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