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Add updated HeadTime charts

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Martin Sumner 2017-11-29 14:31:07 +00:00
parent 41c308c5fd
commit ad36578434
3 changed files with 10 additions and 0 deletions
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docs/LATENCY.md
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@ -38,6 +38,14 @@ The time for Head requests is broken down in two ways: the time based on the lev
The timings for the merge tree are surprisingly high for missing entries (which largely should be covered by bloom filter checks), and there is a surprisingly high gap between the timings for different levels (again given the expected speed of the bloom checks).
This has been improved, first by changing the bloom filter and passing the bloom so that it can be accessed directly without a message pass from the penciller:
![](pics/28Nov_HeadTimeChart.png)
A second improvement was to include a small (32) cache of recently fetched keys in each SST process to be checked before the expensive slot_fetch:
![](pics/29Nov_HeadTimeChart.png)
Within the SST file, the timings are broken down between index_query (the time taken to find the slot in the tree), tiny_bloom (the time taken to check against the bloom filter for that slot), slot_index (the time taken to look up the position in the slot - which may also double as a bloom filter to determine non_presence) and slot_fetch (the time taken to fetch the block from disk and perform binary_to_term and then walk to the position).
![](pics/23Nov_SSTFetchTimeSplit.png)
@ -52,4 +60,6 @@ This can also be broken down for each individual SST file request (note that mos
![](pics/23Nov_SSTFetchCountSplit.png)
80% of file requests exit after the bloom check. Very few false positives are discovered after the bloom check in the slot_index check, so 20% of fetch requests to files result in a slot_fetch.
## Merge Time