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1 : // Copyright 2022 The LevelDB-Go and Pebble Authors. All rights reserved. Use
2 : // of this source code is governed by a BSD-style license that can be found in
3 : // the LICENSE file.
4 :
5 : package base
6 :
7 : import "github.com/cockroachdb/pebble/internal/invariants"
8 :
9 : // A value can have user-defined attributes that are a function of the value
10 : // byte slice. For now, we only support "short attributes", which can be
11 : // encoded in 3 bits. We will likely extend this to "long attributes" later
12 : // for values that are even more expensive to access than those in value
13 : // blocks in the same sstable.
14 : //
15 : // When a sstable writer chooses not to store a value together with the key,
16 : // it can call the ShortAttributeExtractor to extract the attribute and store
17 : // it together with the key. This allows for cheap retrieval of
18 : // AttributeAndLen on the read-path, without doing a more expensive retrieval
19 : // of the value. In general, the extraction code may want to also look at the
20 : // key to decide how to treat the value, hence the key* parameters.
21 : //
22 : // Write path performance: The ShortAttributeExtractor func cannot be inlined,
23 : // so we will pay the cost of this function call. However, we will only pay
24 : // this when (a) the value is not being stored together with the key, and (b)
25 : // the key-value pair is being initially written to the DB, or a compaction is
26 : // transitioning the key-value pair from being stored together to being stored
27 : // separately.
28 :
29 : // ShortAttribute encodes a user-specified attribute of the value.
30 : type ShortAttribute uint8
31 :
32 : // MaxShortAttribute is the maximum value of the short attribute (3 bits).
33 : const MaxShortAttribute = 7
34 :
35 : // ShortAttributeExtractor is an extractor that given the value, will return
36 : // the ShortAttribute.
37 : type ShortAttributeExtractor func(
38 : key []byte, keyPrefixLen int, value []byte) (ShortAttribute, error)
39 :
40 : // AttributeAndLen represents the pair of value length and the short
41 : // attribute.
42 : type AttributeAndLen struct {
43 : ValueLen int32
44 : ShortAttribute ShortAttribute
45 : }
46 :
47 : // LazyValue represents a value that may not already have been extracted.
48 : // Currently, it can represent either an in-place value (stored with the key)
49 : // or a value stored in the value section. However, the interface is general
50 : // enough to support values that are stored in separate files.
51 : //
52 : // LazyValue is used in the InternalIterator interface, such that all
53 : // positioning calls return (*InternalKey, LazyValue). It is also exposed via
54 : // the public Iterator for callers that need to remember a recent but not
55 : // necessarily latest LazyValue, in case they need the actual value in the
56 : // future. An example is a caller that is iterating in reverse and looking for
57 : // the latest MVCC version for a key -- it cannot identify the latest MVCC
58 : // version without stepping to the previous key-value pair e.g.
59 : // storage.pebbleMVCCScanner in CockroachDB.
60 : //
61 : // Performance note: It is important for this struct to not exceed a sizeof 32
62 : // bytes, for optimizing the common case of the in-place value. Prior to
63 : // introducing LazyValue, we were passing around a []byte which is 24 bytes.
64 : // Passing a 40 byte or larger struct causes performance to drop by 75% on
65 : // some benchmarks that do tight iteration loops.
66 : //
67 : // Memory management:
68 : // This is subtle, but important for performance.
69 : //
70 : // A LazyValue returned by an InternalIterator or Iterator is unstable in that
71 : // repositioning the iterator will invalidate the memory inside it. A caller
72 : // wishing to maintain that LazyValue needs to call LazyValue.Clone(). Note
73 : // that this does not fetch the value if it is not in-place. Clone() should
74 : // ideally not be called if LazyValue.Value() has been called, since the
75 : // cloned LazyValue will forget the extracted/fetched value, and calling
76 : // Value() on this clone will cause the value to be extracted again. That is,
77 : // Clone() does not make any promise about the memory stability of the
78 : // underlying value.
79 : //
80 : // A user of an iterator that calls LazyValue.Value() wants as much as
81 : // possible for the returned value []byte to point to iterator owned memory.
82 : //
83 : // 1. [P1] The underlying iterator that owns that memory also needs a promise
84 : // from that user that at any time there is at most one value []byte slice
85 : // that the caller is expecting it to maintain. Otherwise, the underlying
86 : // iterator has to maintain multiple such []byte slices which results in
87 : // more complicated and inefficient code.
88 : //
89 : // 2. [P2] The underlying iterator, in order to make the promise that it is
90 : // maintaining the one value []byte slice, also needs a way to know when
91 : // it is relieved of that promise. One way it is relieved of that promise
92 : // is by being told that it is being repositioned. Typically, the owner of
93 : // the value []byte slice is a sstable iterator, and it will know that it
94 : // is relieved of the promise when it is repositioned. However, consider
95 : // the case where the caller has used LazyValue.Clone() and repositioned
96 : // the iterator (which is actually a tree of iterators). In this case the
97 : // underlying sstable iterator may not even be open. LazyValue.Value()
98 : // will still work (at a higher cost), but since the sstable iterator is
99 : // not open, it does not have a mechanism to know when the retrieved value
100 : // is no longer in use. We refer to this situation as "not satisfying P2".
