diff --git a/cpp/include/kvikio/defaults.hpp b/cpp/include/kvikio/defaults.hpp
index 200f248d79..d214649da7 100644
--- a/cpp/include/kvikio/defaults.hpp
+++ b/cpp/include/kvikio/defaults.hpp
@@ -18,7 +18,10 @@
 
 #include <cstddef>
 #include <cstdlib>
+#include <functional>
 #include <initializer_list>
+#include <map>
+#include <optional>
 #include <sstream>
 #include <stdexcept>
 #include <string>
@@ -35,23 +38,98 @@
  */
 namespace kvikio {
 
+namespace detail {
 template <typename T>
-T getenv_or(std::string_view env_var_name, T default_val)
+std::optional<T> from_string(std::string const& env_val)
 {
+  std::stringstream ss(env_val);
+  T converted_val;
+  ss >> converted_val;
+
+  if (!ss.fail()) { return converted_val; }
+
+  // An exception: for string, empty value is allowed
+  if constexpr (std::is_same_v<T, std::string>) { return std::optional<std::string>{""}; }
+
+  // For all other cases, return std::nullopt
+  return {};
+}
+
+template <typename T>
+[[nodiscard]] T process_single_env_var(
+  std::string_view env_var_name,
+  T default_val,
+  std::function<std::optional<T>(std::string const&)> user_callback,
+  std::map<T, std::vector<std::string>> const& dictionary,
+  bool case_sensitive)
+{
+  // Step 0: If the name does not exist, use default value
   auto const* env_val = std::getenv(env_var_name.data());
   if (env_val == nullptr) { return default_val; }
 
-  std::stringstream sstream(env_val);
-  T converted_val;
-  sstream >> converted_val;
+  // Step 1: try to convert to type T
+  std::optional<T> converted_val;
+  if (user_callback) {
+    converted_val = std::invoke(user_callback, env_val);
+    if (converted_val.has_value()) { return converted_val.value(); }
+  }
+
+  // Step 2: look up in the user-provided dictionary
+  std::string str{env_val};
+  if (!dictionary.empty()) {
+    // Convert to lowercase
+    if (!case_sensitive) {
+      // Special considerations regarding the case conversion:
+      // - std::tolower() is not an addressable function. Passing it to std::transform() as
+      //   a function pointer, if the compile turns out successful, causes the program behavior
+      //   "unspecified (possibly ill-formed)", hence the lambda. ::tolower() is addressable
+      //   and does not have this problem, but the following item still applies.
+      // - To avoid UB in std::tolower() or ::tolower(), the character must be cast to unsigned
+      // char.
+      std::transform(
+        str.begin(), str.end(), str.begin(), [](unsigned char c) { return std::tolower(c); });
+    }
 
-  if constexpr (!std::is_same_v<T, std::string>) {
-    KVIKIO_EXPECT(!sstream.fail(),
-                  "unknown config value " + std::string{env_var_name} + "=" + std::string{env_val},
-                  std::invalid_argument);
+    // Trim whitespaces
+    std::stringstream trimmer;
+    trimmer << str;
+    str.clear();
+    trimmer >> str;
+
+    // Convert the dictionary to an easier format
+    // Example:
+    // dictionary is (v_1, {a, b}), (v_2, {d})
+    // then flat_dictionary is (a, v_1), (b, v_1), (d, v_2)
+    // and there must be no duplicate among a, b, d
+    std::map<std::string, T> flat_dictionary;
+    for (auto const& [dst, src_list] : dictionary) {
+      for (auto const& src : src_list) {
+        if (auto const it = flat_dictionary.find(src); it == flat_dictionary.end()) {
+          flat_dictionary[src] = dst;
+        } else {
+          KVIKIO_FAIL("Duplicate environment variable values.");
+        }
+      }
+    }
+
+    // Look up in the dictionary
+    if (auto it = flat_dictionary.find(str); it != flat_dictionary.end()) {
+      return flat_dictionary[str];
+    }
   }
 
