From a58e71c571065330fa647ed0c4fe04f0b10b14d8 Mon Sep 17 00:00:00 2001
From: user <nobody>
Date: Wed, 22 Jun 2016 18:50:39 +0000
Subject: [PATCH] Add test for user mode context switches

90% more efficient than thread switching, nice.
---
 timectxsw_umode.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++
 timetctxsw2.c     |  2 +-
 2 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 timectxsw_umode.c

diff --git a/timectxsw_umode.c b/timectxsw_umode.c
new file mode 100644
index 0000000..889185d
--- /dev/null
+++ b/timectxsw_umode.c
@@ -0,0 +1,59 @@
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#include <ucontext.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <unistd.h>
+#include <assert.h>
+
+const int iterations = 500000;
+
+static inline long long unsigned time_ns(struct timespec* const ts) {
+  if (clock_gettime(CLOCK_REALTIME, ts)) {
+    exit(1);
+  }
+  return ((long long unsigned) ts->tv_sec) * 1000000000LLU
+    + (long long unsigned) ts->tv_nsec;
+}
+
+ucontext_t main_context, other_context;
+
+void run_other(void) {
+	int i;
+	for(i=0;i<iterations;++i) {
+		swapcontext(&other_context, &main_context);
+	}
+}
+
+int main(void) {
+  struct timespec ts;
+	static char other_stack[0x10000];
+	assert(getcontext(&other_context) >= 0);	
+	other_context.uc_stack.ss_sp = other_stack;
+	other_context.uc_stack.ss_size = sizeof(other_stack);
+	other_context.uc_link = &main_context;
+	makecontext(&other_context, run_other, 1, &main_context);
+
+  const long long unsigned start_ns = time_ns(&ts);
+  for (int i = 0; i < iterations; i++) {
+		assert(swapcontext(&main_context, &other_context) >= 0);
+	}
+  const long long unsigned delta = time_ns(&ts) - start_ns;
+
+  const int nswitches = iterations << 2;
+  printf("%i user mode context switches in %lluns (%.1fns/ctxsw)\n",
+         nswitches, delta, (delta / (float) nswitches));
+  return 0;
+}
diff --git a/timetctxsw2.c b/timetctxsw2.c
index a5cbc18..7bb3e23 100644
--- a/timetctxsw2.c
+++ b/timetctxsw2.c
@@ -57,7 +57,7 @@ int main(void) {
   long long unsigned delta = time_ns(&ts) - start_ns;
 
   const int nswitches = iterations << 2;
-  printf("%i  thread context switches in %lluns (%.1fns/ctxsw)\n",
+  printf("%i  thread sched_yield context switches in %lluns (%.1fns/ctxsw)\n",
          nswitches, delta, (delta / (float) nswitches));
   return 0;
 }