Improving performance and reducing object allocations of java.util.UUID to/from string (original) (raw)
Steven Schlansker stevenschlansker at gmail.com
Sat Dec 29 17:25:06 UTC 2012
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Hello core-libs-dev,
My company uses UUIDs throughout our software. We recently identified that the java.util.UUID implementations of fromString and toString are horridly inefficient.
An incomplete list of the inefficiencies:
fromString uses a regular expression that is not even precompiled
fromString uses a regular expression to parse out the "-" markers, requiring the allocation of many String and String[] objects, when a simple linear scan works just fine
fromString unnecessarily allocates multiple Long objects
toString creates a char[64], a String object which copies that, and a sub-String object for each of the 5 hex digit segments
toString produces a fixed-length result but involves multiple StringBuilder concatenations
Everyone that I have shown the relevant code to has been horrified by the complete lack of care towards object allocations. While I am a big fan of the "object allocation is free until otherwise proven" philosophy, we traced multiple production problems down to these hotspots.
But instead of just whining about it, I wish to contribute a patch which I believe fixes the problem. This is my first attempt to submit anything to the JDK, so please be nice :-)
My initial attempt has yielded micro benchmarks with very favorable outcomes. By taking the appropriate code from java.lang.Long, modifying it to work on a single pre-allocated array+offset rather than returning a String, and scanning for dashes instead of regex splitting I reduced times 3-6x and object allocations to the low single digits.
My Google Caliper benchmark is available here, to ensure that I have not committed any benchmark sins: https://gist.github.com/4356621
benchmark instances Bytes ns linear runtimeJdkUuidFromString 51.00 1544.0 608.2 ============================== NewUuidFromString 2.00 72.0 179.1 ========
JdkUuidToString 31.00 1720.0 321.4 =============== NewUuidToString 3.00 200.0 51.5 ==
In particular, the reduction of object allocations from ~1.5KB to 72/200 bytes dramatically reduces GC pressure when you sit in a tight loop deserializing millions of UUIDs from strings.
I believe the same patch can (and should?) apply to jdk7u and jdk8, as the code does not seem to have changed.
I would appreciate any feedback on the code style, efficiency, or correctness that you can offer. I have run the "java/util/UUID" jtreg suite against this and it passes. We stole the more thorough Apache Harmony tests for this and they pass as well: https://github.com/stevenschlansker/components-ness-core/blob/master/src/test/java/com/nesscomputing/uuid/TestNessUUID.java
I have not yet secured a CLA, but my company is in the process of signing one.
The rest of the message is a "hg export" of my change set, which is current to the tip of jdk7. Happy holidays, and thank you for your time! Steven Schlansker
HG changeset patch
User Steven Schlansker <steven at nesscomputing.com>
Date 1356737383 0
Node ID a812c963b96f08112f6805cbc380b12af196f788
Parent 9b8c96f96a0f9a5801b55530a387fefbe50482a3
java.util.UUID: improve performance of UUID#toString and UUID#fromString by avoiding many unnecessary object allocations.
benchmark instances Bytes ns linear runtimeJdkUuidFromString 51.00 1544.0 608.2 ============================== NewUuidFromString 2.00 72.0 179.1 ========
JdkUuidToString 31.00 1720.0 321.4 =============== NewUuidToString 3.00 200.0 51.5 ==
diff -r 9b8c96f96a0f -r a812c963b96f src/share/classes/java/util/UUID.java --- a/src/share/classes/java/util/UUID.java Mon Jun 27 13:21:34 2011 -0700 +++ b/src/share/classes/java/util/UUID.java Fri Dec 28 23:29:43 2012 +0000 @@ -189,26 +189,79 @@ * described in {@link #toString} * */
- public static UUID fromString(String name) {
String[] components = name.split("-");if (components.length != 5)throw new IllegalArgumentException("Invalid UUID string: "+name);for (int i=0; i<5; i++)components[i] = "0x"+components[i];
- public static UUID fromString(String str) {
int dashCount = 4;int [] dashPos = new int [6];dashPos[0] = -1;dashPos[5] = str.length();
long mostSigBits = Long.decode(components[0]).longValue();
for (int i = str.length()-1; i >= 0; i--) {if (str.charAt(i) == '-') {if (dashCount == 0) {throw new IllegalArgumentException("Invalid UUID string: " + str);}dashPos[dashCount--] = i;}}if (dashCount > 0) {throw new IllegalArgumentException("Invalid UUID string: " + str);}long mostSigBits = decode(str, dashPos, 0) & 0xffffffffL; mostSigBits <<= 16;
mostSigBits |= Long.decode(components[1]).longValue();
mostSigBits |= decode(str, dashPos, 1) & 0xffffL; mostSigBits <<= 16;
mostSigBits |= Long.decode(components[2]).longValue();
mostSigBits |= decode(str, dashPos, 2) & 0xffff);
long leastSigBits = Long.decode(components[3]).longValue();
long leastSigBits = decode(str, dashPos, 3) & 0xffffL; leastSigBits <<= 48;
leastSigBits |= Long.decode(components[4]).longValue();
leastSigBits |= decode(str, dashPos, 4) & 0xffffffffffffL; return new UUID(mostSigBits, leastSigBits);}
private static long decode(final String str, final int [] dashPos, final int field) {
int start = dashPos[field]+1;int end = dashPos[field+1];if (start >= end) {throw new IllegalArgumentException("Invalid UUID string: " + str);}long curr = 0;for (int i = start; i < end; i++) {int x = getNibbleFromChar(str.charAt(i));curr <<= 4;if (curr < 0) {throw new NumberFormatException("long overflow");}curr |= x;}return curr;}
private static final int NUM_ALPHA_DIFF = 'A' - '9' - 1;
private static final int LOWER_UPPER_DIFF = 'a' - 'A';
private static int getNibbleFromChar(final char c)
{
int x = c - '0';if (x > 9) {x -= NUM_ALPHA_DIFF; // difference between '9' and 'A'if (x > 15) {x -= LOWER_UPPER_DIFF; // difference between 'a' and 'A'}if (x < 10) {throw new IllegalArgumentException(c + " is not a valid character for an UUID string");}}if (x < 0 || x > 15) {throw new IllegalArgumentException(c + " is not a valid character for an UUID string");}return x;}
// Field Accessor Methods
/**
@@ -372,19 +425,46 @@ * @return A string representation of this {@code UUID} */ public String toString() {
return (digits(mostSigBits >> 32, 8) + "-" +digits(mostSigBits >> 16, 4) + "-" +digits(mostSigBits, 4) + "-" +digits(leastSigBits >> 48, 4) + "-" +digits(leastSigBits, 12));
}return toString(getMostSignificantBits(), getLeastSignificantBits());
- /** Returns val represented by the specified number of hex digits. */
- private static String digits(long val, int digits) {
long hi = 1L << (digits * 4);return Long.toHexString(hi | (val & (hi - 1))).substring(1);
public static String toString(long msb, long lsb) {
char[] uuidChars = new char[36];hexDigits(uuidChars, 0, 8, msb >> 32);uuidChars[8] = '-';hexDigits(uuidChars, 9, 4, msb >> 16);uuidChars[13] = '-';hexDigits(uuidChars, 14, 4, msb);uuidChars[18] = '-';hexDigits(uuidChars, 19, 4, lsb >> 48);uuidChars[23] = '-';hexDigits(uuidChars, 24, 12, lsb);return new String(uuidChars);}
private static void hexDigits(char[] dest, int offset, int digits, long val) {
long hi = 1L << (digits * 4);toUnsignedString(dest, offset, digits, hi | (val & (hi - 1)), 4);}
private final static char[] HEX_DIGITS = {
'0' , '1' , '2' , '3' , '4' , '5' ,'6' , '7' , '8' , '9' , 'a' , 'b' ,'c' , 'd' , 'e' , 'f'};
private static void toUnsignedString(char[] dest, int offset, int len, long i, int shift) {
int charPos = len;int radix = 1 << shift;long mask = radix - 1;do {dest[offset + --charPos] = HEX_DIGITS[(int)(i & mask)];i >>>= shift;} while (i != 0 && charPos > 0);}
/**- Returns a hash code for this {@code UUID}.
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