-  return converted_val;
+  KVIKIO_FAIL("unknown config value " + std::string{env_var_name} + "=" + str,
+              std::invalid_argument);
+
+  return {};
+}
+}  // namespace detail
+
+template <typename T>
+T getenv_or(std::string_view env_var_name, T default_val)
+{
+  return detail::process_single_env_var(
+    env_var_name, default_val, detail::from_string<T>, {}, false);
 }
 
 template <>
@@ -86,34 +164,42 @@ std::vector<int> getenv_or(std::string_view env_var_name, std::vector<int> defau
  */
 template <typename T>
 std::tuple<std::string_view, T, bool> getenv_or(
-  std::initializer_list<std::string_view> env_var_names, T default_val)
+  std::initializer_list<std::string_view> env_var_names,
+  T default_val,
+  std::function<std::optional<T>(std::string const&)> conversion_callback = detail::from_string<T>,
+  std::map<T, std::vector<std::string>> dictionary                        = {},
+  bool case_sensitive                                                     = false)
 {
   KVIKIO_EXPECT(env_var_names.size() > 0,
                 "`env_var_names` must contain at least one environment variable name.",
                 std::invalid_argument);
   std::string_view env_name_target;
-  std::string_view env_val_target;
+  std::string_view env_val_str_target;
+  T env_val_target;
 
-  for (auto const& env_var_name : env_var_names) {
-    auto const* env_val = std::getenv(env_var_name.data());
-    if (env_val == nullptr) { continue; }
+  for (auto const& current_env_var_name : env_var_names) {
+    auto const* current_env_val_str = std::getenv(current_env_var_name.data());
+    if (current_env_val_str == nullptr) { continue; }
 
-    if (!env_name_target.empty() && env_val_target != env_val) {
+    auto current_env_val = detail::process_single_env_var(
+      current_env_var_name, default_val, conversion_callback, dictionary, case_sensitive);
+
+    if (!env_name_target.empty() && env_val_target != current_env_val) {
       std::stringstream ss;
-      ss << "Environment variable " << env_var_name << " (" << env_val
-         << ") has already been set by its alias " << env_name_target << " (" << env_val_target
+      ss << "Environment variable " << current_env_var_name << " (" << current_env_val_str
+         << ") has already been set by its alias " << env_name_target << " (" << env_val_str_target
          << ") with a different value.";
       KVIKIO_FAIL(ss.str(), std::invalid_argument);
     }
 
-    env_name_target = env_var_name;
-    env_val_target  = env_val;
+    env_name_target    = current_env_var_name;
+    env_val_target     = current_env_val;
+    env_val_str_target = current_env_val_str;
   }
 
   if (env_name_target.empty()) { return {env_name_target, default_val, false}; }
 
-  auto res = getenv_or<T>(env_name_target, default_val);
-  return {env_name_target, res, true};
+  return {env_name_target, env_val_target, true};
 }
 
 /**
@@ -130,6 +216,7 @@ class defaults {
   std::size_t _http_max_attempts;
   long _http_timeout;
   std::vector<int> _http_status_codes;
+  std::size_t _mmap_task_size;
 
   static unsigned int get_num_threads_from_env();
 
@@ -367,6 +454,10 @@ class defaults {
    * @param status_codes The HTTP status codes to retry.
    */
   static void set_http_status_codes(std::vector<int> status_codes);
+
+  [[nodiscard]] static std::size_t mmap_task_size();
+
+  static void set_mmap_task_size(std::size_t nbytes);
 };
 
 }  // namespace kvikio
diff --git a/cpp/src/compat_mode.cpp b/cpp/src/compat_mode.cpp
index 78a96c66be..016c969796 100644
--- a/cpp/src/compat_mode.cpp
+++ b/cpp/src/compat_mode.cpp
@@ -14,7 +14,6 @@
  * limitations under the License.
  */
 
