应用内存升高原因排查

#起因

最近一个部署了 go 应用的服务器出现了 OOM 的现象，内存占用过高。

#原因

通过 Pyroscope 分析得出是因为 Minio 的 go sdk 中的 PutObject 函数占用了大量的内存。 Pyroscope 是什么，前面的文章已经介绍过了，这里就不过多介绍了。

接下来我们通过查看相关的源码来查看是什么原因。

// PutObject creates an object in a bucket.
//
// You must have WRITE permissions on a bucket to create an object.
//
//  - For size smaller than 16MiB PutObject automatically does a
//    single atomic PUT operation.
//
//  - For size larger than 16MiB PutObject automatically does a
//    multipart upload operation.
//
//  - For size input as -1 PutObject does a multipart Put operation
//    until input stream reaches EOF. Maximum object size that can
//    be uploaded through this operation will be 5TiB.
//
//    WARNING: Passing down '-1' will use memory and these cannot
//    be reused for best outcomes for PutObject(), pass the size always.
//
// NOTE: Upon errors during upload multipart operation is entirely aborted.
func (c *Client) PutObject(ctx context.Context, bucketName, objectName string, reader io.Reader, objectSize int64,
	opts PutObjectOptions,
) (info UploadInfo, err error) {
	if objectSize < 0 && opts.DisableMultipart {
		return UploadInfo{}, errors.New("object size must be provided with disable multipart upload")
	}

	err = opts.validate()
	if err != nil {
		return UploadInfo{}, err
	}

	return c.putObjectCommon(ctx, bucketName, objectName, reader, objectSize, opts)
}

从方法的注释可以看出，当传递的大小为 -1 时，会进行多次 put 操作，直到输入流结束。 多次 put 操作的最大大小为 5TiB， 并且不能重用内存，导致占用大量内存。

接下来继续深入，我们看看这个函数的源码。

func OptimalPartInfo(objectSize int64, configuredPartSize uint64) (totalPartsCount int, partSize int64, lastPartSize int64, err error) {
	// object size is '-1' set it to 5TiB.
	var unknownSize bool
	if objectSize == -1 {
		unknownSize = true
		objectSize = maxMultipartPutObjectSize
	}

	// object size is larger than supported maximum.
	if objectSize > maxMultipartPutObjectSize {
		err = errEntityTooLarge(objectSize, maxMultipartPutObjectSize, "", "")
		return
	}

	var partSizeFlt float64
	if configuredPartSize > 0 {
		if int64(configuredPartSize) > objectSize {
			err = errEntityTooLarge(int64(configuredPartSize), objectSize, "", "")
			return
		}

		if !unknownSize {
			if objectSize > (int64(configuredPartSize) * maxPartsCount) {
				err = errInvalidArgument("Part size * max_parts(10000) is lesser than input objectSize.")
				return
			}
		}

		if configuredPartSize < absMinPartSize {
			err = errInvalidArgument("Input part size is smaller than allowed minimum of 5MiB.")
			return
		}

		if configuredPartSize > maxPartSize {
			err = errInvalidArgument("Input part size is bigger than allowed maximum of 5GiB.")
			return
		}

		partSizeFlt = float64(configuredPartSize)
		if unknownSize {
			// If input has unknown size and part size is configured
			// keep it to maximum allowed as per 10000 parts.
			objectSize = int64(configuredPartSize) * maxPartsCount
		}
	} else {
		configuredPartSize = minPartSize
		// Use floats for part size for all calculations to avoid
		// overflows during float64 to int64 conversions.
		partSizeFlt = float64(objectSize / maxPartsCount)
		partSizeFlt = math.Ceil(partSizeFlt/float64(configuredPartSize)) * float64(configuredPartSize)
	}

	// Total parts count.
	totalPartsCount = int(math.Ceil(float64(objectSize) / partSizeFlt))
	// Part size.
	partSize = int64(partSizeFlt)
	// Last part size.
	lastPartSize = objectSize - int64(totalPartsCount-1)*partSize
	return totalPartsCount, partSize, lastPartSize, nil
}

从函数中可以看出，这个函数的作用是计算出最佳的分片大小，并且计算出总的分片数量。 当大小为 -1 时，会使用最大的 5TiB。可以看出当没有指定对象大小时，每次会使用较大的内存。

#小结

在我们使用 MinIO sdk 时，在使用 PutObject 方法时，最好指定要上传的对象的大小，避免造成内存资源的浪费。