forked from Shiloh/githaven
68 lines
2.5 KiB
Go
68 lines
2.5 KiB
Go
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// Copyright (c) 2019 Couchbase, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package zap
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import (
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"fmt"
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)
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// LegacyChunkMode was the original chunk mode (always chunk size 1024)
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// this mode is still used for chunking doc values.
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var LegacyChunkMode uint32 = 1024
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// DefaultChunkMode is the most recent improvement to chunking and should
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// be used by default.
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var DefaultChunkMode uint32 = 1026
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func getChunkSize(chunkMode uint32, cardinality uint64, maxDocs uint64) (uint64, error) {
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switch {
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// any chunkMode <= 1024 will always chunk with chunkSize=chunkMode
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case chunkMode <= 1024:
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// legacy chunk size
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return uint64(chunkMode), nil
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case chunkMode == 1025:
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// attempt at simple improvement
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// theory - the point of chunking is to put a bound on the maximum number of
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// calls to Next() needed to find a random document. ie, you should be able
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// to do one jump to the correct chunk, and then walk through at most
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// chunk-size items
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// previously 1024 was chosen as the chunk size, but this is particularly
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// wasteful for low cardinality terms. the observation is that if there
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// are less than 1024 items, why not put them all in one chunk,
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// this way you'll still achieve the same goal of visiting at most
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// chunk-size items.
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// no attempt is made to tweak any other case
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if cardinality <= 1024 {
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return maxDocs, nil
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}
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return 1024, nil
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case chunkMode == 1026:
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// improve upon the ideas tested in chunkMode 1025
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// the observation that the fewest number of dense chunks is the most
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// desirable layout, given the built-in assumptions of chunking
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// (that we want to put an upper-bound on the number of items you must
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// walk over without skipping, currently tuned to 1024)
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//
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// 1. compute the number of chunks needed (max 1024/chunk)
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// 2. convert to chunkSize, dividing into maxDocs
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numChunks := (cardinality / 1024) + 1
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chunkSize := maxDocs / numChunks
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return chunkSize, nil
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}
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return 0, fmt.Errorf("unknown chunk mode %d", chunkMode)
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}
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