Add ability to set per jemalloc arena dirty and muzzy decay values

This commit adds couple of functions to change "dirty_decay_ms" and
"muzzy_decay_ms" settings on arenas associated with memory contexts.

lib/isc/include/isc/mem.h

@@ -201,13 +201,38 @@ isc__mem_create(isc_mem_t **_ISC_MEM_FLARG);
 void
 isc__mem_create_arena(isc_mem_t **_ISC_MEM_FLARG);
 /*!<
- * \brief Create a memory context that routs all its operations to a dedicated
- * jemalloc arena (when available).
+ * \brief Create a memory context that routs all its operations to a
+ * dedicated jemalloc arena (when available). When jemalloc is not
+ * available, the function is, effectively, an alias to
+ * isc_mem_create().
  *
  * Requires:
  * mctxp != NULL && *mctxp == NULL */
 /*@}*/
 
+isc_result_t
+isc_mem_arena_set_muzzy_decay_ms(isc_mem_t *mctx, const ssize_t decay_ms);
+
+isc_result_t
+isc_mem_arena_set_dirty_decay_ms(isc_mem_t *mctx, const ssize_t decay_ms);
+/*!<
+ * \brief These two functions set the given parameters on the
+ * jemalloc arena associated with the memory context (if there is
+ * one). When jemalloc is not available, these are no-op.
+ *
+ * NOTE: The "muzzy_decay_ms" and "dirty_decay_ms" are the most common
+ * parameters to adjust when the defaults do not work well (per the
+ * official jemalloc tuning guide:
+ * https://github.com/jemalloc/jemalloc/blob/dev/TUNING.md).
+ *
+ * Requires:
+ * mctx - a valid memory context.
+ */
+/*@}*/
+
+void
+isc_mem_attach(isc_mem_t *, isc_mem_t **);
+
 /*@{*/
 void
 isc_mem_attach(isc_mem_t *, isc_mem_t **);

lib/isc/mem.c

@@ -1640,6 +1640,55 @@ isc__mem_create_arena(isc_mem_t **mctxp FLARG) {
 #endif /* ISC_MEM_TRACKLINES */
 }
 
+#ifdef JEMALLOC_API_SUPPORTED
+static bool
+jemalloc_set_ssize_value(const char *valname, ssize_t newval) {
+	int ret;
+
+	ret = mallctl(valname, NULL, NULL, &newval, sizeof(newval));
+	return (ret == 0);
+}
+#endif /* JEMALLOC_API_SUPPORTED */
+
+static isc_result_t
+mem_set_arena_ssize_value(isc_mem_t *mctx, const char *arena_valname,
+			  const ssize_t newval) {
+	REQUIRE(VALID_CONTEXT(mctx));
+#ifdef JEMALLOC_API_SUPPORTED
+	bool ret;
+	char buf[256] = { 0 };
+
+	if (mctx->jemalloc_arena == ISC_MEM_ILLEGAL_ARENA) {
+		return (ISC_R_UNEXPECTED);
+	}
+
+	(void)snprintf(buf, sizeof(buf), "arena.%u.%s", mctx->jemalloc_arena,
+		       arena_valname);
+
+	ret = jemalloc_set_ssize_value(buf, newval);
+
+	if (!ret) {
+		return (ISC_R_FAILURE);
+	}
+
+	return (ISC_R_SUCCESS);
+#else
+	UNUSED(arena_valname);
+	UNUSED(newval);
+	return (ISC_R_NOTIMPLEMENTED);
+#endif
+}
+
+isc_result_t
+isc_mem_arena_set_muzzy_decay_ms(isc_mem_t *mctx, const ssize_t decay_ms) {
+	return (mem_set_arena_ssize_value(mctx, "muzzy_decay_ms", decay_ms));
+}
+
+isc_result_t
+isc_mem_arena_set_dirty_decay_ms(isc_mem_t *mctx, const ssize_t decay_ms) {
+	return (mem_set_arena_ssize_value(mctx, "dirty_decay_ms", decay_ms));
+}
+
 void
 isc__mem_printactive(isc_mem_t *ctx, FILE *file) {
 #if ISC_MEM_TRACKLINES