opt7: prefetching (#226)

libursa/OnDiskDataset.cpp

@@ -83,6 +83,13 @@ QueryResult OnDiskDataset::query(const Query &query,
             }
             throw std::runtime_error("Unexpected ngram type in query");
         },
+        [this, &seen](PrimitiveQuery primitive) {
+            for (auto &ndx : indices) {
+                if (ndx.index_type() == primitive.itype) {
+                    ndx.prefetch(primitive.trigram);
+                }
+            }
+        },
         counters);
 }
 

libursa/OnDiskIndex.cpp

@@ -60,6 +60,12 @@ QueryResult OnDiskIndex::query(TriGram trigram, QueryCounters *counters) const {
     return QueryResult(std::move(query_primitive(trigram, &counters->reads())));
 }
 
+void OnDiskIndex::prefetch(TriGram trigram) const {
+    std::pair<uint64_t, uint64_t> offsets = get_run_offsets(trigram);
+    uint64_t length = offsets.second - offsets.first;
+    ndxfile.prefetch(length, offsets.first);
+}
+
 std::pair<uint64_t, uint64_t> OnDiskIndex::get_run_offsets(
     TriGram trigram) const {
     uint64_t ptrs[2];

libursa/OnDiskIndex.h

@@ -38,6 +38,8 @@ class OnDiskIndex {
     const fs::path &get_fpath() const { return fpath; }
     IndexType index_type() const { return ntype; }
     QueryResult query(TriGram trigram, QueryCounters *counters) const;
+    void prefetch(TriGram trigram) const;
+
     uint64_t real_size() const;
     static void on_disk_merge(const fs::path &db_base, const std::string &fname,
                               IndexType merge_type,

libursa/Query.cpp

@@ -211,17 +211,46 @@ Query Query::plan(const std::unordered_set<IndexType> &types_to_query) const {
     return plan_qstring(types_to_query, value);
 }
 
+// Prefetch the next `howmany` ngrams.
+// This doesn't recurse into other queries. It's not a big problem,
+// because all primitives that we can fetch are in long AND sequences.
+// But in the future we may consider improving this.
+void Query::prefetch(int from_index, int howmany, bool only_last,
+                     const PrefetchFunc &prefetcher) const {
+    for (int i = 0; i < howmany; i++) {
+        int ndx = i + from_index;
+        if (ndx >= queries.size()) {
+            break;
+        }
+        if (queries[ndx].type == QueryType::PRIMITIVE) {
+            if (only_last && (i + 1 != howmany)) {
+                continue;
+            }
+            spdlog::debug("prefetching {}", ndx);
+            prefetcher(queries[ndx].ngram);
+        }
+    }
+}
+
 QueryResult Query::run(const QueryPrimitive &primitive,
+                       const PrefetchFunc &prefetcher,
                        QueryCounters *counters) const {
     // Case: primitive query - reduces to AND with tokens from query plan.
     if (type == QueryType::PRIMITIVE) {
         return primitive(ngram, counters);
     }
+
+    constexpr int PRETECTH_RANGE = 3;
+    prefetch(0, PRETECTH_RANGE, false, prefetcher);
+
     // Case: and. Short circuits when result is already empty.
     if (type == QueryType::AND) {
         auto result = QueryResult::everything();
-        for (const auto &query : queries) {
-            result.do_and(query.run(primitive, counters), &counters->ands());
+        for (int i = 0; i < queries.size(); i++) {
+            prefetch(i + 1, PRETECTH_RANGE, true, prefetcher);
+            const auto &query = queries[i];
+            result.do_and(query.run(primitive, prefetcher, counters),
+                          &counters->ands());
             if (result.is_empty()) {
                 break;
             }
@@ -232,7 +261,8 @@ QueryResult Query::run(const QueryPrimitive &primitive,
     if (type == QueryType::OR) {
         auto result = QueryResult::empty();
         for (const auto &query : queries) {
-            result.do_or(query.run(primitive, counters), &counters->ors());
+            result.do_or(query.run(primitive, prefetcher, counters),
+                         &counters->ors());
             if (result.is_everything()) {
                 break;
             }
@@ -252,7 +282,7 @@ QueryResult Query::run(const QueryPrimitive &primitive,
         int cutoff = count;
         int nonempty_sources = queries.size();
         for (const auto &query : queries) {
-            QueryResult next = query.run(primitive, counters);
+            QueryResult next = query.run(primitive, prefetcher, counters);
             if (next.is_everything()) {
                 cutoff -= 1;
                 if (cutoff <= 0) {

libursa/Query.h

@@ -33,6 +33,8 @@ class PrimitiveQuery {
 using QueryPrimitive =
     std::function<QueryResult(PrimitiveQuery, QueryCounters *counter)>;
 
+using PrefetchFunc = std::function<void(PrimitiveQuery)>;
+
 // Query represents the query as provided by the user.
 // Query can contain subqueries (using AND/OR/MINOF) or be a literal query.
 // There are actually two types of literal query objects - "plain" and
@@ -60,10 +62,14 @@ class Query {
     bool operator==(const Query &other) const;
 
     QueryResult run(const QueryPrimitive &primitive,
+                    const PrefetchFunc &prefetch,
                     QueryCounters *counters) const;
     Query plan(const std::unordered_set<IndexType> &types_to_query) const;
 
    private:
+    void prefetch(int from_index, int howmany, bool only_last,
+                  const PrefetchFunc &prefetch) const;
+
     QueryType type;
     // used for QueryType::PRIMITIVE before plan()
     QString value;

libursa/RawFile.cpp

@@ -44,6 +44,10 @@ void RawFile::pread(void *buf, size_t to_read, off_t offset) const {
     }
 }
 
+void RawFile::prefetch(size_t size, off_t offset) const {
+    ::posix_fadvise(fd, offset, size, POSIX_FADV_WILLNEED);
+}
+
 template <typename T>
 void RawFile::write(const T *buf, size_t count) {
     const auto *buf_raw = reinterpret_cast<const char *>(buf);

libursa/RawFile.h

@@ -13,6 +13,7 @@ class RawFile {
 
     uint64_t size() const;
     void pread(void *buf, size_t to_read, off_t offset) const;
+    void prefetch(size_t size, off_t offse) const;
 
     template <typename T>
     void write(const T *buf, size_t count);

libursa/Version.h.in

@@ -9,5 +9,5 @@ constexpr std::string_view ursadb_format_version = "1.5.0";
 // Project version.
 // Consider updating the version tag when doing PRs.
 // clang-format off
-constexpr std::string_view ursadb_version_string = "@PROJECT_VERSION@+opt6";
+constexpr std::string_view ursadb_version_string = "@PROJECT_VERSION@+opt7";
 // clang-format on