Improving performance and reducing object allocations of java.util.UUID to/from string
Steven Schlansker
stevenschlansker at gmail.com
Sat Dec 29 17:25:06 UTC 2012
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 runtime
JdkUuidFromString 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 runtime
JdkUuidFromString 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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