Details
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Improvement
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Resolution: Fixed
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Critical
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4.0.0
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Security Level: Public
Description
During investigation of an unrelated memcached bug (MB-15007) I noticed that of the fragmented jemalloc memory a significant amount of it (75%) was from the 80 byte size class:
small allocation stats:
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bin size (B) regions pages allocated allocated cur runs % of small % of blame
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per run per run items bytes utilization frag memory (B)
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0 8 512 1 2381756 19054048 4653 100% 0% 4640 0%
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1 16 256 1 2025 32400 116 7% 0% 442736 0%
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2 32 128 1 10857534 347441088 90002 94% 4% 21207104 3%
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3 48 256 3 305749 14675952 2742 44% 0% 19017744 3%
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4 64 64 1 64371 4119744 1632 62% 0% 2564928 0%
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5 80 256 5 11102416 888193280 67517 64% 11% 494554880 75%
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6 96 128 3 5378 516288 62 68% 0% 245568 0%
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7 112 256 7 494100 55339200 3135 62% 1% 34547520 5%
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utilization = allocated / (size * regions_per_run * cur runs)
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% of small = allocated / total allocated
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frag memory = (size * regions_per_run * cur runs) - allocated
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% of blame = frag memory / total frag memory
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I don't definitively know for sure, but I hypothesise that this is the size class being used for StoredValue objects. Currently the defragmenter only reallocates the Blob part of a document; but there may be value in additionally reallocing the StoredValue (i.e. the document key and metadata).
This would likely help cases where a large number of documents with a given key size are deleted, and then new documents are allocated with a different key size. In this case the original size class would not be able to be reused, and hence fragmentation could be high in that class.
My expectation would be that the magnitude of reclaimed memory would be much lower than say the existing Blob defragmentation as keys are typically significantly smaller than values. However for certain use-cases where values are of similar size to the key (and where key sizes vary) this could have noticeable benefit.
Attachments
Issue Links
- relates to
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MB-29879 memcached is OOM-killed during data ingestion due to high heap fragmentation
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- Closed
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