Make it use random lookups before a whole scan

Cargo.lock

@@ -3733,6 +3733,7 @@ dependencies = [
  "indexmap",
  "meilisearch-auth",
  "meilisearch-types",
+ "rand",
  "serde",
  "serde_json",
  "tempfile",

crates/meilitool/Cargo.toml

@@ -17,6 +17,7 @@ file-store = { path = "../file-store" }
 indexmap = { version = "2.7.0", features = ["serde"] }
 meilisearch-auth = { path = "../meilisearch-auth" }
 meilisearch-types = { path = "../meilisearch-types" }
+rand = { version = "0.8", default-features = false }
 serde = { version = "1.0.217", features = ["derive"] }
 serde_json = { version = "1.0.135", features = ["preserve_order"] }
 tempfile = "3.15.0"

crates/meilitool/src/main.rs

@@ -1,7 +1,6 @@
 use std::fs::{read_dir, read_to_string, remove_file, File};
 use std::hint::black_box;
 use std::io::{BufWriter, Write as _};
-use std::ops::Bound;
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::thread;
@@ -24,6 +23,8 @@ use meilisearch_types::milli::{obkv_to_json, BEU32};
 use meilisearch_types::tasks::{Status, Task};
 use meilisearch_types::versioning::{get_version, parse_version};
 use meilisearch_types::Index;
+use rand::rngs::StdRng;
+use rand::{Rng, SeedableRng};
 use serde_json::Value::Object;
 use time::macros::format_description;
 use time::OffsetDateTime;
@@ -652,6 +653,8 @@ fn hair_dryer(
                         let num_keys = database.len(&rtxn)? as usize;
                         let first_entry = database.iter(&rtxn)?.next().transpose()?;
                         let last_entry = database.rev_iter(&rtxn)?.next().transpose()?;
+                        // Visit more keys per thread to ensure wide coverage.
+                        let total_keys_to_visit = num_keys * 100;
                         let keys_by_thread = num_keys / total_threads;
 
                         let Some(((first_key, _), (last_key, _))) = first_entry.zip(last_entry)
@@ -662,24 +665,53 @@ fn hair_dryer(
                         let first_key_num = first_key.try_into().map(u64::from_be_bytes).unwrap();
                         let last_key_num = last_key.try_into().map(u64::from_be_bytes).unwrap();
 
-                        eprintln!("between {first_key_num:x} and {last_key_num:x}");
+                        eprintln!("Iterating over {keys_by_thread} entries by thread ({total_threads}x)...");
-                        eprintln!("Iterating over {keys_by_thread} entries by thread...");
 
                         let progress = AtomicUsize::new(0);
-                        let count = thread::scope(|s| -> anyhow::Result<usize> {
+                        thread::scope(|s| -> anyhow::Result<()> {
                             let mut handles = Vec::new();
 
                             for tid in 0..total_threads {
                                 let index = &index;
                                 let progress = &progress;
-                                let handle = s.spawn(move || -> anyhow::Result<usize> {
+                                let handle = s.spawn(move || -> anyhow::Result<()> {
                                     let rtxn = index.read_txn()?;
-                                    let start = first_key_num + (keys_by_thread * tid) as u64;
-                                    let start_bytes = start.to_be_bytes();
-                                    let range = (Bound::Included(&start_bytes[..]), Bound::Unbounded);
 
-                                    let mut count: usize = 0;
+                                    let mut rng = StdRng::seed_from_u64(tid as u64);
-                                    for result in database.range(&rtxn, &range)?.take(keys_by_thread) {
+                                    for _ in 0..keys_by_thread {
+                                        let random_key_num = rng.gen_range(first_key_num..=last_key_num);
+                                        let random_key = random_key_num.to_be_bytes();
+
+                                        let Some((key, value)) = database.get_greater_than(&rtxn, &random_key)? else {
+                                            continue;
+                                        };
+
+                                        // All of this just to avoid compiler optimizations 🤞
+                                        // We must read all the bytes to make the pages hot in cache.
+                                        // <https://doc.rust-lang.org/std/hint/fn.black_box.html>
+                                        black_box(key.iter().fold(0, |acc, _| acc + 1));
+                                        black_box(value.iter().fold(0, |acc, _| acc + 1));
+
+                                        let current_progress = progress.fetch_add(1, Ordering::Relaxed);
+                                        if current_progress % 10_000 == 0 {
+                                            let perc = (current_progress as f64) / (total_keys_to_visit as f64) * 100.0;
+                                            eprintln!("Visited {current_progress}/{total_keys_to_visit} ({perc:.2}%) keys");
+                                        }
+                                    }
+
+                                    Ok(())
+                                });
+
+                                handles.push(handle);
+                            }
+
+                            handles.into_iter().try_for_each(|h| h.join().unwrap())
+                        })?;
+
+                        eprintln!("Doing a last pass on all the keys...");
+
+                        let mut count = 0;
+                        for (i, result) in database.iter(&rtxn)?.enumerate() {
                             let (key, value) = result?;
 
                             // All of this just to avoid compiler optimizations 🤞
@@ -688,26 +720,12 @@ fn hair_dryer(
                             count += black_box(key.iter().fold(0, |acc, _| acc + 1));
                             count += black_box(value.iter().fold(0, |acc, _| acc + 1));
 
-                                        let current_progress = progress.fetch_add(1, Ordering::Relaxed);
+                            if i % 10_000 == 0 {
-                                        if current_progress % 10_000 == 0 {
+                                let perc = (i as f64) / (total_keys_to_visit as f64) * 100.0;
-                                            let perc = (current_progress as f64) / (num_keys as f64) * 100.0;
+                                eprintln!("Visited {i}/{total_keys_to_visit} ({perc:.2}%) keys");
-                                            eprintln!("Visited {current_progress}/{num_keys} ({perc:.2}%) keys");
                             }
                         }
 
-                                    Ok(count)
-                                });
-
-                                handles.push(handle);
-                            }
-
-                            let mut count = 0usize;
-                            for handle in handles {
-                                count += handle.join().unwrap()?;
-                            }
-                            Ok(count)
-                        })?;
-
                         eprintln!("Done hair drying a total of at least {count} bytes.");
                     }
                 }