Introduce a way to store the HNSW on multiple LMDB entries

milli/src/index.rs

@@ -22,6 +22,7 @@ use crate::heed_codec::facet::{
     FieldIdCodec, OrderedF64Codec,
 };
 use crate::heed_codec::{ScriptLanguageCodec, StrBEU16Codec, StrRefCodec};
+use crate::readable_slices::ReadableSlices;
 use crate::{
     default_criteria, CboRoaringBitmapCodec, Criterion, DocumentId, ExternalDocumentsIds,
     FacetDistribution, FieldDistribution, FieldId, FieldIdWordCountCodec, GeoPoint, ObkvCodec,
@@ -47,7 +48,10 @@ pub mod main_key {
     pub const FIELDS_IDS_MAP_KEY: &str = "fields-ids-map";
     pub const GEO_FACETED_DOCUMENTS_IDS_KEY: &str = "geo-faceted-documents-ids";
     pub const GEO_RTREE_KEY: &str = "geo-rtree";
-    pub const VECTOR_HNSW_KEY: &str = "vector-hnsw";
+    /// The prefix of the key that is used to store the, potential big, HNSW structure.
+    /// It is concatenated with a big-endian encoded number (non-human readable).
+    /// e.g. vector-hnsw0x0032.
+    pub const VECTOR_HNSW_KEY_PREFIX: &str = "vector-hnsw";
     pub const HARD_EXTERNAL_DOCUMENTS_IDS_KEY: &str = "hard-external-documents-ids";
     pub const NUMBER_FACETED_DOCUMENTS_IDS_PREFIX: &str = "number-faceted-documents-ids";
     pub const PRIMARY_KEY_KEY: &str = "primary-key";
@@ -517,19 +521,49 @@ impl Index {
 
     /// Writes the provided `hnsw`.
     pub(crate) fn put_vector_hnsw(&self, wtxn: &mut RwTxn, hnsw: &Hnsw) -> heed::Result<()> {
-        self.main.put::<_, Str, SerdeBincode<Hnsw>>(wtxn, main_key::VECTOR_HNSW_KEY, hnsw)
+        // We must delete all the chunks before we write the new HNSW chunks.
+        self.delete_vector_hnsw(wtxn)?;
+
+        let chunk_size = 1024 * 1024 * (1024 + 512); // 1.5 GiB
+        let bytes = bincode::serialize(hnsw).map_err(|_| heed::Error::Encoding)?;
+        for (i, chunk) in bytes.chunks(chunk_size).enumerate() {
+            let i = i as u32;
+            let mut key = main_key::VECTOR_HNSW_KEY_PREFIX.as_bytes().to_vec();
+            key.extend_from_slice(&i.to_be_bytes());
+            self.main.put::<_, ByteSlice, ByteSlice>(wtxn, &key, chunk)?;
+        }
+        Ok(())
     }
 
     /// Delete the `hnsw`.
     pub(crate) fn delete_vector_hnsw(&self, wtxn: &mut RwTxn) -> heed::Result<bool> {
-        self.main.delete::<_, Str>(wtxn, main_key::VECTOR_HNSW_KEY)
+        let mut iter = self.main.prefix_iter_mut::<_, ByteSlice, DecodeIgnore>(
+            wtxn,
+            main_key::VECTOR_HNSW_KEY_PREFIX.as_bytes(),
+        )?;
+        let mut deleted = false;
+        while let Some(_) = iter.next().transpose()? {
+            // We do not keep a reference to the key or the value.
+            unsafe { deleted |= iter.del_current()? };
+        }
+        Ok(deleted)
     }
 
     /// Returns the `hnsw`.
     pub fn vector_hnsw(&self, rtxn: &RoTxn) -> Result<Option<Hnsw>> {
-        match self.main.get::<_, Str, SerdeBincode<Hnsw>>(rtxn, main_key::VECTOR_HNSW_KEY)? {
+        let mut slices = Vec::new();
-            Some(hnsw) => Ok(Some(hnsw)),
+        for result in
-            None => Ok(None),
+            self.main.prefix_iter::<_, Str, ByteSlice>(rtxn, main_key::VECTOR_HNSW_KEY_PREFIX)?
+        {
+            let (_, slice) = result?;
+            slices.push(slice);
+        }
+
+        if slices.is_empty() {
+            Ok(None)
+        } else {
+            let readable_slices: ReadableSlices<_> = slices.into_iter().collect();
+            Ok(Some(bincode::deserialize_from(readable_slices).map_err(|_| heed::Error::Decoding)?))
         }
     }
 

milli/src/lib.rs

@@ -18,6 +18,7 @@ mod fields_ids_map;
 pub mod heed_codec;
 pub mod index;
 pub mod proximity;
+mod readable_slices;
 pub mod score_details;
 mod search;
 pub mod update;

milli/src/readable_slices.rs

@@ -0,0 +1,84 @@
+use std::io::{self, Read};
+use std::iter::FromIterator;
+
+pub struct ReadableSlices<A> {
+    inner: Vec<A>,
+    pos: u64,
+}
+
+impl<A> FromIterator<A> for ReadableSlices<A> {
+    fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
+        ReadableSlices { inner: iter.into_iter().collect(), pos: 0 }
+    }
+}
+
+impl<A: AsRef<[u8]>> Read for ReadableSlices<A> {
+    fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
+        let original_buf_len = buf.len();
+
+        // We explore the list of slices to find the one where we must start reading.
+        let mut pos = self.pos;
+        let index = match self
+            .inner
+            .iter()
+            .map(|s| s.as_ref().len() as u64)
+            .position(|size| pos.checked_sub(size).map(|p| pos = p).is_none())
+        {
+            Some(index) => index,
+            None => return Ok(0),
+        };
+
+        let mut inner_pos = pos as usize;
+        for slice in &self.inner[index..] {
+            let slice = &slice.as_ref()[inner_pos..];
+
+            if buf.len() > slice.len() {
+                // We must exhaust the current slice and go to the next one there is not enough here.
+                buf[..slice.len()].copy_from_slice(slice);
+                buf = &mut buf[slice.len()..];
+                inner_pos = 0;
+            } else {
+                // There is enough in this slice to fill the remaining bytes of the buffer.
+                // Let's break just after filling it.
+                buf.copy_from_slice(&slice[..buf.len()]);
+                buf = &mut [];
+                break;
+            }
+        }
+
+        let written = original_buf_len - buf.len();
+        self.pos += written as u64;
+        Ok(written)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::ReadableSlices;
+    use std::io::Read;
+
+    #[test]
+    fn basic() {
+        let data: Vec<_> = (0..100).collect();
+        let splits: Vec<_> = data.chunks(3).collect();
+        let mut rdslices: ReadableSlices<_> = splits.into_iter().collect();
+
+        let mut output = Vec::new();
+        let length = rdslices.read_to_end(&mut output).unwrap();
+        assert_eq!(length, data.len());
+        assert_eq!(output, data);
+    }
+
+    #[test]
+    fn small_reads() {
+        let data: Vec<_> = (0..u8::MAX).collect();
+        let splits: Vec<_> = data.chunks(27).collect();
+        let mut rdslices: ReadableSlices<_> = splits.into_iter().collect();
+
+        let buffer = &mut [0; 45];
+        let length = rdslices.read(buffer).unwrap();
+        let expected: Vec<_> = (0..buffer.len() as u8).collect();
+        assert_eq!(length, buffer.len());
+        assert_eq!(buffer, &expected[..]);
+    }
+}