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Fix _dd.p.ksr formatting to use 6 decimal places #11068

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Merged
gh-worker-dd-mergequeue-cf854d merged 1 commit into from
Apr 9, 2026
Merged

Fix _dd.p.ksr formatting to use 6 decimal places #11068

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12 changes: 7 additions & 5 deletions ...e/src/jmh/java/datadog/trace/core/propagation/ptags/KnuthSamplingRateFormatBenchmark.java
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Original file line number Diff line number Diff line change
Expand Up @@ -22,8 +22,8 @@
/**
* Benchmarks for formatting the Knuth sampling rate (_dd.p.ksr tag value).
*
* <p>The format requirement is %.6g semantics: 6 significant figures, no trailing zeros, using
* fixed notation for values in [1e-4, 1] and scientific notation for smaller values.
* <p>The format requirement is 6 decimal digits of precision, no trailing zeros (fixed notation
* only; values below 0.0000005 round to "0").
*
* <p>Run with:
*
Expand Down Expand Up @@ -94,10 +94,12 @@ public void updateRateFreshTrace(Blackhole bh) {
bh.consume(ptags.getKnuthSamplingRateTagValue());
}

// ---- old implementation for comparison ----
// ---- old implementation for comparison (%.6f with trailing zero removal) ----

static String stringFormatImpl(double rate) {
String formatted = String.format(Locale.ROOT, "%.6g", rate);
if (rate <= 0.0) return "0";
if (rate >= 1.0) return "1";
String formatted = String.format(Locale.ROOT, "%.6f", rate);
int dotIndex = formatted.indexOf('.');
if (dotIndex >= 0) {
int end = formatted.length();
Expand All @@ -109,6 +111,6 @@ static String stringFormatImpl(double rate) {
}
formatted = formatted.substring(0, end);
}
return formatted;
return "0".equals(formatted) ? "0" : formatted;
}
}
115 changes: 10 additions & 105 deletions dd-trace-core/src/main/java/datadog/trace/core/propagation/ptags/PTagsFactory.java
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Expand Up @@ -298,57 +298,28 @@ public void updateKnuthSamplingRate(double rate) {
}

/**
* Formats a sampling rate with up to 6 significant digits and no trailing zeros, matching
* {@code %.6g} semantics (fixed notation for values in [1e-4, 1], scientific for smaller).
* Formats a sampling rate with up to 6 decimal digits of precision and no trailing zeros.
*
* <p>Values below 0.0000005 (which round to zero at 6 decimal places) return {@code "0"}.
* Values at or above 0.9999995 return {@code "1"}.
*
* <p>Uses char-array arithmetic to avoid {@link java.util.Formatter} allocations entirely.
*/
static String formatKnuthSamplingRate(double rate) {
if (rate <= 0.0) return "0";
if (rate >= 1.0) return "1";

if (rate < 1e-4) {
return formatScientific6g(rate);
}

return formatFixed6g(rate);
}

/** Fixed notation for rates in [1e-4, 1): "0.DDDDDDDDD" with trailing zeros trimmed. */
private static String formatFixed6g(double rate) {
// Choose a multiplier so Math.round(rate * multiplier) is a 6-significant-figure integer.
// For rate in [10^-k, 10^-(k-1)) the first sig fig is at decimal position k, so we need
// k+5 total fractional digits:
// [0.1, 1.0) -> scale=6, multiplier=1e6
// [0.01, 0.1) -> scale=7, multiplier=1e7
// [0.001, 0.01) -> scale=8, multiplier=1e8
// [1e-4, 0.001) -> scale=9, multiplier=1e9
final int scale;
final long multiplier;
if (rate >= 0.1) {
scale = 6;
multiplier = 1_000_000L;
} else if (rate >= 0.01) {
scale = 7;
multiplier = 10_000_000L;
} else if (rate >= 0.001) {
scale = 8;
multiplier = 100_000_000L;
} else {
scale = 9;
multiplier = 1_000_000_000L;
}

long rounded = Math.round(rate * multiplier);
// Round to 6 decimal places.
long rounded = Math.round(rate * 1_000_000L);
if (rounded == 0) return "0";
if (rounded >= multiplier) return "1"; // rounding pushed value to 1.0
if (rounded >= 1_000_000L) return "1";

// Build "0." + <scale digits> and trim trailing zeros in a single right-to-left pass.
char[] buf = new char[2 + scale];
// Build "0.DDDDDD" and trim trailing zeros in a single right-to-left pass.
char[] buf = new char[8]; // "0." + 6 digits
buf[0] = '0';
buf[1] = '.';
int end = 2; // exclusive end; updated on first non-zero digit found from the right
for (int i = 2 + scale - 1; i >= 2; i--) {
for (int i = 7; i >= 2; i--) {
int d = (int) (rounded % 10);
rounded /= 10;
buf[i] = (char) ('0' + d);
Expand All @@ -360,72 +331,6 @@ private static String formatFixed6g(double rate) {
return new String(buf, 0, end);
}

