perf(workers): scaled embedding preprocessing and idle-only backoff

- Embedding and tagging workers now sleep their backoff interval
  (500ms/750ms) only when the queue is empty, the model is not ready,
  or a batch errored — previously every batch paid the sleep even with
  work pending.
- CLIP preprocessing no longer decodes originals at full resolution:
  decode_for_thumbnail is generalized into decode_image_scaled with a
  cover mode (shortest side >= target) and the embedder decodes JPEGs
  at the smallest DCT scale covering its 224px fill-crop, in parallel
  via rayon. The forward pass, not image decode, is now the dominant
  embedding cost.
- EXIF orientation is now applied before embedding, so rotated photos
  embed the way they are displayed (previously embedded sideways).
  Existing stored embeddings are unaffected.

src-tauri/src/embedder.rs

@@ -191,9 +191,11 @@ fn resolve_device() -> Result<Device> {
 }
 
 fn load_image(path: &Path, image_size: usize) -> Result<Tensor> {
-    let image = image::ImageReader::open(path)?
-        .with_guessed_format()?
-        .decode()?;
+    // Scaled decode: CLIP only needs image_size² pixels, so decoding a large
+    // JPEG at full resolution is wasted work. Cover mode keeps the shortest
+    // side at or above image_size for the fill-crop below. Also applies EXIF
+    // orientation, so rotated photos embed the way they are displayed.
+    let image = crate::thumbnail::decode_image_scaled(path, image_size as u32, true)?;
     let image = image.resize_to_fill(
         image_size as u32,
         image_size as u32,
@@ -208,10 +210,11 @@ fn load_image(path: &Path, image_size: usize) -> Result<Tensor> {
 }
 
 fn load_images(paths: &[PathBuf], image_size: usize) -> Result<Tensor> {
-    let mut images = Vec::with_capacity(paths.len());
-    for path in paths {
-        images.push(load_image(path, image_size)?);
-    }
+    use rayon::prelude::*;
+    let images = paths
+        .par_iter()
+        .map(|path| load_image(path, image_size))
+        .collect::<Result<Vec<_>>>()?;
     Ok(Tensor::stack(&images, 0)?)
 }
 

src-tauri/src/indexer.rs

@@ -231,11 +231,17 @@ pub fn start_embedding_worker(app: AppHandle, pool: DbPool) {
         let mut embedder: Option<ClipImageEmbedder> = None;
         println!("Embedding worker started.");
         loop {
-            if let Err(error) = process_embedding_batch(&app, &pool, &mut embedder) {
+            // Only back off when the queue is empty (or errored); while jobs
+            // are pending, claim the next batch immediately.
+            match process_embedding_batch(&app, &pool, &mut embedder) {
+                Ok(true) => {}
+                Ok(false) => std::thread::sleep(std::time::Duration::from_millis(500)),
+                Err(error) => {
                     eprintln!("Embedding worker error: {}", error);
-            }
                     std::thread::sleep(std::time::Duration::from_millis(500));
                 }
+            }
+        }
     });
 }
 
@@ -270,14 +276,18 @@ pub fn start_tagging_worker(app: AppHandle, pool: DbPool, app_data_dir: PathBuf)
                 println!("Tagging worker: acceleration setting changed — resetting session.");
                 tagger_instance = None;
             }
-            if let Err(error) =
-                process_tagging_batch(&app, &pool, &app_data_dir, &mut tagger_instance)
-            {
+            // Only back off when the queue is empty (or errored); while jobs
+            // are pending, claim the next batch immediately.
+            match process_tagging_batch(&app, &pool, &app_data_dir, &mut tagger_instance) {
+                Ok(true) => {}
+                Ok(false) => std::thread::sleep(std::time::Duration::from_millis(750)),
+                Err(error) => {
                     eprintln!("Tagging worker error: {}", error);
                     tagger_instance = None;
-            }
                     std::thread::sleep(std::time::Duration::from_millis(750));
                 }
+            }
+        }
     });
 }
 
