update on single doc is slower than 2.6 (both WT and mmapv1)

Test

• standalone mongod
• test with following benchRun script:

  var run = function() {

  var d = db.getSiblingDB("test");

  d.foo.insert({_id:1,a:0});

  res = benchRun( {

  ops : [{

  ns : "test.foo",

  op : "update" ,

  query: {_id: 1},

  update : { $inc: {a: 1} } ,

  w: 1,

  writeCmd : true }],

  seconds : 60,

  totals : true,

  writeCmd : true,

  w: 1,

  parallel : 40

  });

  return res

  }

  var s = db.serverStatus();

  var v = db.serverBuildInfo();

  print("Version: " + v.version)

  print("git version: " + v.gitVersion)

  if ( s.storageEngine !== undefined) {

  print("storageEngine: " + db.serverStatus().storageEngine.name)

  }

  print("Update RPS: " + run().update);

Results:

• the throughput from both WT and mmapv1 is slower than that of 2.6.7

  test	rc9 + wt	rc9 + mmapv1	2.6.7 + mmapv0
  contenedUpdate (update with 40 thread)	23082	22061	25975
  update with 1 thread	8947	10849	10816

Observation

• WT update is slow, this probably related how update is done with WT. With single thread update, it is 17.3% slower than 2.6.7, and 11.2% with 40 thread update
• mmapv1 is slower when there is contention, and in par with mmapv0 (single thread update)