-#include <algorithm>
 #include <cassert>
 #include <stdexcept>
 
@@ -27,29 +26,6 @@
 
 namespace kvikio {
 
-namespace detail {
-CompatMode parse_compat_mode_str(std::string_view compat_mode_str)
-{
-  KVIKIO_NVTX_FUNC_RANGE();
-  // Convert to lowercase
-  std::string tmp{compat_mode_str};
-  std::transform(
-    tmp.begin(), tmp.end(), tmp.begin(), [](unsigned char c) { return std::tolower(c); });
-
-  if (tmp == "on" || tmp == "true" || tmp == "yes" || tmp == "1") {
-    return CompatMode::ON;
-  } else if (tmp == "off" || tmp == "false" || tmp == "no" || tmp == "0") {
-    return CompatMode::OFF;
-  } else if (tmp == "auto") {
-    return CompatMode::AUTO;
-  } else {
-    KVIKIO_FAIL("Unknown compatibility mode: " + std::string{tmp}, std::invalid_argument);
-  }
-  return {};
-}
-
-}  // namespace detail
-
 CompatMode CompatModeManager::infer_compat_mode_if_auto(CompatMode compat_mode) noexcept
 {
   KVIKIO_NVTX_FUNC_RANGE();
diff --git a/cpp/src/defaults.cpp b/cpp/src/defaults.cpp
index 24a4de0898..4e585072bf 100644
--- a/cpp/src/defaults.cpp
+++ b/cpp/src/defaults.cpp
@@ -16,10 +16,9 @@
 
 #include <cstddef>
 #include <cstdlib>
-#include <regex>
-#include <sstream>
 #include <stdexcept>
 #include <string>
+#include <string_view>
 
 #include <BS_thread_pool.hpp>
 
@@ -28,62 +27,45 @@
 #include <kvikio/error.hpp>
 #include <kvikio/http_status_codes.hpp>
 #include <kvikio/shim/cufile.hpp>
-#include <string_view>
 
 namespace kvikio {
+
 template <>
 bool getenv_or(std::string_view env_var_name, bool default_val)
 {
   KVIKIO_NVTX_FUNC_RANGE();
-  auto const* env_val = std::getenv(env_var_name.data());
-  if (env_val == nullptr) { return default_val; }
-  try {
-    // Try parsing `env_var_name` as a integer
-    return static_cast<bool>(std::stoi(env_val));
-  } catch (std::invalid_argument const&) {
-  }
-  // Convert to lowercase
-  std::string str{env_val};
-  // Special considerations regarding the case conversion:
-  // - std::tolower() is not an addressable function. Passing it to std::transform() as
-  //   a function pointer, if the compile turns out successful, causes the program behavior
-  //   "unspecified (possibly ill-formed)", hence the lambda. ::tolower() is addressable
-  //   and does not have this problem, but the following item still applies.
-  // - To avoid UB in std::tolower() or ::tolower(), the character must be cast to unsigned char.
-  std::transform(
-    str.begin(), str.end(), str.begin(), [](unsigned char c) { return std::tolower(c); });
-  // Trim whitespaces
-  std::stringstream trimmer;
-  trimmer << str;
-  str.clear();
-  trimmer >> str;
-  // Match value
-  if (str == "true" || str == "on" || str == "yes") { return true; }
-  if (str == "false" || str == "off" || str == "no") { return false; }
-  KVIKIO_FAIL("unknown config value " + std::string{env_var_name} + "=" + std::string{env_val},
-              std::invalid_argument);
-  return {};
+  return detail::process_single_env_var<bool>(
+    env_var_name,
+    default_val,
+    {},
+    {{true, {"true", "on", "yes", "1"}}, {false, {"false", "off", "no", "0"}}},
+    false);
 }
 
 template <>
 CompatMode getenv_or(std::string_view env_var_name, CompatMode default_val)
 {
   KVIKIO_NVTX_FUNC_RANGE();
-  auto* env_val = std::getenv(env_var_name.data());
-  if (env_val == nullptr) { return default_val; }
-  return detail::parse_compat_mode_str(env_val);
+  return detail::process_single_env_var<CompatMode>(env_var_name,
+                                                    default_val,
+                                                    {},
+                                                    {{CompatMode::ON, {"true", "on", "yes", "1"}},
+                                                     {CompatMode::OFF, {"false", "off", "no", "0"}},
+                                                     {CompatMode::AUTO, {"auto"}}},
+                                                    false);
 }
 
