Merge #3544

3544: Attempt to use default vram budget for faster startup r=Kerollmops a=dureuill

# Pull Request

## Related issue
Follow-up to #3382: addresses the added startup time on Windows/macOS.

## What does this PR do?
- Attempt to skip budget calculation by using "known good values" instead
- Perform dichotomic budget calculation as fallback only when the known value is not actually good.


Co-authored-by: Louis Dureuil <louis@meilisearch.com>

index-scheduler/src/lib.rs

@@ -435,9 +435,19 @@ impl IndexScheduler {
         mut task_db_size: usize,
         max_index_count: usize,
     ) -> IndexBudget {
-        let budget = utils::dichotomic_search(base_map_size, |map_size| {
+        #[cfg(windows)]
+        const DEFAULT_BUDGET: usize = 6 * 1024 * 1024 * 1024 * 1024; // 6 TiB, 1 index
+        #[cfg(not(windows))]
+        const DEFAULT_BUDGET: usize = 80 * 1024 * 1024 * 1024 * 1024; // 80 TiB, 18 indexes
+
+        let budget = if Self::is_good_heed(tasks_path, DEFAULT_BUDGET) {
+            DEFAULT_BUDGET
+        } else {
+            log::debug!("determining budget with dichotomic search");
+            utils::dichotomic_search(DEFAULT_BUDGET / 2, |map_size| {
                 Self::is_good_heed(tasks_path, map_size)
-        });
+            })
+        };
 
         log::debug!("memmap budget: {budget}B");
         let mut budget = budget / 2;