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[WIP] Partial bot skeleton, non-functional though

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eleksir
2024-02-09 19:40:54 +03:00
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vendor/github.com/cockroachdb/pebble/objstorage/readahead.go generated vendored Normal file
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// Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
package objstorage
const (
// Constants for dynamic readahead of data blocks. Note that the size values
// make sense as some multiple of the default block size; and they should
// both be larger than the default block size.
minFileReadsForReadahead = 2
// TODO(bilal): Have the initial size value be a factor of the block size,
// as opposed to a hardcoded value.
initialReadaheadSize = 64 << 10 /* 64KB */
maxReadaheadSize = 256 << 10 /* 256KB */
)
// readaheadState contains state variables related to readahead. Updated on
// file reads.
type readaheadState struct {
// Number of sequential reads.
numReads int64
// Size issued to the next call to Prefetch. Starts at or above
// initialReadaheadSize and grows exponentially until maxReadaheadSize.
size int64
// prevSize is the size used in the last Prefetch call.
prevSize int64
// The byte offset up to which the OS has been asked to read ahead / cached.
// When reading ahead, reads up to this limit should not incur an IO
// operation. Reads after this limit can benefit from a new call to
// Prefetch.
limit int64
}
func makeReadaheadState() readaheadState {
return readaheadState{
size: initialReadaheadSize,
}
}
func (rs *readaheadState) recordCacheHit(offset, blockLength int64) {
currentReadEnd := offset + blockLength
if rs.numReads >= minFileReadsForReadahead {
if currentReadEnd >= rs.limit && offset <= rs.limit+maxReadaheadSize {
// This is a read that would have resulted in a readahead, had it
// not been a cache hit.
rs.limit = currentReadEnd
return
}
if currentReadEnd < rs.limit-rs.prevSize || offset > rs.limit+maxReadaheadSize {
// We read too far away from rs.limit to benefit from readahead in
// any scenario. Reset all variables.
rs.numReads = 1
rs.limit = currentReadEnd
rs.size = initialReadaheadSize
rs.prevSize = 0
return
}
// Reads in the range [rs.limit - rs.prevSize, rs.limit] end up
// here. This is a read that is potentially benefitting from a past
// readahead.
return
}
if currentReadEnd >= rs.limit && offset <= rs.limit+maxReadaheadSize {
// Blocks are being read sequentially and would benefit from readahead
// down the line.
rs.numReads++
return
}
// We read too far ahead of the last read, or before it. This indicates
// a random read, where readahead is not desirable. Reset all variables.
rs.numReads = 1
rs.limit = currentReadEnd
rs.size = initialReadaheadSize
rs.prevSize = 0
}
// maybeReadahead updates state and determines whether to issue a readahead /
// prefetch call for a block read at offset for blockLength bytes.
// Returns a size value (greater than 0) that should be prefetched if readahead
// would be beneficial.
func (rs *readaheadState) maybeReadahead(offset, blockLength int64) int64 {
currentReadEnd := offset + blockLength
if rs.numReads >= minFileReadsForReadahead {
// The minimum threshold of sequential reads to justify reading ahead
// has been reached.
// There are two intervals: the interval being read:
// [offset, currentReadEnd]
// as well as the interval where a read would benefit from read ahead:
// [rs.limit, rs.limit + rs.size]
// We increase the latter interval to
// [rs.limit, rs.limit + maxReadaheadSize] to account for cases where
// readahead may not be beneficial with a small readahead size, but over
// time the readahead size would increase exponentially to make it
// beneficial.
if currentReadEnd >= rs.limit && offset <= rs.limit+maxReadaheadSize {
// We are doing a read in the interval ahead of
// the last readahead range. In the diagrams below, ++++ is the last
// readahead range, ==== is the range represented by
// [rs.limit, rs.limit + maxReadaheadSize], and ---- is the range
// being read.
//
// rs.limit rs.limit + maxReadaheadSize
// ++++++++++|===========================|
//
// |-------------|
// offset currentReadEnd
//
// This case is also possible, as are all cases with an overlap
// between [rs.limit, rs.limit + maxReadaheadSize] and [offset,
// currentReadEnd]:
//
// rs.limit rs.limit + maxReadaheadSize
// ++++++++++|===========================|
//
// |-------------|
// offset currentReadEnd
//
//
rs.numReads++
rs.limit = offset + rs.size
rs.prevSize = rs.size
// Increase rs.size for the next read.
rs.size *= 2
if rs.size > maxReadaheadSize {
rs.size = maxReadaheadSize
}
return rs.prevSize
}
if currentReadEnd < rs.limit-rs.prevSize || offset > rs.limit+maxReadaheadSize {
// The above conditional has rs.limit > rs.prevSize to confirm that
// rs.limit - rs.prevSize would not underflow.
// We read too far away from rs.limit to benefit from readahead in
// any scenario. Reset all variables.
// The case where we read too far ahead:
//
// (rs.limit - rs.prevSize) (rs.limit) (rs.limit + maxReadaheadSize)
// |+++++++++++++|=============|
//
// |-------------|
// offset currentReadEnd
//
// Or too far behind:
//
// (rs.limit - rs.prevSize) (rs.limit) (rs.limit + maxReadaheadSize)
// |+++++++++++++|=============|
//
// |-------------|
// offset currentReadEnd
//
rs.numReads = 1
rs.limit = currentReadEnd
rs.size = initialReadaheadSize
rs.prevSize = 0
return 0
}
// Reads in the range [rs.limit - rs.prevSize, rs.limit] end up
// here. This is a read that is potentially benefitting from a past
// readahead, but there's no reason to issue a readahead call at the
// moment.
//
// (rs.limit - rs.prevSize) (rs.limit + maxReadaheadSize)
// |+++++++++++++|===============|
// (rs.limit)
//
// |-------|
// offset currentReadEnd
//
rs.numReads++
return 0
}
if currentReadEnd >= rs.limit && offset <= rs.limit+maxReadaheadSize {
// Blocks are being read sequentially and would benefit from readahead
// down the line.
//
// (rs.limit) (rs.limit + maxReadaheadSize)
// |=============|
//
// |-------|
// offset currentReadEnd
//
rs.numReads++
return 0
}
// We read too far ahead of the last read, or before it. This indicates
// a random read, where readahead is not desirable. Reset all variables.
//
// (rs.limit - maxReadaheadSize) (rs.limit) (rs.limit + maxReadaheadSize)
// |+++++++++++++|=============|
//
// |-------|
// offset currentReadEnd
//
rs.numReads = 1
rs.limit = currentReadEnd
rs.size = initialReadaheadSize
rs.prevSize = 0
return 0
}