Details
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Bug
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Resolution: Fixed
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Major
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6.5.0
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None
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Triaged
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No
Description
Summary
The checkpoint expelling code added in Mad-Hatter can fail to expel an item from a Checkpoint if there's only 1 candidate item to expel.
This can result in increased memory usage (see MB-37294), which is more significant for large Documents / small memory quotas.
(Separated out from MB-37294.)
Steps to Reproduce
Difficult to precisely trigger this scenario in full-stack tests, however you want to attempt to end up with a Checkpoint as follows, and then have expelling be attempted:
CheckpointManager[0x105dfa740] with numItems:3 checkpoints:1
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Checkpoint[0x105f3b000] with id:1 seqno:{1001,1002} snap:{0,1002, visible:1002} state:CHECKPOINT_OPEN numCursors:1 type:Memory hcs:-- items:[
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{1001,empty,cid:0x1:empty,110,[m]}
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{1001,checkpoint_start,cid:0x1:checkpoint_start,121,[m]}
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{1001,mutation,cid:0x0:key0,109,}
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{1002,mutation,cid:0x0:key1,109,}
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]
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cursors:[
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persistence: CheckpointCursor[0x105c5d998] with name:persistence currentCkpt:{id:1 state:CHECKPOINT_OPEN} currentPos:1002
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]
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- Two adjacent mutations (at seqnos 1001 and 1002).
- The the first mutation shares its seqno with the remainder of the items in the checkpoint (here with meta-items checkpoint_start & empty).
Note that checkpoint state can be examined via:
cbstats localhost:11210 -u Administrator -p <PASSWORD> raw "_checkpoint-dump <VBID>
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Any set regular workload under DGM (e.g. pillowfight) should trigger this.
Expected Results
Such checkpoints should allow all but the last item (1002) to be expelled:
CheckpointManager[0x105efb740] with numItems:3 checkpoints:1
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Checkpoint[0x10803c000] with id:1 seqno:{1002,1002} snap:{0,1002, visible:1002} state:CHECKPOINT_OPEN numCursors:1 type:Memory hcs:-- items:[
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{1001,empty,cid:0x1:empty,110,[m]}
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{1002,mutation,cid:0x0:key1,109,}
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]
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cursors:[
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persistence: CheckpointCursor[0x105c5e998] with name:persistence currentCkpt:{id:1 state:CHECKPOINT_OPEN} currentSeq:1002 distance:1
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]
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Actual Results
No items are expelled.
