Patch file to add +sfwi tuning to an R15B01 Erlang VM. Should be present without the patch in R16B01+. It's possible that this patch will apply to other releases. Use at your own risk.

rg-sfwi-R15B01.patch

From 005be2e442074ea529ae0a188399eda553cb8783 Mon Sep 17 00:00:00 2001
From: evan <evan@basho.com>
Date: Tue, 18 Jun 2013 17:52:54 -0400
Subject: [PATCH] backport sfwi to R15B01 

---
 erts/emulator/beam/erl_init.c    |   16 +++++++
 erts/emulator/beam/erl_process.c |   89 ++++++++++++++++++++++++++++++++++++--
 erts/emulator/beam/erl_process.h |    9 ++++
 erts/etc/common/erlexec.c        |    1 +
 4 files changed, 112 insertions(+), 3 deletions(-)

diff --git a/erts/emulator/beam/erl_init.c b/erts/emulator/beam/erl_init.c
index ca4385d..4c2aef3 100644
--- a/erts/emulator/beam/erl_init.c
+++ b/erts/emulator/beam/erl_init.c
@@ -1283,6 +1283,22 @@ erl_start(int argc, char **argv)
     		("suggested scheduler thread stack size %d kilo words\n",
 			 erts_sched_thread_suggested_stack_size));
 	    }
+	    else if (has_prefix("fwi", sub_param)) {
+		long val;
+		arg = get_arg(sub_param+3, argv[i+1], &i);
+		errno = 0;
+		val = strtol(arg, NULL, 10);
+		if (errno != 0 || val < 0) {
+		    erts_fprintf(stderr,
+				 "bad scheduler forced wakeup "
+				 "interval %s\n",
+				 arg);
+		    erts_usage();
+		}
+#ifdef ERTS_SMP
+		erts_runq_supervision_interval = val;
+#endif
+	    }
 	    else {
 		erts_fprintf(stderr, "bad scheduling option %s\n", argv[i]);
 		erts_usage();
diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index 95d408f..fec9160 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -211,6 +211,10 @@ static erts_smp_atomic32_t function_calls;
 #ifdef ERTS_SMP
 static erts_smp_atomic32_t doing_sys_schedule;
 static erts_smp_atomic32_t no_empty_run_queues;
+long erts_runq_supervision_interval = 0;
+static ethr_event runq_supervision_event;
+static erts_tid_t runq_supervisor_tid;
+static erts_atomic_t runq_supervisor_sleeping;
 #else /* !ERTS_SMP */
 ErtsSchedulerData *erts_scheduler_data;
 #endif
@@ -1794,7 +1798,13 @@ empty_runq(ErtsRunQueue *rq)
 	 */
 	ASSERT(0 <= empty && empty < 2*erts_no_run_queues);
 #endif
-	erts_smp_atomic32_inc_relb(&no_empty_run_queues);
+	if (!erts_runq_supervision_interval)
+	    erts_smp_atomic32_inc_relb(&no_empty_run_queues);
+	else {
+	    erts_smp_atomic32_inc_mb(&no_empty_run_queues);
+	    if (erts_atomic_read_nob(&runq_supervisor_sleeping))
+		ethr_event_set(&runq_supervision_event);
+	}
     }
 }
 
@@ -1812,7 +1822,14 @@ non_empty_runq(ErtsRunQueue *rq)
 	 */
 	ASSERT(0 < empty && empty <= 2*erts_no_run_queues);
 #endif
-	erts_smp_atomic32_dec_relb(&no_empty_run_queues);
+	if (!erts_runq_supervision_interval)
+	    erts_smp_atomic32_dec_relb(&no_empty_run_queues);
+	else {
+	    erts_aint32_t no;
+	    no = erts_smp_atomic32_dec_read_mb(&no_empty_run_queues);
+	    if (no > 0 && erts_atomic_read_nob(&runq_supervisor_sleeping))
+		ethr_event_set(&runq_supervision_event);
+	}
     }
 }
 
@@ -2412,7 +2429,6 @@ try_inc_no_active_runqs(int active)
     return 0;
 }
 
-
 static ERTS_INLINE int
 chk_wake_sched(ErtsRunQueue *crq, int ix, int activate)
 {
@@ -3875,6 +3891,57 @@ erts_get_scheduler_data(void)
     return (ErtsSchedulerData *) erts_tsd_get(sched_data_key);
 }
 
