Count context switches on appending writes

diff --git a/fsqual.cc b/fsqual.cc
new file mode 100644 (file)
index 0000000..3bd3292
--- /dev/null
+++ b/fsqual.cc
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2016 ScyllaDB
+ */
+
+
+#include <libaio.h>
+#include <sys/time.h>
+#include <sys/resource.h>
+#include <fcntl.h>
+#include <iostream>
+#include <unistd.h>
+#include <cstdlib>
+
+template <typename Counter, typename Func>
+auto
+with_ctxsw_counting(Counter& counter, Func&& func) {
+    struct count_guard {
+        Counter& counter;
+        count_guard(Counter& counter) : counter(counter) {
+            counter -= nvcsw();
+        }
+        ~count_guard() {
+            counter += nvcsw();
+        }
+        static Counter nvcsw() {
+            struct rusage usage;
+            getrusage(RUSAGE_THREAD, &usage);
+            return usage.ru_nvcsw;
+        }
+    };
+    count_guard g(counter);
+    return func();
+}
+
+int main(int ac, char** av) {
+    io_context_t ioctx = {};
+    io_setup(1, &ioctx);
+    auto fname = "fsqual.tmp";
+    int fd = open(fname, O_CREAT|O_EXCL|O_RDWR|O_DIRECT, 0600);
+    unlink(fname);
+    auto nr = 1000;
+    auto bufsize = 4096;
+    auto ctxsw = 0;
+    auto buf = aligned_alloc(4096, 4096);
+    for (int i = 0; i < nr; ++i) {
+        struct iocb cmd;
+        io_prep_pwrite(&cmd, fd, buf, bufsize, bufsize*i);
+        struct iocb* cmds[1] = { &cmd };
+        with_ctxsw_counting(ctxsw, [&] {
+            io_submit(ioctx, 1, cmds);
+        });
+        struct io_event ioev;
+        io_getevents(ioctx, 1, 1, &ioev, nullptr);
+    }
+    std::cout << "context switch per appending io: " << float(ctxsw) / nr << "\n";
+}