 template <>
 std::vector<int> getenv_or(std::string_view env_var_name, std::vector<int> default_val)
 {
   KVIKIO_NVTX_FUNC_RANGE();
-  auto* const env_val = std::getenv(env_var_name.data());
-  if (env_val == nullptr) { return std::move(default_val); }
-  std::string const int_str(env_val);
-  if (int_str.empty()) { return std::move(default_val); }
-
-  return detail::parse_http_status_codes(env_var_name, int_str);
+  auto callback = [=](std::string const& env_val) -> std::optional<std::vector<int>> {
+    std::string const int_str(env_val);
+    if (int_str.empty()) { return default_val; }
+    return detail::parse_http_status_codes(env_var_name, int_str);
+  };
+  return detail::process_single_env_var<std::vector<int>>(
+    env_var_name, default_val, callback, {}, false);
 }
 
 unsigned int defaults::get_num_threads_from_env()
diff --git a/cpp/tests/test_defaults.cpp b/cpp/tests/test_defaults.cpp
index 9c283d658a..14449817cf 100644
--- a/cpp/tests/test_defaults.cpp
+++ b/cpp/tests/test_defaults.cpp
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <map>
 #include <stdexcept>
 #include <vector>
 
@@ -27,38 +28,38 @@
 using ::testing::HasSubstr;
 using ::testing::ThrowsMessage;
 
-TEST(DefaultsTest, parse_compat_mode_str)
-{
-  {
-    std::vector<std::string> inputs{
-      "ON", "on", "On", "TRUE", "true", "True", "YES", "yes", "Yes", "1"};
-    for (auto const& input : inputs) {
-      EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::ON);
-    }
-  }
+// TEST(DefaultsTest, parse_compat_mode_str)
+// {
+//   {
+//     std::vector<std::string> inputs{
+//       "ON", "on", "On", "TRUE", "true", "True", "YES", "yes", "Yes", "1"};
+//     for (auto const& input : inputs) {
+//       EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::ON);
+//     }
+//   }
 
-  {
-    std::vector<std::string> inputs{
-      "OFF", "off", "oFf", "FALSE", "false", "False", "NO", "no", "No", "0"};
-    for (auto const& input : inputs) {
-      EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::OFF);
-    }
-  }
+//   {
+//     std::vector<std::string> inputs{
+//       "OFF", "off", "oFf", "FALSE", "false", "False", "NO", "no", "No", "0"};
+//     for (auto const& input : inputs) {
+//       EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::OFF);
+//     }
+//   }
 
-  {
-    std::vector<std::string> inputs{"AUTO", "auto", "aUtO"};
-    for (auto const& input : inputs) {
-      EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::AUTO);
-    }
-  }
+//   {
+//     std::vector<std::string> inputs{"AUTO", "auto", "aUtO"};
+//     for (auto const& input : inputs) {
+//       EXPECT_EQ(kvikio::detail::parse_compat_mode_str(input), kvikio::CompatMode::AUTO);
+//     }
+//   }
 
-  {
-    std::vector<std::string> inputs{"", "invalidOption", "11", "*&^Yes"};
-    for (auto const& input : inputs) {
-      EXPECT_THROW(kvikio::detail::parse_compat_mode_str(input), std::invalid_argument);
-    }
-  }
-}
+//   {
+//     std::vector<std::string> inputs{"", "invalidOption", "11", "*&^Yes"};
+//     for (auto const& input : inputs) {
+//       EXPECT_THROW(kvikio::detail::parse_compat_mode_str(input), std::invalid_argument);
+//     }
+//   }
+// }
 
