AWS S3分段上传不会并行加载

Question

我的上传时间大约需要30秒才能上传20MB的文件，所以我切换到分段上传，上传20MB文件仍需要30秒左右。

我觉得它与这个功能有关：

  function createMultipartUpload(s3, multiPartParams, buffer) {
    partNum = 0;
    startTime = new Date();
    var deferred = $q.defer();
    numPartsLeft = Math.ceil(buffer.byteLength / partSize);

    s3.createMultipartUpload(multiPartParams, function(mpErr, multipart){
      if (mpErr) { console.log('Error!', mpErr); return; }
      console.log("Got upload ID", multipart.UploadId);

      // Grab each partSize chunk and upload it as a part
      for (var rangeStart = 0; rangeStart < buffer.byteLength; rangeStart += partSize) {
        (function(rangeStart) {
          console.log("PART ");
          console.log(rangeStart);
          partNum++;
          var end = Math.min(rangeStart + partSize, buffer.byteLength),
              partParams = {
                Body: buffer.slice(rangeStart, end),
                Bucket: multiPartParams.Bucket,
                Key: multiPartParams.Key,
                PartNumber: String(partNum),
                UploadId: multipart.UploadId
              };

          // Send a single part
          console.log('Uploading part: #', partParams.PartNumber, ', Range start:', rangeStart);
          uploadPart(s3, multipart, partParams).then(function() {
            console.log('1');
          });
          var delta = (new Date() - startTime) / 1000;
          console.log('This part took', delta, 'seconds');
          if(rangeStart > buffer.byteLength - partSize) {
            deferred.resolve();
          }
        }(rangeStart));
      }
    });
    return deferred.promise;
  }

当它下载到uploadPart时，它不会异步执行：

如果你把这几秒加起来就可以得到30秒左右。如果它是并行的，它应该需要大约7.5s。

我不确定为什么会发生这种情况，因为函数是在一个循环内部调用的，因为它应该是异步的吗？

以下是完整的代码：

var startTime;
  var partNum = 0;
  var partSize = 1024 * 1024 * 5; // Minimum 5MB per chunk (except the last part) http://docs.aws.amazon.com/AmazonS3/latest/API/mpUploadComplete.html
  var numPartsLeft = 0
  var maxUploadTries = 3;
  var multipartMap = {
      Parts: []
  };

  function completeMultipartUpload(s3, doneParams) {
    var deferred = $q.defer();
    s3.completeMultipartUpload(doneParams, function(err, data) {
      if (err) {
        console.log("An error occurred while completing the multipart upload");
        console.log(err);
        deferred.reject();
      } else {
        var delta = (new Date() - startTime) / 1000;
        console.log('Completed upload in', delta, 'seconds');
        console.log('Final upload data:', data);
        deferred.resolve();
      }
    });
    return deferred.promise;
  }

  function uploadPart(s3, multipart, partParams, tryNum) {
    console.log("UPLOADING PART...");
    var deferred = $q.defer();
    var tryNum = tryNum || 1;
    s3.uploadPart(partParams, function(multiErr, mData) {
      if (multiErr){
        console.log('multiErr, upload part error:', multiErr);
        if (tryNum < maxUploadTries) {
          console.log('Retrying upload of part: #', partParams.PartNumber)
          return uploadPart(s3, multipart, partParams, tryNum + 1);
        } else {
          console.log('Failed uploading part: #', partParams.PartNumber)
        }
        return;
      }
      multipartMap.Parts[this.request.params.PartNumber - 1] = {
        ETag: mData.ETag,
        PartNumber: Number(this.request.params.PartNumber)
      };
      console.log("Completed part", this.request.params.PartNumber);
      console.log('mData', mData);
      if (--numPartsLeft > 0) deferred.resolve(); // complete only when all parts uploaded

      var doneParams = {
        Bucket: partParams.Bucket,
        Key: partParams.Key,
        MultipartUpload: multipartMap,
        UploadId: multipart.UploadId
      };

      console.log("Completing upload...");
      completeMultipartUpload(s3, doneParams).then(function() {
        deferred.resolve();
      });
    });
    return deferred.promise;
  }

  // Multipart
  function createMultipartUpload(s3, multiPartParams, buffer) {
    partNum = 0;
    startTime = new Date();
    var deferred = $q.defer();
    numPartsLeft = Math.ceil(buffer.byteLength / partSize);

    s3.createMultipartUpload(multiPartParams, function(mpErr, multipart){
      if (mpErr) { console.log('Error!', mpErr); return; }
      console.log("Got upload ID", multipart.UploadId);

      // Grab each partSize chunk and upload it as a part
      for (var rangeStart = 0; rangeStart < buffer.byteLength; rangeStart += partSize) {
        (function(rangeStart) {
          console.log("PART ");
          console.log(rangeStart);
          partNum++;
          var end = Math.min(rangeStart + partSize, buffer.byteLength),
              partParams = {
                Body: buffer.slice(rangeStart, end),
                Bucket: multiPartParams.Bucket,
                Key: multiPartParams.Key,
                PartNumber: String(partNum),
                UploadId: multipart.UploadId
              };

          // Send a single part
          console.log('Uploading part: #', partParams.PartNumber, ', Range start:', rangeStart);
          uploadPart(s3, multipart, partParams).then(function() {
            console.log('1');
          });
          var delta = (new Date() - startTime) / 1000;
          console.log('This part took', delta, 'seconds');
          if(rangeStart > buffer.byteLength - partSize) {
            deferred.resolve();
          }
        }(rangeStart));
      }
    });
    return deferred.promise;
  }

代码来自这个人：https://gist.github.com/sevastos/5804803