HPA: Hugification hysteresis.

We wait a while after deciding a huge extent should get hugified to see if it
gets purged before long.  This avoids hugifying extents that might shortly get
dehugified for purging.

Rename and use the hpa_dehugification_threshold option support code for this,
since it's now ignored.

include/jemalloc/internal/hpa_hooks.h

@@ -8,6 +8,7 @@ struct hpa_hooks_s {
 	void (*purge)(void *ptr, size_t size);
 	void (*hugify)(void *ptr, size_t size);
 	void (*dehugify)(void *ptr, size_t size);
+	void (*curtime)(nstime_t *r_time);
 };
 
 extern hpa_hooks_t hpa_hooks_default;

include/jemalloc/internal/hpa_opts.h

@@ -17,16 +17,13 @@ struct hpa_shard_opts_s {
 	 * any allocation request.
 	 */
 	size_t slab_max_alloc;
+
 	/*
 	 * When the number of active bytes in a hugepage is >=
 	 * hugification_threshold, we force hugify it.
 	 */
 	size_t hugification_threshold;
-	/*
-	 * When the number of dirty bytes in a hugepage is >=
-	 * dehugification_threshold, we force dehugify it.
-	 */
-	size_t dehugification_threshold;
+
 	/*
 	 * The HPA purges whenever the number of pages exceeds dirty_mult *
 	 * active_pages.  This may be set to (fxp_t)-1 to disable purging.
@@ -40,6 +37,12 @@ struct hpa_shard_opts_s {
 	 * ourselves for encapsulation purposes.
 	 */
 	bool deferral_allowed;
+
+	/*
+	 * How long a hugepage has to be a hugification candidate before it will
+	 * actually get hugified.
+	 */
+	uint64_t hugify_delay_ms;
 };
 
 #define HPA_SHARD_OPTS_DEFAULT {					\
@@ -47,8 +50,6 @@ struct hpa_shard_opts_s {
 	64 * 1024,							\
 	/* hugification_threshold */					\
 	HUGEPAGE * 95 / 100,						\
-	/* dehugification_threshold */					\
-	HUGEPAGE * 20 / 100,						\
 	/* dirty_mult */						\
 	FXP_INIT_PERCENT(25),						\
 	/*								\
@@ -58,7 +59,9 @@ struct hpa_shard_opts_s {
 	 * or by an hpa_shard_set_deferral_allowed call, so the value	\
 	 * we put here doesn't matter.					\
 	 */								\
-	false								\
+	false,								\
+	/* hugify_delay_ms */						\
+	10 * 1000							\
 }
 
 #endif /* JEMALLOC_INTERNAL_HPA_OPTS_H */

include/jemalloc/internal/hpdata.h

@@ -61,6 +61,8 @@ struct hpdata_s {
 
 	/* And with hugifying. */
 	bool h_hugify_allowed;
+	/* When we became a hugification candidate. */
+	nstime_t h_time_hugify_allowed;
 	bool h_in_psset_hugify_container;
 
 	/* Whether or not a purge or hugify is currently happening. */
@@ -175,8 +177,8 @@ hpdata_purge_allowed_get(const hpdata_t *hpdata) {
 
 static inline void
 hpdata_purge_allowed_set(hpdata_t *hpdata, bool purge_allowed) {
-	assert(purge_allowed == false || !hpdata->h_mid_purge);
-	hpdata->h_purge_allowed = purge_allowed;
+       assert(purge_allowed == false || !hpdata->h_mid_purge);
+       hpdata->h_purge_allowed = purge_allowed;
 }
 
 static inline bool
@@ -185,9 +187,20 @@ hpdata_hugify_allowed_get(const hpdata_t *hpdata) {
 }
 
 static inline void
-hpdata_hugify_allowed_set(hpdata_t *hpdata, bool hugify_allowed) {
-	assert(hugify_allowed == false || !hpdata->h_mid_hugify);
-	hpdata->h_hugify_allowed = hugify_allowed;
+hpdata_allow_hugify(hpdata_t *hpdata, nstime_t now) {
+	assert(!hpdata->h_mid_hugify);
+	hpdata->h_hugify_allowed = true;
+	hpdata->h_time_hugify_allowed = now;
+}
+
+static inline nstime_t
+hpdata_time_hugify_allowed(hpdata_t *hpdata) {
+	return hpdata->h_time_hugify_allowed;
+}
+
+static inline void
+hpdata_disallow_hugify(hpdata_t *hpdata) {
+	hpdata->h_hugify_allowed = false;
 }
 
 static inline bool