 TEST(DefaultsTest, parse_http_status_codes)
 {
@@ -104,7 +105,7 @@ TEST(DefaultsTest, alias_for_getenv_or)
       {{"KVIKIO_TEST_ALIAS_1", ""}, {"KVIKIO_TEST_ALIAS_2", ""}}};
     EXPECT_THAT(
       [=] { kvikio::getenv_or({"KVIKIO_TEST_ALIAS_1", "KVIKIO_TEST_ALIAS_2"}, 123); },
-      ThrowsMessage<std::invalid_argument>(HasSubstr("unknown config value KVIKIO_TEST_ALIAS_2=")));
+      ThrowsMessage<std::invalid_argument>(HasSubstr("unknown config value KVIKIO_TEST_ALIAS_1=")));
   }
 
   // String env var has an empty value
@@ -188,34 +189,42 @@ TEST(DefaultsTest, alias_for_getenv_or)
   }
 
   // Special type: bool
+  std::map<bool, std::vector<std::string>> dictionary_bool{{true, {"true", "on", "yes", "1"}},
+                                                           {false, {"false", "off", "no", "0"}}};
   {
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "yes"}}};
-    auto const [env_var_name, result, has_found] = kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, false);
+    auto const [env_var_name, result, has_found] =
+      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, false, {}, dictionary_bool);
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     EXPECT_TRUE(result);
     EXPECT_TRUE(has_found);
   }
   {
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "OFF"}}};
-    auto const [env_var_name, result, has_found] = kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, false);
+    auto const [env_var_name, result, has_found] =
+      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, false, {}, dictionary_bool);
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     EXPECT_FALSE(result);
     EXPECT_TRUE(has_found);
   }
 
   // Special type: CompatMode
+  std::map<kvikio::CompatMode, std::vector<std::string>> dictionary_compat_mode{
+    {kvikio::CompatMode::ON, {"true", "on", "yes", "1"}},
+    {kvikio::CompatMode::OFF, {"false", "off", "no", "0"}},
+    {kvikio::CompatMode::AUTO, {"auto"}}};
   {
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "yes"}}};
-    auto const [env_var_name, result, has_found] =
-      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::AUTO);
+    auto const [env_var_name, result, has_found] = kvikio::getenv_or(
+      {"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::AUTO, {}, dictionary_compat_mode);
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     EXPECT_EQ(result, kvikio::CompatMode::ON);
     EXPECT_TRUE(has_found);
   }
   {
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "FALSE"}}};
-    auto const [env_var_name, result, has_found] =
-      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::AUTO);
+    auto const [env_var_name, result, has_found] = kvikio::getenv_or(
+      {"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::AUTO, {}, dictionary_compat_mode);
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     EXPECT_EQ(result, kvikio::CompatMode::OFF);
     EXPECT_TRUE(has_found);
@@ -223,7 +232,7 @@ TEST(DefaultsTest, alias_for_getenv_or)
   {
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "aUtO"}}};
     auto const [env_var_name, result, has_found] =
-      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::ON);
+      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, kvikio::CompatMode::ON, {}, dictionary_compat_mode);
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     EXPECT_EQ(result, kvikio::CompatMode::AUTO);
     EXPECT_TRUE(has_found);
@@ -231,9 +240,15 @@ TEST(DefaultsTest, alias_for_getenv_or)
 
   // Special type: std::vector<int>
   {
+    std::vector<int> default_val{111, 112, 113};
+    auto callback = [=](std::string const& env_val) -> std::optional<std::vector<int>> {
+      std::string const int_str(env_val);
+      if (int_str.empty()) { return default_val; }
+      return kvikio::detail::parse_http_status_codes("KVIKIO_TEST_ALIAS", int_str);
+    };
     kvikio::test::EnvVarContext env_var_ctx{{{"KVIKIO_TEST_ALIAS", "109, 108, 107"}}};
     auto const [env_var_name, result, has_found] =
-      kvikio::getenv_or({"KVIKIO_TEST_ALIAS"}, std::vector<int>{111, 112, 113});
+      kvikio::getenv_or<std::vector<int>>({"KVIKIO_TEST_ALIAS"}, default_val, callback, {});
     EXPECT_EQ(env_var_name, std::string_view{"KVIKIO_TEST_ALIAS"});
     std::vector<int> expected{109, 108, 107};
     EXPECT_EQ(result, expected);