AHeise commented on a change in pull request #10358: [FLINK-14346]
[serialization] faster implementation of StringValue writeString and readString
URL: https://github.com/apache/flink/pull/10358#discussion_r353372023
##########
File path: flink-core/src/main/java/org/apache/flink/types/StringValue.java
##########
@@ -759,56 +761,142 @@ public static String readString(DataInput in) throws
IOException {
}
len |= curr << shift;
}
-
+
// subtract one for the null length
len -= 1;
-
- final char[] data = new char[len];
- for (int i = 0; i < len; i++) {
- int c = in.readUnsignedByte();
- if (c < HIGH_BIT) {
- data[i] = (char) c;
- } else {
+ /* as we have no idea about byte-length of the serialized
string, we cannot fully
+ * read it into memory buffer. But we can do it in an
optimistic way:
+ * 1. In a happy case when the string is an us-ascii one, then
byte_len == char_len
+ * 2. If we spot at least one character with code >= 127, then
we reallocate the buffer
+ * to accommodate for the next characters.
+ */
+
+ // happily assume that the string is an 7 bit us-ascii one
+ byte[] buf = new byte[len];
+ in.readFully(buf);
+
+ final char[] data = new char[len];
+ int charPosition = 0;
+ int bufSize = len;
+ int bytePosition = 0;
+
+ while (charPosition < len) {
+ // there is at least `char count - char position` bytes
left in case if all the
+ // remaining characters are 7 bit.
+ int remainingBytesEstimation = len - charPosition;
+ int c;
+ if (bytePosition == bufSize) {
+ // need to expand the buffer as we are already
reached the end.
+ // we also reuse the old buffer, as it's
capacity must be enough in any case.
+ in.readFully(buf, 0, remainingBytesEstimation);
+ bytePosition = 0;
+ bufSize = remainingBytesEstimation;
+ }
+ c = buf[bytePosition++] & 255;
+ // non 7-bit path
+ if (c >= HIGH_BIT) {
int shift = 7;
int curr;
c = c & 0x7f;
- while ((curr = in.readUnsignedByte()) >=
HIGH_BIT) {
+ if (bytePosition == bufSize) {
+ // need to expand the buffer, also
reusing the buffer again.
+ in.readFully(buf, 0,
remainingBytesEstimation);
+ bytePosition = 0;
+ bufSize = remainingBytesEstimation;
+ }
+
+ while ((curr = buf[bytePosition++] & 255) >=
HIGH_BIT) {
c |= (curr & 0x7f) << shift;
shift += 7;
+ if (bytePosition == bufSize) {
+ // may need to expand the
buffer if char bytes are split between the buffers.
+ in.readFully(buf, 0,
remainingBytesEstimation);
+ bytePosition = 0;
+ bufSize =
remainingBytesEstimation;
+ }
}
c |= curr << shift;
- data[i] = (char) c;
}
+ data[charPosition++] = (char) c;
}
-
return new String(data, 0, len);
}
public static final void writeString(CharSequence cs, DataOutput out)
throws IOException {
if (cs != null) {
// the length we write is offset by one, because a
length of zero indicates a null value
- int lenToWrite = cs.length()+1;
+ int position = 0;
+ int strlen = cs.length();
+
+ int lenToWrite = strlen + 1;
if (lenToWrite < 0) {
throw new
IllegalArgumentException("CharSequence is too long.");
}
-
- // write the length, variable-length encoded
- while (lenToWrite >= HIGH_BIT) {
- out.write(lenToWrite | HIGH_BIT);
- lenToWrite >>>= 7;
- }
- out.write(lenToWrite);
-
- // write the char data, variable length encoded
- for (int i = 0; i < cs.length(); i++) {
- int c = cs.charAt(i);
-
- while (c >= HIGH_BIT) {
- out.write(c | HIGH_BIT);
- c >>>= 7;
+
+ // on average, for strings shorter than 6 characters
the cost of allocating the buffer
+ // is higher than writing it directly. See benchmarks
in https://github.com/apache/flink/pull/10358 for
+ // examples.
+ if (strlen < 6) {
+ // write the length, variable-length encoded
+ while (lenToWrite >= HIGH_BIT) {
Review comment:
Would loop unrolling also help here? Could be extracted as a private method
which JIT can inline.
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