@@ -759,11 +769,13 @@ fn process_metadata_batch(app: &AppHandle, pool: &DbPool, media_tools: &MediaToo
     Ok(())
 }
 
+/// Returns `Ok(true)` if a batch was claimed and processed, `Ok(false)` if
+/// the queue was empty.
 fn process_embedding_batch(
     app: &AppHandle,
     pool: &DbPool,
     embedder: &mut Option<ClipImageEmbedder>,
-) -> Result<()> {
+) -> Result<bool> {
     let batch_started_at = Instant::now();
     let claim_started_at = Instant::now();
     let paused_folders = paused_folder_ids("embedding");
@@ -774,7 +786,7 @@ fn process_embedding_batch(
     let claim_elapsed = claim_started_at.elapsed();
 
     if jobs.is_empty() {
-        return Ok(());
+        return Ok(false);
     }
 
     if embedder.is_none() {
@@ -902,7 +914,7 @@ fn process_embedding_batch(
         EMBEDDING_BATCH_SIZE, claim_elapsed, infer_elapsed, write_elapsed, batch_elapsed
     );
 
-    Ok(())
+    Ok(true)
 }
 
 fn process_caption_batch(
@@ -1007,14 +1019,16 @@ fn process_caption_batch(
     Ok(())
 }
 
+/// Returns `Ok(true)` if a batch was claimed and processed, `Ok(false)` if
+/// the queue was empty or the model is not ready.
 fn process_tagging_batch(
     app: &AppHandle,
     pool: &DbPool,
     app_data_dir: &Path,
     tagger_instance: &mut Option<WdTagger>,
-) -> Result<()> {
+) -> Result<bool> {
     if !tagger::tagger_model_status(app_data_dir).ready {
-        return Ok(());
+        return Ok(false);
     }
 
     let paused_folders = paused_folder_ids("tagging");
@@ -1025,7 +1039,7 @@ fn process_tagging_batch(
     })?;
 
     if jobs.is_empty() {
-        return Ok(());
+        return Ok(false);
     }
 
     if tagger_instance.is_none() {
@@ -1133,7 +1147,7 @@ fn process_tagging_batch(
         emit_folder_job_progress(app, pool, &folder_ids.into_iter().collect::<Vec<_>>(), true);
     }
 
-    Ok(())
+    Ok(true)
 }
 
 fn active_indexing_folders() -> HashSet<i64> {

src-tauri/src/thumbnail.rs

@@ -62,13 +62,26 @@ pub fn generate_image_thumbnail(image_path: &Path, cache_dir: &Path) -> Result<G
 }
 
 /// Decodes an image for thumbnailing, with EXIF orientation already applied.
+fn decode_for_thumbnail(image_path: &Path) -> Result<image::DynamicImage> {
+    decode_image_scaled(image_path, THUMB_SIZE, false)
+}
+
+/// Decodes an image at reduced resolution, with EXIF orientation applied.
 ///
 /// JPEGs take a fast path that decodes at reduced resolution (DCT scaling),
 /// which skips most of the IDCT/upsampling work for large photos. Anything
-/// that path can't handle falls back to the `image` crate.
-fn decode_for_thumbnail(image_path: &Path) -> Result<image::DynamicImage> {
+/// that path can't handle falls back to the `image` crate at full size.
+///
+/// `cover` selects which side must stay at or above `target`: `false` keeps
+/// the longest side (for aspect-fit consumers), `true` keeps the shortest
+/// side (for fill-crop consumers like the CLIP preprocessor).
+pub fn decode_image_scaled(
+    image_path: &Path,
+    target: u32,
+    cover: bool,
+) -> Result<image::DynamicImage> {
     if is_jpeg(image_path) {
-        if let Some(img) = decode_jpeg_scaled(image_path) {
+        if let Some(img) = decode_jpeg_scaled(image_path, target, cover) {
             return Ok(img);
         }
     }
@@ -87,16 +100,21 @@ fn is_jpeg(path: &Path) -> bool {
         .is_some_and(|ext| ext.eq_ignore_ascii_case("jpg") || ext.eq_ignore_ascii_case("jpeg"))
 }
 