/** Scientific notation for rates below 1e-4: "X.XXXXXe-YY" with mantissa zeros trimmed. */
private static String formatScientific6g(double rate) {
// Normalize to [1, 10) by repeated multiply — at most ~15 iterations for realistic rates.
int exp = 0;
double normalized = rate;
while (normalized < 1.0) {
normalized *= 10;
exp--;
}

// Round mantissa to 6 significant figures (integer in [100000, 999999]).
long sig = Math.round(normalized * 100000.0);
if (sig >= 1000000) {
sig /= 10;
exp++;
if (exp >= -4) {
// Rounding pushed the value into fixed-notation range (always exactly 0.0001).
return "0.0001";
}
}

// Build "X.XXXXXe-YY" trimming mantissa trailing zeros.
// Max: "X.XXXXXe-XXX" = 13 chars.
char[] buf = new char[13];
int pos = 0;

// Integer part (always a single digit 1-9).
buf[pos++] = (char) ('0' + (int) (sig / 100000));
sig %= 100000;

// Fractional part (5 digits, trim trailing zeros).
if (sig > 0) {
buf[pos++] = '.';
char[] frac = new char[5];
int fracEnd = 0;
for (int i = 4; i >= 0; i--) {
frac[i] = (char) ('0' + (int) (sig % 10));
sig /= 10;
if (frac[i] != '0' && fracEnd == 0) {
fracEnd = i + 1;
}
}
for (int i = 0; i < fracEnd; i++) {
buf[pos++] = frac[i];
}
}

// Exponent: "e-YY" (always negative here, at least 2 digits).
buf[pos++] = 'e';
buf[pos++] = '-';
int absExp = -exp;
if (absExp < 10) {
buf[pos++] = '0';
buf[pos++] = (char) ('0' + absExp);
} else if (absExp < 100) {
buf[pos++] = (char) ('0' + absExp / 10);
buf[pos++] = (char) ('0' + absExp % 10);
} else {
buf[pos++] = (char) ('0' + absExp / 100);
buf[pos++] = (char) ('0' + (absExp / 10) % 10);
buf[pos++] = (char) ('0' + absExp % 10);
}

return new String(buf, 0, pos);
}

TagValue getKnuthSamplingRateTagValue() {
return knuthSamplingRateTagValue;
}
Expand Down
31 changes: 18 additions & 13 deletions dd-trace-core/src/test/groovy/datadog/trace/core/KnuthSamplingRateTest.groovy
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Original file line number Diff line number Diff line change
Expand Up @@ -39,25 +39,30 @@ class KnuthSamplingRateTest extends DDCoreSpecification {
0.123456d | "0.123456"
0.100000d | "0.1"
0.250d | "0.25"
// [0.01, 0.1) magnitude bucket (scale=7)
0.05d | "0.05"
0.0123456789d | "0.0123457"
// [0.001, 0.01) magnitude bucket (scale=8)
// 6 decimal places: round(0.0123456789 * 1e6) = round(12345.6789) = 12346
0.0123456789d | "0.012346"
0.001d | "0.001"
0.00500d | "0.005"
0.00123456789d | "0.00123457"
// [0.0001, 0.001) magnitude bucket (scale=9)
// 6 decimal places: round(0.00123456789 * 1e6) = round(1234.56789) = 1235
0.00123456789d | "0.001235"
0.0001d | "0.0001"
0.000500d | "0.0005"
0.000123456789d | "0.000123457"
// rounding boundary: 0.9999995 rounds up to 1.0
// 6 decimal places: round(0.000123456789 * 1e6) = round(123.456789) = 123
0.000123456789d | "0.000123"
// rounding boundary: round(0.9999995 * 1e6) = round(999999.5) = 1000000 >= 1e6 -> "1"
0.9999995d | "1"
// scientific notation (rate < 1e-4)
0.00001d | "1e-05"
0.000050d | "5e-05"
1.23456789e-5d | "1.23457e-05"
1e-7d | "1e-07"
5.5e-10d | "5.5e-10"
// values in (0, 1e-4): fixed 6 decimal places, no scientific notation
0.00001d | "0.00001"
0.000050d | "0.00005"
// round(1.23456789e-5 * 1e6) = round(12.3456789) = 12
1.23456789e-5d | "0.000012"
// below 6-decimal-place precision: round to 0
1e-7d | "0"
5.5e-10d | "0"
// system-tests Test_Knuth_Sample_Rate boundary cases
0.000001d | "0.000001" // six_decimal_precision_boundary
0.00000051d | "0.000001" // rounds_up_to_one_millionth
}

def "agent rate sampler sets ksr propagated tag"() {
Expand Down
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