mdbx: add MDBX_opt_loose_limit runtime option.

More for https://github.com/erthink/libmdbx/issues/128

Change-Id: I298bf1eef24575df72615ee07e93f5073405e8e0

mdbx.h

@@ -1852,18 +1852,31 @@ enum MDBX_option_t {
    * process. Default is 262144, it is usually enough for most cases. */
   MDBX_opt_rp_augment_limit,
 
-  /** \brief Controls the in-process limit to grow a list of
+  /** \brief Controls the in-process limit to grow a cache of dirty
-   * pre-allocated/reserved dirty pages.
+   * pages for reuse in the current transaction.
+   *
+   * \details A 'dirty page' refers to a page that has been updated in memory
+   * only, the changes to a dirty page are not yet stored on disk.
+   * To reduce overhead, it is reasonable to release not all such pages
+   * immediately, but to leave some ones in cache for reuse in the current
+   * transaction.
+   *
+   * The `MDBX_opt_loose_limit` allows you to set a limit for such cache inside
+   * the current process. Should be in the range 0..255, default is 64. */
+  MDBX_opt_loose_limit,
+
+  /** \brief Controls the in-process limit of a pre-allocated memory items
+   * for dirty pages.
    *
    * \details A 'dirty page' refers to a page that has been updated in memory
    * only, the changes to a dirty page are not yet stored on disk.
    * Without \ref MDBX_WRITEMAP dirty pages are allocated from memory and
    * released when a transaction is committed. To reduce overhead, it is
-   * reasonable to release not all pages, but to leave some ones in reserve for
+   * reasonable to release not all ones, but to leave some allocations in
-   * reuse in the next transaction.
+   * reserve for reuse in the next transaction(s).
    *
-   * The `MDBX_opt_dp_reserve_limit` allows you to set a limit for such a
+   * The `MDBX_opt_dp_reserve_limit` allows you to set a limit for such reserve
-   * reserve inside the current process. Default is 1024. */
+   * inside the current process. Default is 1024. */
   MDBX_opt_dp_reserve_limit,
 
   /** \brief Controls the in-process limit of dirty pages

src/core.c

@@ -4287,9 +4287,14 @@ static int mdbx_page_loose(MDBX_txn *txn, MDBX_page *mp) {
         goto wrong_dirty;
 
       /* If suitable we can reuse it through loose list */
-      if (likely(npages == 1 &&
+      if (likely(
-                 txn->tw.loose_count < 9 /* TODO: options.threshold */
+              npages == 1 &&
-                 /* && txn->mt_next_pgno > pgno + TODO: txn->tw.loose_gap */)) {
+              txn->tw.loose_count < txn->mt_env->me_options.dp_loose_limit &&
+              (!MDBX_ENABLE_REFUND ||
+               /* skip pages near to the end in favor of compactification */
+               txn->mt_next_pgno >
+                   pgno + txn->mt_env->me_options.dp_loose_limit ||
+               txn->mt_next_pgno <= txn->mt_env->me_options.dp_loose_limit))) {
         mdbx_debug("loosen dirty page %" PRIaPGNO, pgno);
         mp->mp_flags = P_LOOSE | P_DIRTY;
         mp->mp_next = txn->tw.loose_pages;
@@ -9932,6 +9937,7 @@ __cold int mdbx_env_create(MDBX_env **penv) {
   env->me_options.spill_max_denominator = 8;
   env->me_options.spill_min_denominator = 8;
   env->me_options.spill_parent4child_denominator = 0;
+  env->me_options.dp_loose_limit = 64;
 
   int rc;
   const size_t os_psize = mdbx_syspagesize();
@@ -20475,6 +20481,12 @@ __cold int mdbx_env_set_option(MDBX_env *env, const MDBX_option_t option,
     env->me_options.spill_parent4child_denominator = (uint8_t)value;
     break;
 
+  case MDBX_opt_loose_limit:
+    if (unlikely(value > 255))
+      return MDBX_EINVAL;
+    env->me_options.dp_loose_limit = (uint8_t)value;
+    break;
+
   default:
     return MDBX_EINVAL;
   }
@@ -20538,6 +20550,10 @@ __cold int mdbx_env_get_option(const MDBX_env *env, const MDBX_option_t option,
     *value = env->me_options.spill_parent4child_denominator;
     break;
 
+  case MDBX_opt_loose_limit:
+    *value = env->me_options.dp_loose_limit;
+    break;
+
   default:
     return MDBX_EINVAL;
   }

src/internals.h

@@ -983,6 +983,7 @@ struct MDBX_env {
     unsigned rp_augment_limit;
     unsigned dp_limit;
     unsigned dp_initial;
+    uint8_t dp_loose_limit;
     uint8_t spill_max_denominator;
     uint8_t spill_min_denominator;
     uint8_t spill_parent4child_denominator;