-/// Decodes a JPEG at the smallest DCT scale that still covers `THUMB_SIZE`.
+/// Decodes a JPEG at the smallest DCT scale that still covers `target`.
 /// Returns `None` on any failure (corrupt file, CMYK without conversion
 /// support, etc.) so the caller can fall back to the generic decoder.
 /// mozjpeg reports errors by panicking, hence the `catch_unwind`.
-fn decode_jpeg_scaled(image_path: &Path) -> Option<image::DynamicImage> {
+fn decode_jpeg_scaled(image_path: &Path, target: u32, cover: bool) -> Option<image::DynamicImage> {
     let path = image_path.to_path_buf();
     let decoded = std::panic::catch_unwind(move || -> std::io::Result<(Vec<u8>, u32, u32)> {
         let mut decompress = mozjpeg::Decompress::new_path(&path)?;
         let (width, height) = decompress.size();
-        decompress.scale(scale_numerator(width.max(height)));
+        let reference = if cover {
+            width.min(height)
+        } else {
+            width.max(height)
+        };
+        decompress.scale(scale_numerator(reference, target));
         let mut started = decompress.rgb()?;
         let (out_width, out_height) = (started.width() as u32, started.height() as u32);
         let pixels = started.read_scanlines::<u8>()?;
@@ -115,11 +133,11 @@ fn decode_jpeg_scaled(image_path: &Path) -> Option<image::DynamicImage> {
     Some(img)
 }
 
-/// Smallest numerator (of /8) that keeps the longest side at or above
-/// `THUMB_SIZE`, so the subsequent resize never upscales.
-fn scale_numerator(max_dimension: usize) -> u8 {
+/// Smallest numerator (of /8) that keeps `reference` at or above `target`,
+/// so the subsequent resize never upscales.
+fn scale_numerator(reference: usize, target: u32) -> u8 {
     for numerator in 1..=8u8 {
-        if max_dimension * numerator as usize / 8 >= THUMB_SIZE as usize {
+        if reference * numerator as usize / 8 >= target as usize {
             return numerator;
         }
     }
@@ -248,10 +266,12 @@ mod tests {
 
     #[test]
     fn scale_numerator_picks_smallest_sufficient() {
-        assert_eq!(scale_numerator(6000), 1);
-        assert_eq!(scale_numerator(640), 4);
-        assert_eq!(scale_numerator(320), 8);
-        assert_eq!(scale_numerator(100), 8);
+        assert_eq!(scale_numerator(6000, THUMB_SIZE), 1);
+        assert_eq!(scale_numerator(640, THUMB_SIZE), 4);
+        assert_eq!(scale_numerator(320, THUMB_SIZE), 8);
+        assert_eq!(scale_numerator(100, THUMB_SIZE), 8);
+        // Cover mode reference: shortest side must reach 224 for CLIP.
+        assert_eq!(scale_numerator(1200, 224), 2);
     }
 
     #[test]
@@ -264,9 +284,15 @@ mod tests {
         img.save(&src_path).unwrap();
 
         // 1600 max dim -> numerator 2 -> 400x300 decode output
-        let decoded = decode_jpeg_scaled(&src_path).expect("fast path should handle plain JPEG");
+        let decoded = decode_jpeg_scaled(&src_path, THUMB_SIZE, false)
+            .expect("fast path should handle plain JPEG");
         assert_eq!((decoded.width(), decoded.height()), (400, 300));
 
+        // Cover mode: shortest side (1200) must stay >= 224 -> numerator 2
+        let covered = decode_jpeg_scaled(&src_path, 224, true)
+            .expect("fast path should handle plain JPEG");
+        assert_eq!((covered.width(), covered.height()), (400, 300));
+
         let cache = dir.join("cache");
         let _ = std::fs::remove_file(thumb_path(&cache, &src_path.to_string_lossy()));
         let result = generate_image_thumbnail(&src_path, &cache).unwrap();