internal/index: call madvise every 512 KB to keep RSS limited

With this change, the pos writing phase of index merging consumes
1GB-2GB of RSS instead of up to 20 GB of RSS!

Author: stapelberg

internal/index/merge.go

@@ -238,7 +238,6 @@ func ConcatN(destdir string, srcdirs []string) error {
 		start = time.Now()
 	}
 
-	// TODO(performance): The following phase accumulates up to 20 GB (!) in memory mappings.
 	if err := writePos(destdir, trigrams, idxDocid, idxMetaPos); err != nil {
 		return err
 	}
@@ -466,6 +465,8 @@ func writePos(destdir string, trigrams []Trigram, idxDocid map[Trigram][]uint32,
 					}
 				}
 			}
+
+			idx.rd.maybeMadvise(meta.OffsetData)
 		}
 
 		if err := dw.Flush(); err != nil {

internal/index/read.go

@@ -140,6 +140,24 @@ func (sr *PForReader) Close() error {
 	return nil
 }
 
+const pageSize = 4096
+const pagesPerAdvise = 128 // release every 512KB
+
+// maybeMadvise calls MADV_DONTNEED on processed pages to reduce RSS.
+func (sr *PForReader) maybeMadvise(offset int64) {
+	if page := int(offset) / pageSize; page > 0 && page%pagesPerAdvise == 0 {
+		sr.madviseDontNeed(page)
+	}
+}
+
+func (sr *PForReader) madviseDontNeed(page int) {
+	// release pages up to the current page-aligned offset
+	releaseUpTo := page * pageSize
+	if releaseUpTo <= len(sr.data.Data) {
+		unix.Madvise(sr.data.Data[:releaseUpTo], unix.MADV_DONTNEED)
+	}
+}
+
 func (sr *PForReader) metaEntry(trigram Trigram) (*MetaEntry, *MetaEntry, error) {
 	num := len(sr.meta.Data) / metaEntrySize
 	d := sr.meta.Data