 -/* Entry encoding */ -#define ZIP_ENC_RAW 0 -#define ZIP_ENC_INT16 1 -#define ZIP_ENC_INT32 2 -#define ZIP_ENC_INT64 3 -#define ZIP_ENCODING(p) ((p)[0] >> 6) +/* Different encoding/length possibilities */ +#define ZIP_STR_06B (0 << 6) +#define ZIP_STR_14B (1 << 6) +#define ZIP_STR_32B (2 << 6) +#define ZIP_INT_16B (0xc0 | 0<<4) +#define ZIP_INT_32B (0xc0 | 1<<4) +#define ZIP_INT_64B (0xc0 | 2<<4) -/* Length encoding for raw entries */ -#define ZIP_LEN_INLINE 0 -#define ZIP_LEN_UINT16 1 -#define ZIP_LEN_UINT32 2 +/* Macro's to determine type */ +#define ZIP_IS_STR(enc) (((enc) & 0xc0) < 0xc0) +#define ZIP_IS_INT(enc) (!ZIP_IS_STR(enc) && ((enc) & 0x30) < 0x30) /* Utility macros */ #define ZIPLIST_BYTES(zl) (*((uint32_t*)(zl))) @@ -67,14 +108,25 @@ typedef struct zlentry { unsigned char *p; } zlentry; +/* Return the encoding pointer to by 'p'. */ +static unsigned int zipEntryEncoding(unsigned char *p) { + /* String encoding: 2 MSBs */ + unsigned char b = p[0] & 0xc0; + if (b < 0xc0) { + return b; + } else { + /* Integer encoding: 4 MSBs */ + return p[0] & 0xf0; + } + assert(NULL); +} + /* Return bytes needed to store integer encoded by 'encoding' */ -static unsigned int zipEncodingSize(unsigned char encoding) { - if (encoding == ZIP_ENC_INT16) { - return sizeof(int16_t); - } else if (encoding == ZIP_ENC_INT32) { - return sizeof(int32_t); - } else if (encoding == ZIP_ENC_INT64) { - return sizeof(int64_t); +static unsigned int zipIntSize(unsigned char encoding) { + switch(encoding) { + case ZIP_INT_16B: return sizeof(int16_t); + case ZIP_INT_32B: return sizeof(int32_t); + case ZIP_INT_64B: return sizeof(int64_t); } assert(NULL); } @@ -82,23 +134,28 @@ static unsigned int zipEncodingSize(unsigned char encoding) { /* Decode the encoded length pointed by 'p'. If a pointer to 'lensize' is * provided, it is set to the number of bytes required to encode the length. */ static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) { - unsigned char encoding = ZIP_ENCODING(p), lenenc; + unsigned char encoding = zipEntryEncoding(p); unsigned int len; - if (encoding == ZIP_ENC_RAW) { - lenenc = (p[0] >> 4) & 0x3; - if (lenenc == ZIP_LEN_INLINE) { - len = p[0] & 0xf; + if (ZIP_IS_STR(encoding)) { + switch(encoding) { + case ZIP_STR_06B: + len = p[0] & 0x3f; if (lensize) *lensize = 1; - } else if (lenenc == ZIP_LEN_UINT16) { - len = p[1] | (p[2] << 8); - if (lensize) *lensize = 3; - } else { - len = p[1] | (p[2] << 8) | (p[3] << 16) | (p[4] << 24); + break; + case ZIP_STR_14B: + len = ((p[0] & 0x3f) << 6) | p[1]; + if (lensize) *lensize = 2; + break; + case ZIP_STR_32B: + len = (p[1] << 24) | (p[2] << 16) | (p[3] << 8) | p[4]; if (lensize) *lensize = 5; + break; + default: + assert(NULL); } } else { - len = zipEncodingSize(encoding); + len = zipIntSize(encoding); if (lensize) *lensize = 1; } return len; @@ -106,34 +163,36 @@ static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) { /* Encode the length 'l' writing it in 'p'. If p is NULL it just returns * the amount of bytes required to encode such a length. */ -static unsigned int zipEncodeLength(unsigned char *p, char encoding, unsigned int rawlen) { - unsigned char len = 1, lenenc, buf[5]; - if (encoding == ZIP_ENC_RAW) { - if (rawlen <= 0xf) { +static unsigned int zipEncodeLength(unsigned char *p, unsigned char encoding, unsigned int rawlen) { + unsigned char len = 1, buf[5]; + + if (ZIP_IS_STR(encoding)) { + /* Although encoding is given it may not be set for strings, + * so we determine it here using the raw length. */ + if (rawlen <= 0x3f) { if (!p) return len; - lenenc = ZIP_LEN_INLINE; - buf[0] = rawlen; - } else if (rawlen <= 0xffff) { - len += 2; + buf[0] = ZIP_STR_06B | rawlen; + } else if (rawlen <= 0x3fff) { + len += 1; if (!p) return len; - lenenc = ZIP_LEN_UINT16; - buf[1] = (rawlen ) & 0xff; - buf[2] = (rawlen >> 8) & 0xff; + buf[0] = ZIP_STR_14B | ((rawlen >> 8) & 0x3f); + buf[1] = rawlen & 0xff; } else { len += 4; if (!p) return len; - lenenc = ZIP_LEN_UINT32; - buf[1] = (rawlen ) & 0xff; - buf[2] = (rawlen >> 8) & 0xff; - buf[3] = (rawlen >> 16) & 0xff; - buf[4] = (rawlen >> 24) & 0xff; + buf[0] = ZIP_STR_32B; + buf[1] = (rawlen >> 24) & 0xff; + buf[2] = (rawlen >> 16) & 0xff; + buf[3] = (rawlen >> 8) & 0xff; + buf[4] = rawlen & 0xff; } - buf[0] = (lenenc << 4) | (buf[0] & 0xf); + } else { + /* Implies integer encoding, so length is always 1. */ + if (!p) return len; + buf[0] = encoding; } - if (!p) return len; - /* Apparently we need to store the length in 'p' */ - buf[0] = (encoding << 6) | (buf[0] & 0x3f); + /* Store this length at p */ memcpy(p,buf,len); return len; } @@ -198,11 +257,11 @@ static int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long /* Great, the string can be encoded. Check what's the smallest * of our encoding types that can hold this value. */ if (value >= INT16_MIN && value <= INT16_MAX) { - *encoding = ZIP_ENC_INT16; + *encoding = ZIP_INT_16B; } else if (value >= INT32_MIN && value <= INT32_MAX) { - *encoding = ZIP_ENC_INT32; + *encoding = ZIP_INT_32B; } else { - *encoding = ZIP_ENC_INT64; + *encoding = ZIP_INT_64B; } *v = value; return 1; @@ -215,13 +274,13 @@ static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encodi int16_t i16; int32_t i32; int64_t i64; - if (encoding == ZIP_ENC_INT16) { + if (encoding == ZIP_INT_16B) { i16 = value; memcpy(p,&i16,sizeof(i16)); - } else if (encoding == ZIP_ENC_INT32) { + } else if (encoding == ZIP_INT_32B) { i32 = value; memcpy(p,&i32,sizeof(i32)); - } else if (encoding == ZIP_ENC_INT64) { + } else if (encoding == ZIP_INT_64B) { i64 = value; memcpy(p,&i64,sizeof(i64)); } else { @@ -234,13 +293,13 @@ static int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) { int16_t i16; int32_t i32; int64_t i64, ret; - if (encoding == ZIP_ENC_INT16) { + if (encoding == ZIP_INT_16B) { memcpy(&i16,p,sizeof(i16)); ret = i16; - } else if (encoding == ZIP_ENC_INT32) { + } else if (encoding == ZIP_INT_32B) { memcpy(&i32,p,sizeof(i32)); ret = i32; - } else if (encoding == ZIP_ENC_INT64) { + } else if (encoding == ZIP_INT_64B) { memcpy(&i64,p,sizeof(i64)); ret = i64; } else { @@ -255,7 +314,7 @@ static zlentry zipEntry(unsigned char *p) { e.prevrawlen = zipPrevDecodeLength(p,&e.prevrawlensize); e.len = zipDecodeLength(p+e.prevrawlensize,&e.lensize); e.headersize = e.prevrawlensize+e.lensize; - e.encoding = ZIP_ENCODING(p+e.prevrawlensize); + e.encoding = zipEntryEncoding(p+e.prevrawlensize); e.p = p; return e; } @@ -327,7 +386,7 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig unsigned int curlen = ZIPLIST_BYTES(zl), reqlen, prevlen = 0; unsigned int offset, nextdiff = 0; unsigned char *tail; - unsigned char encoding = ZIP_ENC_RAW; + unsigned char encoding = 0; long long value; zlentry entry; @@ -344,11 +403,13 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig /* See if the entry can be encoded */ if (zipTryEncoding(s,slen,&value,&encoding)) { - reqlen = zipEncodingSize(encoding); + /* 'encoding' is set to the appropriate integer encoding */ + reqlen = zipIntSize(encoding); } else { + /* 'encoding' is untouched, however zipEncodeLength will use the + * string length to figure out how to encode it. */ reqlen = slen; } - /* We need space for both the length of the previous entry and * the length of the payload. */ reqlen += zipPrevEncodeLength(NULL,prevlen); @@ -380,10 +441,10 @@ static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsig /* Write the entry */ p += zipPrevEncodeLength(p,prevlen); p += zipEncodeLength(p,encoding,slen); - if (encoding != ZIP_ENC_RAW) { - zipSaveInteger(p,value,encoding); - } else { + if (ZIP_IS_STR(encoding)) { memcpy(p,s,slen); + } else { + zipSaveInteger(p,value,encoding); } ZIPLIST_INCR_LENGTH(zl,1); return zl; @@ -463,7 +524,7 @@ unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *sl if (sstr) *sstr = NULL; entry = zipEntry(p); - if (entry.encoding == ZIP_ENC_RAW) { + if (ZIP_IS_STR(entry.encoding)) { if (sstr) { *slen = entry.len; *sstr = p+entry.headersize; @@ -510,7 +571,7 @@ unsigned int ziplistCompare(unsigned char *p, unsigned char *sstr, unsigned int if (p[0] == ZIP_END) return 0; entry = zipEntry(p); - if (entry.encoding == ZIP_ENC_RAW) { + if (ZIP_IS_STR(entry.encoding)) { /* Raw compare */ if (entry.len == slen) { return memcmp(p+entry.headersize,sstr,slen) == 0; @@ -562,7 +623,7 @@ void ziplistRepr(unsigned char *zl) { entry = zipEntry(p); printf("{offset %ld, header %u, payload %u} ",p-zl,entry.headersize,entry.len); p += entry.headersize; - if (entry.encoding == ZIP_ENC_RAW) { + if (ZIP_IS_STR(entry.encoding)) { fwrite(p,entry.len,1,stdout); } else { printf("%lld", (long long) zipLoadInteger(p,entry.encoding));