add comments #18
add comments #18
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pixels-cache/src/main/java/cn/edu/ruc/iir/pixels/cache/PixelsCacheReader.java
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| @@ -137,6 +137,7 @@ private PixelsCacheIdx search(ByteBuffer keyBuffer) | |||
| int bytesMatchedInNodeFound = 0; | |||
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| // get root | |||
| // TODO: does root node have an edge? | |||
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Yes. root node can also have an edge.
The first entry in the tree is stored in the root node.
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Yes. root node can also have an edge.
The first entry in the tree is stored in the root node.
Where is the root edge processed?
| indexFile.getBytes(currentNodeOffset + 4 + currentNodeChildrenNum * 8, | ||
| currentNodeEdge, 0, currentNodeEdgeSize); | ||
| dramAccessCounter++; | ||
| // TODO: numEdgeBytes seems redundant. |
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Yeah. It can be replaced by currentNodeEdgeSize.
| @@ -179,10 +180,13 @@ private PixelsCacheIdx search(ByteBuffer keyBuffer) | |||
| dramAccessCounter++; | |||
| currentNodeChildrenNum = currentNodeHeader & 0x000001FF; | |||
| currentNodeEdgeSize = (currentNodeHeader & 0x7FFFFE00) >>> 9; | |||
| // TODO: does max length of edge = 12? can we move currentNodeEdge allocation out before this loop? | |||
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The max length of edge is far larger than 12. It takes 22 bits for the size of edge in the node.
Node Header: isKey(1 bit) + edgeSize(22 bits) + childrenSize(9 bits).
I think we can move currentNodeEdge out of the loop and make it a buffer only allocated once, if we make it large enough.
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The max length of edge is far larger than 12. It takes 22 bits for the size of edge in the node.
Node Header: isKey(1 bit) + edgeSize(22 bits) + childrenSize(9 bits).
I think we can move
currentNodeEdgeout of the loop and make it a buffer only allocated once, if we make it large enough.
What is an edge? is it the key part to match on a node? 22bit -> 4MB edge size, it is so large.
Currently, we have search key = 8 byte block id + 2 byte row group id + 2 byte column id.
| byte[] currentNodeEdge = new byte[currentNodeEdgeSize]; | ||
| // TODO: can we get header, edge and children of a node in one memory access? |
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Currently, no, we can't do this.
Since the size of edge and children are all stored in header.
We cannot access edge and children without accessing the header first.
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Currently, no, we can't do this.
Since the size of edge and children are all stored in header.
We cannot access edge and children without accessing the header first.
I see. but can we merge the children and edge into one memory access?
Review PixelsCacheReader.