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GH-41: BaseVariableWidthViewVector setZero only if necessary #557

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GH-41: BaseVariableWidthViewVector setZero only if necessary #557

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BaseVariableWidthViewVector setZero only if necessary
ViggoC Jan 23, 2025
16c9e2f
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ViggoC Jan 24, 2025
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20 changes: 12 additions & 8 deletions vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthViewVector.java
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Original file line number Diff line number Diff line change
Expand Up @@ -1367,11 +1367,13 @@ protected ArrowBuf allocateOrGetLastDataBuffer(int length) {
protected final void setBytes(int index, byte[] value, int start, int length) {
int writePosition = index * ELEMENT_SIZE;

// to clear the memory segment of view being written to
// this is helpful in case of overwriting the value
viewBuffer.setZero(writePosition, ELEMENT_SIZE);

if (length <= INLINE_SIZE) {
// Check if the memory segment has been written, and clear it if it has been set.
// It is recommended to batch initialize the viewBuffer before setBytes.
if (viewBuffer.getLong(writePosition) != 0 || viewBuffer.getLong(writePosition + 8) != 0) {
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So the basic effect is that if the view buffer is all 0, we skip setting it back to 0? And basically, for freshly allocated memory, this skips a redundant memset? Can we explain it a little more in the comment?

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Yes, it's redundant to setZero if the data is already zero. Checking will introduce some overhead, but it is much smaller than that of setMemory.

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Update in 16c9e2f

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Is it safe to check only these positions? IIUC, ZERO is a valid value to be set.

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But if all 16 bytes are 0, there is no need to set them to 0 again, I think is the point.

viewBuffer.setZero(writePosition, ELEMENT_SIZE);
}

// allocate inline buffer
// set length
viewBuffer.setInt(writePosition, length);
Expand Down Expand Up @@ -1411,11 +1413,13 @@ protected final void setBytes(int index, byte[] value, int start, int length) {
protected final void setBytes(int index, ArrowBuf valueBuf, int start, int length) {
int writePosition = index * ELEMENT_SIZE;

// to clear the memory segment of view being written to
// this is helpful in case of overwriting the value
viewBuffer.setZero(writePosition, ELEMENT_SIZE);

if (length <= INLINE_SIZE) {
// Check if the memory segment has been written, and clear it if it has been set.
// It is recommended to batch initialize the viewBuffer before setBytes.
if (viewBuffer.getLong(writePosition) != 0 || viewBuffer.getLong(writePosition + 8) != 0) {
viewBuffer.setZero(writePosition, ELEMENT_SIZE);
}

// allocate inline buffer
// set length
viewBuffer.setInt(writePosition, length);
Expand Down
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