+#ifdef ERTS_SMP
+
+static int
+no_runqs_to_supervise(void)
+{
+    int used;
+    erts_aint32_t nerq = erts_smp_atomic32_read_acqb(&no_empty_run_queues);
+    if (nerq <= 0)
+	return 0;
+    get_no_runqs(NULL, &used);
+    if (nerq >= used)
+	return 0;
+    return used;
+}
+
+static void *
+runq_supervisor(void *unused)
+{
+    while (1) {
+	int ix, no_rqs;
+
+	erts_milli_sleep(erts_runq_supervision_interval);
+	no_rqs = no_runqs_to_supervise();
+	if (!no_rqs) {
+	    erts_atomic_set_nob(&runq_supervisor_sleeping, 1);
+	    while (1) {
+		ethr_event_reset(&runq_supervision_event);
+		no_rqs = no_runqs_to_supervise();
+		if (no_rqs) {
+		    erts_atomic_set_nob(&runq_supervisor_sleeping, 0);
+		    break;
+		}
+		ethr_event_wait(&runq_supervision_event);
+	    }
+	}
+
+	for (ix = 0; ix < no_rqs; ix++) {
+	    ErtsRunQueue *rq = ERTS_RUNQ_IX(ix);
+	    if (ERTS_RUNQ_IFLGS_GET(rq) & ERTS_RUNQ_IFLG_NONEMPTY) {
+		erts_smp_runq_lock(rq);
+		if (rq->len != 0) { 
+		    wake_scheduler_on_empty_runq(rq); /* forced wakeup... */
+		}
+		erts_smp_runq_unlock(rq);
+	    }
+	}
+    }
+    return NULL;
+}
+
+#endif
 #endif
 
 static int remove_proc_from_runq(ErtsRunQueue *rq, Process *p, int to_inactive);
@@ -4851,6 +4918,22 @@ erts_start_schedulers(void)
     ethr_thr_opts opts = ETHR_THR_OPTS_DEFAULT_INITER;
 
     opts.detached = 1;
+
+#ifdef ERTS_SMP
+    if (erts_runq_supervision_interval) {
+	opts.suggested_stack_size = 16;
+	erts_atomic_init_nob(&runq_supervisor_sleeping, 0);
+	if (0 != ethr_event_init(&runq_supervision_event))
+	    erl_exit(1, "Failed to create run-queue supervision event\n");
+	if (0 != ethr_thr_create(&runq_supervisor_tid,
+				 runq_supervisor,
+				 NULL,
+				 &opts))
+	    erl_exit(1, "Failed to create run-queue supervision thread\n");
+
+    }
+#endif
+
     opts.suggested_stack_size = erts_sched_thread_suggested_stack_size;
 
     if (wanted < 1)
diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index cff0783..e1f4f1e 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -148,6 +148,8 @@ extern int erts_sched_thread_suggested_stack_size;
   (((Uint32) 1) << (ERTS_RUNQ_FLG_BASE2 + 3))
 #define ERTS_RUNQ_FLG_INACTIVE \
   (((Uint32) 1) << (ERTS_RUNQ_FLG_BASE2 + 4))
+#define ERTS_RUNQ_FLG_NONEMPTY \
+  (((Uint32) 1) << (ERTS_RUNQ_FLG_BASE2 + 5))
 
 #define ERTS_RUNQ_FLGS_MIGRATION_QMASKS	\
   (ERTS_RUNQ_FLGS_EMIGRATE_QMASK	\
@@ -186,6 +188,9 @@ extern int erts_sched_thread_suggested_stack_size;
 #define ERTS_UNSET_RUNQ_FLG_EVACUATE(FLGS, PRIO) \
   ((FLGS) &= ~ERTS_RUNQ_FLG_EVACUATE((PRIO)))
 
+#define ERTS_RUNQ_IFLGS_GET(RQ)						\
+    ((Uint32) erts_smp_atomic32_read_acqb(&(RQ)->info_flags))
+
 #define ERTS_RUNQ_IFLG_SUSPENDED		(((erts_aint32_t) 1) << 0)
 #define ERTS_RUNQ_IFLG_NONEMPTY			(((erts_aint32_t) 1) << 1)
 
@@ -370,6 +375,10 @@ struct ErtsRunQueue_ {
     } ports;
 };
 
+#ifdef ERTS_SMP
+extern long erts_runq_supervision_interval;
+#endif
+
 typedef union {
     ErtsRunQueue runq;
     char align[ERTS_ALC_CACHE_LINE_ALIGN_SIZE(sizeof(ErtsRunQueue))];
diff --git a/erts/etc/common/erlexec.c b/erts/etc/common/erlexec.c
index 19b3bb8..ac4e509 100644
--- a/erts/etc/common/erlexec.c
+++ b/erts/etc/common/erlexec.c
@@ -123,6 +123,7 @@ static char *pluss_val_switches[] = {
     "bt",
     "cl",
     "ct",
+    "fwi",
     "wt",
     "ss",
     NULL
-- 
1.7.9.5