Reduce repo contention issues

If there were multiple processes (or threads) trying to write to
the repo, we were spending a lot of time in raw_spin_lock, resulting
in almost no forward progress. The new test case runs in about 8
seconds on my dev server, with a debug build, but didnt complete in
20 minutes when using the current release build of hhvm.

I also removed the 20 thread limit on the test runner, which
drops the time for SlowJit from about 1:45 to 1:30, and should
no longer cause repo-contention issues.

hphp/runtime/vm/repo.cpp

@@ -14,10 +14,10 @@
    +----------------------------------------------------------------------+
 */
 
+#include "runtime/vm/repo.h"
 #include "util/logger.h"
 #include "util/trace.h"
 #include "util/repo_schema.h"
-#include "runtime/vm/repo.h"
 
 namespace HPHP {
 namespace VM {
@@ -425,7 +425,7 @@ static int busyHandler(void* opaque, int nCalls) {
   Repo* repo UNUSED = static_cast<Repo*>(opaque);
   // yield to allow other threads access to the machine
   // spin-wait can starve other threads.
-  sched_yield();
+  usleep(1000 * nCalls);
   return 1; // Tell SQLite to retry.
 }
 

hphp/test/slow/program_functions/repo_contention.php

@@ -0,0 +1,34 @@
+<?php
+
+function foobar($i) {
+  usleep(500*1000);
+}
+
+function main($num) {
+  $cmd = file_get_contents("/proc/self/cmdline");
+  $cmd = str_replace("\000", " ", $cmd);
+  $start = microtime(true);
+
+  echo "Starting $num procs\n";
+  for ($i = 0; $i < $num; $i++) {
+    $handles[] = proc_open($cmd." $i", array(), $pipes);
+  }
+  while ($handles) {
+    foreach ($handles as $i => $handle) {
+      $status = proc_get_status($handle);
+      if (!$status['running']) {
+        proc_close($handle);
+        unset($handles[$i]);
+      }
+    }
+  }
+  echo "Done $num procs\n";
+  return microtime(true) - $start;
+}
+if (count($argv) > 1) {
+  foobar($argv[1]);
+} else {
+  $x10 = main(10);
+  $x200 = main(200);
+  echo $x10 * 20 < $x200 ? "Failed" : "Passed", "\n";
+}

hphp/test/slow/program_functions/repo_contention.php.expect

@@ -0,0 +1,5 @@
+Starting 10 procs
+Done 10 procs
+Starting 200 procs
+Done 200 procs
+Passed

hphp/test/verify

@@ -32,9 +32,6 @@ $opt_server = 0;
 $opt_port = 8080;
 $opt_home = ".";
 $opt_threads = `cat /proc/cpuinfo | grep '^processor' | wc -l`;
-if ($opt_threads > 20) {
-    $opt_threads = 20;
-}
 $opt_reduce = "";
 $opt_coverage = "";
 $opt_hhvm = "hhvm";