[perf] try to improve small_string_map eviction

Author: tstack

src/base/small_string_map.cc

@@ -29,6 +29,8 @@
 
 #include "small_string_map.hh"
 
+#include "lnav_log.hh"
+
 namespace lnav {
 
 std::optional<uint32_t>
@@ -47,6 +49,7 @@ small_string_map::lookup(const string_fragment& in)
                 || this->ssm_keys[index * MAX_KEY_SIZE + in.length()] == '\0'))
         {
             this->ssm_start_index = index;
+            this->ssm_age[index] = true;
             return this->ssm_values[index];
         }
         index = (index + 1) % MAP_SIZE;
@@ -57,18 +60,27 @@ small_string_map::lookup(const string_fragment& in)
 void
 small_string_map::insert(const string_fragment& key, uint32_t value)
 {
-    if (key.length() >= MAX_KEY_SIZE) {
+    if (key.empty() || key.length() > MAX_KEY_SIZE) {
         return;
     }
 
-    auto key_index = this->ssm_used_keys < MAP_SIZE ? this->ssm_used_keys++
-                                                    : key.hash() % MAP_SIZE;
+    auto key_index = (this->ssm_start_index + 1) % MAP_SIZE;
+    for (auto lpc = 0; lpc < MAP_SIZE; lpc++) {
+        if (!this->ssm_age[lpc]) {
+            key_index = lpc;
+        } else {
+            this->ssm_age[lpc] = false;
+        }
+    }
+    this->ssm_age[this->ssm_start_index] = true;
+    this->ssm_age[key_index] = true;
 
     memset(&this->ssm_keys[key_index * MAX_KEY_SIZE],
            0,
            MAX_KEY_SIZE * sizeof(char));
     memcpy(&this->ssm_keys[key_index * MAX_KEY_SIZE], key.data(), key.length());
     this->ssm_values[key_index] = value;
+    this->ssm_start_index = key_index;
 }
 
 }  // namespace lnav

src/base/small_string_map.hh

@@ -38,12 +38,12 @@
 namespace lnav {
 
 struct small_string_map {
-    static constexpr auto MAP_SIZE = 4;
+    static constexpr auto MAP_SIZE = 8;
     static constexpr auto MAX_KEY_SIZE = 8;
     char ssm_keys[MAP_SIZE * MAX_KEY_SIZE]{};
     uint32_t ssm_values[MAP_SIZE]{};
-    uint32_t ssm_used_keys{0};
     uint32_t ssm_start_index{0};
+    bool ssm_age[MAP_SIZE];
 
     std::optional<uint32_t> lookup(const string_fragment& in);
     void insert(const string_fragment& key, uint32_t value);

src/base/small_string_map.tests.cc

@@ -45,8 +45,10 @@ TEST_CASE("basic lookup")
 {
     auto ssm = lnav::small_string_map{};
 
-    ssm.insert("test"_frag, 123);
-    auto res = ssm.lookup("test"_frag);
+    ssm.insert("info"_frag, 123);
+    ssm.insert("304"_frag, 123);
+    ssm.insert("404"_frag, 123);
+    auto res = ssm.lookup("info"_frag);
     CHECK(res.has_value());
     CHECK(123 == res.value());