Github user cloud-fan commented on the pull request:
https://github.com/apache/spark/pull/791#issuecomment-43738827
@mridulm I checked the code of BlockManager#doPut.
val putBlockInfo = {
val tinfo = new BlockInfo(level, tellMaster)
// Do atomically !
val oldBlockOpt = blockInfo.putIfAbsent(blockId, tinfo)
if (oldBlockOpt.isDefined) {
if (oldBlockOpt.get.waitForReady()) {
logWarning("Block " + blockId + " already exists on this machine;
not re-adding it")
return updatedBlocks
}
// TODO: So the block info exists - but previous attempt to load it
(?) failed.
// What do we do now ? Retry on it ?
oldBlockOpt.get
} else {
tinfo
}
}
BlockManger will create a BlockInfo for the block to be added, and `val
oldBlockOpt = blockInfo.putIfAbsent(blockId, tinfo)`, so if multi threads are
adding same block, one thread will put the BlockInfo successfully and the
other will fail and stop to put.
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