101 : // To handle this situation, the LazyValue.Value() method accepts a caller
102 : // owned buffer, that the callee will use if needed. The callee explicitly
103 : // tells the caller whether the []byte slice for the value is now owned by
104 : // the caller. This will be true if the callee attempted to use buf and
105 : // either successfully used it or allocated a new []byte slice.
106 : //
107 : // To ground the above in reality, we consider three examples of callers of
108 : // LazyValue.Value():
109 : //
110 : // - Iterator: it calls LazyValue.Value for its own use when merging values.
111 : // When merging during reverse iteration, it may have cloned the LazyValue.
112 : // In this case it calls LazyValue.Value() on the cloned value, merges it,
113 : // and then calls LazyValue.Value() on the current iterator position and
114 : // merges it. So it is honoring P1.
115 : //
116 : // - Iterator on behalf of Iterator clients: The Iterator.Value() method
117 : // needs to call LazyValue.Value(). The client of Iterator is satisfying P1
118 : // because of the inherent Iterator interface constraint, i.e., it is calling
119 : // Iterator.Value() on the current Iterator position. It is possible that
120 : // the Iterator has cloned this LazyValue (for the reverse iteration case),
121 : // which the client is unaware of, so the underlying sstable iterator may
122 : // not be able to satisfy P2. This is ok because Iterator will call
123 : // LazyValue.Value with its (reusable) owned buffer.
124 : //
125 : // - CockroachDB's pebbleMVCCScanner: This will use LazyValues from Iterator
126 : // since during reverse iteration in order to find the highest version that
127 : // satisfies a read it needs to clone the LazyValue, step back the iterator
128 : // and then decide whether it needs the value from the previously cloned
129 : // LazyValue. The pebbleMVCCScanner will satisfy P1. The P2 story is
130 : // similar to the previous case in that it will call LazyValue.Value with
131 : // its (reusable) owned buffer.
132 : //
133 : // Corollary: callers that directly use InternalIterator can know that they
134 : // have done nothing to interfere with promise P2 can pass in a nil buf and be
135 : // sure that it will not trigger an allocation.
136 : //
137 : // Repeated calling of LazyValue.Value:
138 : // This is ok as long as the caller continues to satisfy P1. The previously
139 : // fetched value will be remembered inside LazyValue to avoid fetching again.
140 : // So if the caller's buffer is used the first time the value was fetched, it
141 : // is still in use.
142 : //
143 : // LazyValue fields are visible outside the package for use in
144 : // InternalIterator implementations and in Iterator, but not meant for direct
145 : // use by users of Pebble.
146 : type LazyValue struct {
147 : // ValueOrHandle represents a value, or a handle to be passed to ValueFetcher.
148 : // - Fetcher == nil: ValueOrHandle is a value.
149 : // - Fetcher != nil: ValueOrHandle is a handle and Fetcher.Attribute is
150 : // initialized.
151 : // The ValueOrHandle exposed by InternalIterator or Iterator may not be stable
152 : // if the iterator is stepped. To make it stable, make a copy using Clone.
153 : ValueOrHandle []byte
154 : // Fetcher provides support for fetching an actually lazy value.
155 : Fetcher *LazyFetcher
156 : }
157 :
158 : // LazyFetcher supports fetching a lazy value.
159 : //
160 : // Fetcher and Attribute are to be initialized at creation time. The fields
161 : // are arranged to reduce the sizeof this struct.
162 : type LazyFetcher struct {
163 : // Fetcher, given a handle, returns the value.
164 : Fetcher ValueFetcher
165 : err error
166 : value []byte
167 : // Attribute includes the short attribute and value length.
168 : Attribute AttributeAndLen
169 : fetched bool
170 : callerOwned bool
171 : }
172 :
173 : // ValueFetcher is an interface for fetching a value.
174 : type ValueFetcher interface {
175 : // Fetch returns the value, given the handle. It is acceptable to call the
176 : // ValueFetcher.Fetch as long as the DB is open. However, one should assume
177 : // there is a fast-path when the iterator tree has not moved off the sstable
178 : // iterator that initially provided this LazyValue. Hence, to utilize this
179 : // fast-path the caller should try to decide whether it needs the value or
180 : // not as soon as possible, with minimal possible stepping of the iterator.
181 : //
182 : // buf will be used if the fetcher cannot satisfy P2 (see earlier comment).
183 : // If the fetcher attempted to use buf *and* len(buf) was insufficient, it
184 : // will allocate a new slice for the value. In either case it will set
185 : // callerOwned to true.
186 : Fetch(
187 : handle []byte, valLen int32, buf []byte) (val []byte, callerOwned bool, err error)
188 : }
189 :
190 : // Value returns the underlying value.
