Durability RFC

RFC0005-Durability.md

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+# Summary
+
+- Introduce a user-visibile concept of `Durability`
+- Adjusting the "durability" of an input can allow salsa to skip a lot of validation work
+- Garbage collection -- particularly of interned values -- however becomes more complex
+- Possible future expansion: automatic detection of more "durable" input values
+
+# Motivation
+
+## Making validation faster by optimizing for "durability"
+
+Presently, salsa's validation logic requires traversing all
+dependencies to check that they have not changed. This can sometimes
+be quite costly in practice: rust-analyzer for example sometimes
+spends as much as 90ms revalidating the results from a no-op
+change. One option to improve this is simply optimization --
+[salsa#176] for example reduces validation times significantly, and
+there remains opportunity to do better still. However, even if we are
+able to traverse the dependency graph more efficiently, it will still
+be an O(n) process. It would be nice if we could do better.
+
+[salsa#176]: https://github.com/salsa-rs/salsa/pull/176
+
+One observation is that, in practice, there are often input values
+that are known to change quite infrequently. For example, in
+rust-analyzer, the standard library and crates downloaded from
+crates.io are unlikely to change (though changes are possible; see
+below). Similarly, the `Cargo.toml` file for a project changes
+relatively infrequently compared to the sources. We say then that
+these inputs are more **durable** -- that is, they change less frequently.
+
+This RFC proposes a mechanism to take advantage of durability for
+optimization purposes. Imagine that we have some query Q that depends
+solely on the standard library. The idea is that we can track the last
+revision R when the standard library was changed. Then, when
+traversing dependencies, we can skip traversing the dependencies of Q
+if it was last validated after the revision R. Put another way, we
+only need to traverse the dependencies of Q when the standard library
+changes -- which is unusual. If the standard library *does* change,
+for example by user's tinkering with the internal sources, then yes we
+walk the dependencies of Q to see if it is affected.
+
+# User's guide
+
+## The durability type
+
+We add a new type `salsa::Durability` which has there associated constants:
+
+```rust
+#[derive(Copy, Clone, Debug, Ord)]
+pub struct Durability(..);
+
+impl Durability {
+  // Values that change regularly, like the source to the current crate.
+  pub const LOW: Durability;
+
+  // Values that change infrequently, like Cargo.toml.
+  pub const MEDIUM: Durability;
+
+  // Values that are not expected to change, like sources from crates.io or the stdlib.
+  pub const HIGH: Durability;
+}
+```
+
+h## Specifying the durability of an input
+
+When setting an input `foo`, one can now invoke a method
+`set_foo_with_durability`, which takes a `Durability` as the final
+argument:
+
+```rust
+// db.set_foo(key, value) is equivalent to:
+db.set_foo_with_durability(key, value, Durability::LOW);
+
+// This would indicate that `foo` is not expected to change: 
+db.set_foo_with_durability(key, value, Durability::HIGH);
+```
+
+## Durability of interned values
+
+Interned values are always considered `Durability::HIGH`. This makes
+sense as many queries that only use high durability inputs will also
+make use of interning internally. A consequence of this is that they
+will not be garbage collected unless you use the specific patterns
+recommended below.
+
+## Synthetic writes
+
+Finally, we add one new method, `synthetic_write(durability)`, 
+available on the salsa runtime:
+
+```rust
+db.salsa_runtime().synthetic_write(Durability::HIGH)
+```
+
+As the name suggests, `synthetic_write` causes salsa to act *as
+though* a write to an input of the given durability had taken
+place. This can be used for benchmarking, but it's also important to
+controlling what values get garbaged collected, as described below.
+
+## Tracing and garbage collection
+
+Durability affects garbage collection. The `SweepStrategy` struct is
+modified as follows:
+
+```rust
+/// Sweeps values which may be outdated, but which have not
+/// been verified since the start of the current collection.
+/// These are typically memoized values from previous computations
+/// that are no longer relevant.
+pub fn sweep_outdated(self) -> SweepStrategy;
+
+/// Sweeps values which have not been verified since the start 
+/// of the current collection, even if they are known to be 
+/// up to date. This can be used to collect "high durability" values
+/// that are not *directly* used by the main query.
+///
+/// So, for example, imagine a main query `result` which relies
+/// on another query `threshold` and (indirectly) on a `threshold_inner`:
+///
+/// ```
+/// result(10) [durability: Low]
+///    |
+///    v
+/// threshold(10) [durability: High]
+///    |
+///    v
+/// threshold_inner(10)  [durability: High]
+/// ```
+///
+/// If you modify a low durability input and then access `result`,
+/// then `result(10)` and its *immediate* dependencies will 
+/// be considered "verified". However, because `threshold(10)` 
+/// has high durability and no high durability input was modified,
+/// we will not verify *its* dependencies, so `threshold_inner` is not 
+/// verified (but it is also not outdated).
+///
+/// Collecting unverified things would therefore collect `threshold_inner(10)`.
+/// Collecting only *outdated* things (i.e., with `sweep_outdated`)
+/// would collect nothing -- but this does mean that some high durability
+/// queries that are no longer relevant to your main query may stick around.
+/// 
+/// To get the most precise garbage collection, do a synthetic write with
+/// high durability -- this will force us to verify *all* values. You can then
+/// sweep unverified values.
+pub fn sweep_unverified(self) -> SweepStrategy;
+```
+
+# Reference guide
+
+## Review: The need for GC to collect outdated values
+
+In general, salsa's lazy validation scheme can lead to the accumulation
+of garbage that is no longer needed. Consider a query like this one:
+
+```rust
+fn derived1(db: &impl Database, start: usize) {
+  let middle = self.input(start);
+  self.derived2(middle)
+}
+```
+
+Now imagine that, on some particular run, we compute `derived1(22)`:
+
+- `derived1(22)`
+  - executes `input(22)`, which returns `44`
+  - then executes `derived2(44)`
+
+The end result of this execution will be a dependency graph
+like:
+
+```
+derived1(22) -> derived2(44)
+  |
+  v
+input(22)
+```
+
+Now. imagine that the user modifies `input(22)` to have the value `45`.
+The next time `derived1(22)` executes, it will load `input(22)` as before,
+but then execute `derived2(45)`. This leaves us with a dependency
+graph as follows:
+
+```
+derived1(22) -> derived2(45)
+  |
+  v
+input(22)       derived2(44)
+```
+
+Notice that we still see `derived2(44)` in the graph. This is because
+we memoized the result in last round and then simply had no use for it
+in this round. The role of GC is to collect "outdated" values like
+this one.
+
+###Review: Tracing and GC before durability
+
+In the absence of durability, when you execute a query Q in some new
+revision where Q has not previously executed, salsa must trace back
+through all the queries that Q depends on to ensure that they are
+still up to date. As each of Q's dependencies is validated, we mark it
+to indicate that it has been checked in the current revision (and
+thus, within a particular revision, we would never validate or trace a
+particular query twice).
+
+So, to continue our example, when we first executed `derived1(22)`
+in revision R1, we might have had a graph like:
+
+
+```
+derived1(22)   -> derived2(44)
+[verified: R1]    [verified: R1]
+  |
+  v
+input(22)
+```
+
+Now, after we modify `input(22)` and execute `derived1(22)` again, we 
+would have a graph like:
+
+```
+derived1(22)   -> derived2(45)
+[verified: R2]    [verified: R2]
+  |
+  v
+input(22)         derived2(44)
+                  [verified: R1]
+```
+
+Note that `derived2(44)`, the outdated value, never had its "verified"
+revision updated, because we never accessed it.
+
+Salsa leverages this validation stamp to serve as the "marking" phase
+of a simple mark-sweep garbage collector. The idea is that the sweep
+method can collect any values that are "outdated" (whose "verified"
+revision is less than the current revision).
+
+The intended model is that one can do a "mark-sweep" style garbage
+collection like so:
+
+```rust
+// Modify some input, triggering a new revision.
+db.set_input(22, 45);
+
+// The **mark** phase: execute the "main query", with the intention
+// that we wish to retain all the memoized values needed to compute
+// this main query, but discard anything else. For example, in an IDE
+// context, this might be a "compute all errors" query.
+db.derived1(22);
+
+// The **sweep** phase: discard anything that was not traced during
+// the mark phase.
+db.sweep_all(...);
+```
+
+In the case of our example, when we execute `sweep_all`, it would
+collect `derived2(44)`.
+
+## Challenge: Durability lets us avoid tracing
+
+This tracing model is affected by the move to durability. Now, if some
+derived value has a high durability, we may skip tracing its
+descendants altogether. This means that they would never be "verified"
+-- that is, their "verified date" would never be updated.
+
+This is why we modify the definition of "outdated" as follows:
+
+- For a query value `Q` with durability `D`, let `R_lc` be the revision when
+  values of durability `D` last changed. Let `R_v` be the revision when
+  `Q` was last verified.
+- `Q` is outdated if `R_v < R_lc`.
+    - In other words, if `Q` may have changed since it was last verified.
+
+## Collecting interned and untracked values
+
+Most values can be collected whenever we like without influencing
+correctness.  However, interned values and those with untracked
+dependencies are an exception -- **they can only be collected when
+outdated**.  This is because their values may not be reproducible --
+in other words, re-executing an interning query (or one with untracked
+dependencies, which can read arbitrary program state) twice in a row
+may produce a different value. In the case of an interning query, for
+example, we may wind up using a different integer than we did before.
+If the query is outdated, this is not a problem: anything that
+dependend on its result must also be outdated, and hence would be
+re-executed and would observe the new value. But if the query is *not*
+outdated, then we could get inconsistent result.s
+
+# Alternatives and future work
+
+## Rejected: Arbitrary durabilities
+
+We considered permitting arbitrary "levels" of durability -- for
+example, allowing the user to specify a number -- rather than offering
+just three. Ultimately it seemed like that level of control wasn't
+really necessary and that having just three levels would be sufficient
+and simpler.
+
+## Rejected: Durability lattices
+
+We also considered permitting a "lattice" of durabilities -- e.g., to
+mirror the crate DAG in rust-analyzer -- but this is tricky because
+the lattice itself would be dependent on other inputs.
+