94c21e5c05
Single-row reads from large partition issue 64 KiB reads to the data file, which is equal to the default span of the promoted index block in the data file. If users would want to increase selectivity of the index to speed up single-row reads, this won't be effective. The reason is that the reader uses promoted index to look up the start position in the data file of the read, but end position will in practice extend to the next partition, and amount of I/O will be determined by the underlying file input stream implementation and its read-ahead heuristics. By default, that results in at least 2 IOs 32KB each. There is already infrastructure to lookup end position based on upper bound of the read, in anticipation for sharing the promoted index cache, but it's not effective becasue it's a non-populating lookup and the upper bound cursor has its own private cached_promoted_index, which is cold when positions are computed. It's non-populating on purpose, to avoid extra index file IO to read upper bound. In case upper bound is far-enough from the lower bound, this will only increase the cost of the read. The solution employed here is to warm up the lower bound cursor's cache before positions are computed, and use that cursor for non-populating lookup of the upper bound. We use the lower bound cursor and the slice's lower bound so that we read the same blocks as later lower-bound slicing would, so that we don't incur extra IO for cases where looking up upper bound is not worth it, that is when upper bound is far from the lower bound. If upper bound is near lower bound, then warming up using lower bound will populate cached_promoted_index with blocks which will allow us to locate the upper bound block accurately. This is especially important for single-row reads, where the bounds are around the same key. In this case we want to read the data file range which belongs to a single promoted index block. It doesn't matter that the upper bound is not exactly the same. They both will likely lie in the same block, and if not, binary search will bring adjacent blocks into cache. Even if upper bound is not near, the binary search will populate the cache with blocks which can be used to narrow down the data file range somewhat. Fixes #10030. The change was tested with perf-fast-forward. I populated the data set with `column_index_size_in_kb` set to 1 scylla perf-fast-forward --populate --run-tests=large-partition-slicing --column-index-size-in-kb=1 Test run: build/release/scylla perf-fast-forward --run-tests=large-partition-select-few-rows -c1 --keep-cache-across-test-cases --test-case-duration=0 This test issues two reads of subsequent keys from the middle of a large partition (1M rows in total). The first read will miss in the index file page cache, the second read will hit. Notice that before the change, the second read issued 2 aio requests worth of 64KiB in total. After the change, the second read issued 1 aio worth of 2 KiB. That's because promoted index block is larger than 1 KiB. I verified using logging that the data file range matches a single promoted index block. Also, the first read which misses in cache is still faster after the change. Before: ``` running: large-partition-select-few-rows on dataset large-part-ds1 Testing selecting few rows from a large partition: stride rows time (s) iterations frags frag/s mad f/s max f/s min f/s avg aio aio (KiB) blocked dropped idx hit idx miss idx blk c hit c miss c blk allocs tasks insns/f cpu 500000 1 0.009802 1 1 102 0 102 102 21.0 21 196 2 1 0 1 1 0 0 0 568 269 4716050 53.4% 500001 1 0.000321 1 1 3113 0 3113 3113 2.0 2 64 1 0 1 0 0 0 0 0 116 26 555110 45.0% ``` After: ``` running: large-partition-select-few-rows on dataset large-part-ds1 Testing selecting few rows from a large partition: stride rows time (s) iterations frags frag/s mad f/s max f/s min f/s avg aio aio (KiB) blocked dropped idx hit idx miss idx blk c hit c miss c blk allocs tasks insns/f cpu 500000 1 0.009609 1 1 104 0 104 104 20.0 20 137 2 1 0 1 1 0 0 0 561 268 4633407 43.1% 500001 1 0.000217 1 1 4602 0 4602 4602 1.0 1 2 1 0 1 0 0 0 0 0 110 26 313882 64.1% ``` Backports: none, not a regression Closes scylladb/scylladb#20522 * github.com:scylladb/scylladb: perf: perf_fast_forward: Add test case for querying missing rows perf-fast-forward: Allow overriding promoted index block size perf-fast-forward: Test subsequent key reads from the middle in test_large_partition_select_few_rows perf-fast-forward: Allow adding key offset in test_large_partition_select_few_rows perf-fast-forward: Use single-partition reads in test_large_partition_select_few_rows sstables: bsearch_clustered_cursor: Add more tracing points sstables: reader: Log data file range sstables: bsearch_clustered_cursor: Unify skip_info logging sstables: bsearch_clustered_cursor: Narrow down range using "end" position of the block sstables: bsearch_clustered_cursor: Skip even to the first block test: sstables: sstable_3_x_test: Improve failure message sstables: mx: writer: Never include partition_end marker in promoted index block width sstables: Reduce amount of I/O for clustering-key-bounded reads from large partitions sstables: clustered_cursor: Track current block
Scylla developer documentation
This folder contains developer-oriented documentation concerning the ScyllaDB codebase. We also have a wiki, which contains additional developer-oriented documentation. There is currently no clear definition of what goes where, so when looking for something be sure to check both.
Seastar documentation can be found here.
User documentation can be found on docs.scylladb.com
For information on how to build Scylla and how to contribute visit HACKING.md and CONTRIBUTING.md.