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This commit is contained in:
Julien Cretin
2020-10-13 12:46:25 +02:00
parent eede767fe3
commit f512645837
2 changed files with 24 additions and 7 deletions
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19
libraries/persistent_store/src/lib.rs
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@@ -95,7 +95,7 @@
//! The _total lifetime_ of the store is below `L = ((E + 1) * N - 1) * (P - 2)` and
//! above `L - M` words, where `E` is the maximum number of erase cycles. The
//! lifetime is used when capacity is used, including transiently, as well as when
//! compaction occurs. Compaction frequency and litetime consumption are positively
//! compaction occurs. Compaction frequency and lifetime consumption are positively
//! correlated to the store load factor (the ratio of used capacity to total capacity).
//!
//! It is possible to approximate the cost of transient words in terms of capacity:
@@ -125,8 +125,8 @@
//! It is preferred to use `Clear` with a threshold of 0 to keep the lifetime
//! tracking.
//!
//! The store properties may still hold outside some of those assumptions but with
//! weaker probabilities as the usage diverges from them.
//! The store properties may still hold outside some of those assumptions, but with
//! an increasing chance of failure.
//!
//! # Implementation
//!
@@ -301,9 +301,18 @@
//! erased word. Marking an entry as padding or deleted is a single bit operation,
//! so the property trivially holds. For those cases, the proof relies on the fact
//! that there is exactly one bit equal to 0 in the 3 first bits. Either the 3 first
//! bits are still `111` in which case we expect the remaining bits to bit equal
//! bits are still `111` in which case we expect the remaining bits to be equal
//! to 1. Otherwise we can use the checksum of the given type of entry because those
//! 2 types of entries are not reachable from each other.
//! 2 types of entries are not reachable from each other. Here is a visualization of
//! the partitioning based on the first 3 bits:
//!
//! | First 3 bits | Description | How to check |
//! | ------------:| ------------------ | ---------------------------- |
//! | `111` | Erased word | All bits set to `1` |
//! | `101` | User entry | Contains a checksum |
//! | `110` | Internal entry | Contains a checksum |
//! | `100` | Deleted user entry | No check, atomically written |
//! | `0??` | Padding entry | No check, atomically written |
//!
//! To show that valid entries of a given type are not reachable from each other, we
//! show 3 lemmas: