Mercurial > public > mercurial-scm > hg-stable

diff rust/hg-core/examples/nodemap/index.rs @ 44417:8f7c6656ac79

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
rust-nodemap: pure Rust example To run, use `cargo run --release --example nodemap` This demonstrates that simple scenarios entirely written in Rust can content themselves with `NodeTree<T>`. The example mmaps both the nodemap file and the changelog index. We had of course to include an implementation of `RevlogIndex` directly, which isn't much at this stage. It felt a bit prematurate to include it in the lib. Here are some first performance measurements, obtained with this example, on a clone of mozilla-central with 440000 changesets: (create) Nodemap constructed in RAM in 153.638305ms (query CAE63161B68962) found in 22.362us: Ok(Some(269489)) (bench) Did 3 queries in 36.418?s (mean 12.139?s) (bench) Did 50 queries in 184.318?s (mean 3.686?s) (bench) Did 100000 queries in 31.053461ms (mean 310ns) To be fair, even between bench runs, results tend to depend whether the file is still in kernel caches, and it's not so easy to get back to a real cold start. The worst we've seen was in the 50us ballpark. In any busy server setting, the pages would always be in RAM. We hope it's good enough not to be significantly slower on any concrete Mercurial operation than the C nodetree when fully in RAM, and of course this implementation has the serious headstart advantage of persistence. Differential Revision: https://phab.mercurial-scm.org/D7797
author Georges Racinet <georges.racinet@octobus.net>
date Tue, 18 Feb 2020 19:11:15 +0100
parents
children e834b79def74
line wrap: on
line diff
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-core/examples/nodemap/index.rs	Tue Feb 18 19:11:15 2020 +0100
@@ -0,0 +1,95 @@
+// Copyright 2019-2020 Georges Racinet <georges.racinet@octobus.net>
+//
+// This software may be used and distributed according to the terms of the
+// GNU General Public License version 2 or any later version.
+
+//! Minimal `RevlogIndex`, readable from standard Mercurial file format
+use hg::*;
+use memmap::*;
+use std::fs::File;
+use std::ops::Deref;
+use std::path::Path;
+use std::slice;
+
+pub struct Index {
+    data: Box<dyn Deref<Target = [IndexEntry]> + Send>,
+}
+
+/// A fixed sized index entry. All numbers are big endian
+#[repr(C)]
+pub struct IndexEntry {
+    not_used_yet: [u8; 24],
+    p1: Revision,
+    p2: Revision,
+    node: Node,
+    unused_node: [u8; 12],
+}
+
+pub const INDEX_ENTRY_SIZE: usize = 64;
+
+impl IndexEntry {
+    fn parents(&self) -> [Revision; 2] {
+        [Revision::from_be(self.p1), Revision::from_be(self.p1)]
+    }
+}
+
+impl RevlogIndex for Index {
+    fn len(&self) -> usize {
+        self.data.len()
+    }
+
+    fn node(&self, rev: Revision) -> Option<&Node> {
+        if rev == NULL_REVISION {
+            return None;
+        }
+        let i = rev as usize;
+        if i >= self.len() {
+            None
+        } else {
+            Some(&self.data[i].node)
+        }
+    }
+}
+
+impl Graph for &Index {
+    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
+        let [p1, p2] = (*self).data[rev as usize].parents();
+        let len = (*self).len();
+        if p1 < NULL_REVISION
+            || p2 < NULL_REVISION
+            || p1 as usize >= len
+            || p2 as usize >= len
+        {
+            return Err(GraphError::ParentOutOfRange(rev));
+        }
+        Ok([p1, p2])
+    }
+}
+
+struct IndexMmap(Mmap);
+
+impl Deref for IndexMmap {
+    type Target = [IndexEntry];
+
+    fn deref(&self) -> &[IndexEntry] {
+        let ptr = self.0.as_ptr() as *const IndexEntry;
+        // Any misaligned data will be ignored.
+        debug_assert_eq!(
+            self.0.len() % std::mem::align_of::<IndexEntry>(),
+            0,
+            "Misaligned data in mmap"
+        );
+        unsafe { slice::from_raw_parts(ptr, self.0.len() / INDEX_ENTRY_SIZE) }
+    }
+}
+
+impl Index {
+    pub fn load_mmap(path: impl AsRef<Path>) -> Self {
+        let file = File::open(path).unwrap();
+        let msg = "Index file is missing, or missing permission";
+        let mmap = unsafe { MmapOptions::new().map(&file) }.expect(msg);
+        Self {
+            data: Box::new(IndexMmap(mmap)),
+        }
+    }
+}