mdbx: add MDBX_opt_spill_parent4child_denominator runtime option.

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

Change-Id: If1549c3855eb59c107eb115e6dbca515e81644f9

mdbx.h

@@ -1917,6 +1917,29 @@ enum MDBX_option_t {
    * bottom. Default is 8, i.e. at least the 1/8 of the current dirty pages
    * should be spilled when reached the condition described above. */
   MDBX_opt_spill_min_denominator,
+
+  /** \brief Controls the in-process how part of the parent transaction dirty
+   * pages will be spilled while start each child transaction.
+   *
+   * \details The `MDBX_opt_spill_parent4child_denominator` defines the
+   * denominator to determine how part of parent transaction dirty pages will be
+   * spilled explicitly while start each child transaction.
+   * Exactly `pages_to_spill = dirty_pages / N`,
+   * where `N` is the value set by `MDBX_opt_spill_parent4child_denominator`.
+   *
+   * For a stack of nested transactions each dirty page could be spilled only
+   * once, and parent's dirty pages couldn't be spilled while child
+   * transaction(s) are running. Therefore a child transaction could reach
+   * \ref MDBX_TXN_FULL when parent(s) transaction has  spilled too less (and
+   * child reach the limit of dirty pages), either when parent(s) has spilled
+   * too more (since child can't spill already spilled pages). So there is no
+   * universal golden ratio.
+   *
+   * Should be in the range 0..255, where zero means no explicit spilling will
+   * be performed during starting nested transactions.
+   * Default is 0, i.e. by default no spilling performed during starting nested
+   * transactions, that correspond historically behaviour. */
+  MDBX_opt_spill_parent4child_denominator,
 };
 #ifndef __cplusplus
 /** \ingroup c_settings */

src/core.c

@@ -6893,6 +6893,15 @@ int mdbx_txn_begin_ex(MDBX_env *env, MDBX_txn *parent, MDBX_txn_flags_t flags,
                       MDBX_TXN_RDONLY | MDBX_WRITEMAP | MDBX_TXN_BLOCKED);
     if (unlikely(rc != MDBX_SUCCESS))
       return rc;
+
+    if (env->me_options.spill_parent4child_denominator) {
+      /* Spill dirty-pages of parent to provide dirtyroom for child txn */
+      rc = mdbx_txn_spill(parent, nullptr,
+                          parent->tw.dirtylist->length /
+                              env->me_options.spill_parent4child_denominator);
+      if (unlikely(rc != MDBX_SUCCESS))
+        return rc;
+    }
     mdbx_tassert(parent, mdbx_audit_ex(parent, 0, false) == 0);
 
     flags |= parent->mt_flags & (MDBX_TXN_RW_BEGIN_FLAGS | MDBX_TXN_SPILLS);
@@ -9922,6 +9931,7 @@ __cold int mdbx_env_create(MDBX_env **penv) {
     env->me_options.dp_initial = env->me_options.dp_limit;
   env->me_options.spill_max_denominator = 8;
   env->me_options.spill_min_denominator = 8;
+  env->me_options.spill_parent4child_denominator = 0;
 
   int rc;
   const size_t os_psize = mdbx_syspagesize();
@@ -20462,6 +20472,11 @@ __cold int mdbx_env_set_option(MDBX_env *env, const MDBX_option_t option,
       return MDBX_EINVAL;
     env->me_options.spill_min_denominator = (uint8_t)value;
     break;
+  case MDBX_opt_spill_parent4child_denominator:
+    if (unlikely(value > 255))
+      return MDBX_EINVAL;
+    env->me_options.spill_parent4child_denominator = (uint8_t)value;
+    break;
 
   default:
     return MDBX_EINVAL;
@@ -20522,6 +20537,9 @@ __cold int mdbx_env_get_option(const MDBX_env *env, const MDBX_option_t option,
   case MDBX_opt_spill_min_denominator:
     *value = env->me_options.spill_min_denominator;
     break;
+  case MDBX_opt_spill_parent4child_denominator:
+    *value = env->me_options.spill_parent4child_denominator;
+    break;
 
   default:
     return MDBX_EINVAL;

src/internals.h

@@ -985,6 +985,7 @@ struct MDBX_env {
     unsigned dp_initial;
     uint8_t spill_max_denominator;
     uint8_t spill_min_denominator;
+    uint8_t spill_parent4child_denominator;
   } me_options;
   struct {
 #if MDBX_LOCKING > 0