191 2 : func (lv *LazyValue) Value(buf []byte) (val []byte, callerOwned bool, err error) {
192 2 : if lv.Fetcher == nil {
193 2 : return lv.ValueOrHandle, false, nil
194 2 : }
195 : // Do the rest of the work in a separate method to attempt mid-stack
196 : // inlining of Value(). Unfortunately, this still does not inline since the
197 : // cost of 85 exceeds the budget of 80.
198 : //
199 : // TODO(sumeer): Packing the return values into a struct{[]byte error bool}
200 : // causes it to be below the budget. Consider this if we need to recover
201 : // more performance. I suspect that inlining this only matters in
202 : // micro-benchmarks, and in actual use cases in CockroachDB it will not
203 : // matter because there is substantial work done with a fetched value.
204 2 : return lv.fetchValue(buf)
205 : }
206 :
207 : // INVARIANT: lv.Fetcher != nil
208 2 : func (lv *LazyValue) fetchValue(buf []byte) (val []byte, callerOwned bool, err error) {
209 2 : f := lv.Fetcher
210 2 : if !f.fetched {
211 2 : f.fetched = true
212 2 : f.value, f.callerOwned, f.err = f.Fetcher.Fetch(
213 2 : lv.ValueOrHandle, lv.Fetcher.Attribute.ValueLen, buf)
214 2 : }
215 2 : return f.value, f.callerOwned, f.err
216 : }
217 :
218 : // InPlaceValue returns the value under the assumption that it is in-place.
219 : // This is for Pebble-internal code.
220 2 : func (lv *LazyValue) InPlaceValue() []byte {
221 2 : if invariants.Enabled && lv.Fetcher != nil {
222 0 : panic("value must be in-place")
223 : }
224 2 : return lv.ValueOrHandle
225 : }
226 :
227 : // Len returns the length of the value.
228 2 : func (lv *LazyValue) Len() int {
229 2 : if lv.Fetcher == nil {
230 2 : return len(lv.ValueOrHandle)
231 2 : }
232 2 : return int(lv.Fetcher.Attribute.ValueLen)
233 : }
234 :
235 : // TryGetShortAttribute returns the ShortAttribute and a bool indicating
236 : // whether the ShortAttribute was populated.
237 1 : func (lv *LazyValue) TryGetShortAttribute() (ShortAttribute, bool) {
238 1 : if lv.Fetcher == nil {
239 1 : return 0, false
240 1 : }
241 1 : return lv.Fetcher.Attribute.ShortAttribute, true
242 : }
243 :
244 : // Clone creates a stable copy of the LazyValue, by appending bytes to buf.
245 : // The fetcher parameter must be non-nil and may be over-written and used
246 : // inside the returned LazyValue -- this is needed to avoid an allocation.
247 : // Most callers have at most K cloned LazyValues, where K is hard-coded, so
248 : // they can have a pool of exactly K LazyFetcher structs they can reuse in
249 : // these calls. The alternative of allocating LazyFetchers from a sync.Pool is
250 : // not viable since we have no code trigger for returning to the pool
251 : // (LazyValues are simply GC'd).
252 : //
253 : // NB: It is highly preferable that LazyValue.Value() has not been called,
254 : // since the Clone will forget any previously extracted value, and a future
255 : // call to Value will cause it to be fetched again. We do this since we don't
256 : // want to reason about whether or not to clone an already extracted value
257 : // inside the Fetcher (we don't). Property P1 applies here too: if lv1.Value()
258 : // has been called, and then lv2 is created as a clone of lv1, then calling
259 : // lv2.Value() can invalidate any backing memory maintained inside the fetcher
260 : // for lv1 (even though these are the same values). We initially prohibited
261 : // calling LazyValue.Clone() if LazyValue.Value() has been called, but there
262 : // is at least one complex caller (pebbleMVCCScanner inside CockroachDB) where
263 : // it is not easy to prove this invariant.
264 2 : func (lv *LazyValue) Clone(buf []byte, fetcher *LazyFetcher) (LazyValue, []byte) {
265 2 : var lvCopy LazyValue
266 2 : if lv.Fetcher != nil {
267 2 : *fetcher = LazyFetcher{
268 2 : Fetcher: lv.Fetcher.Fetcher,
269 2 : Attribute: lv.Fetcher.Attribute,
270 2 : // Not copying anything that has been extracted.
271 2 : }
272 2 : lvCopy.Fetcher = fetcher
273 2 : }
274 2 : vLen := len(lv.ValueOrHandle)
275 2 : if vLen == 0 {
276 2 : return lvCopy, buf
277 2 : }
278 2 : bufLen := len(buf)
279 2 : buf = append(buf, lv.ValueOrHandle...)
280 2 : lvCopy.ValueOrHandle = buf[bufLen : bufLen+vLen]
281 2 : return lvCopy, buf
282 : }
283 :
284 : // MakeInPlaceValue constructs an in-place value.
285 2 : func MakeInPlaceValue(val []byte) LazyValue {
286 2 : return LazyValue{ValueOrHandle: val}
287 2 : }
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