[jira] [Created] (HADOOP-14937) initial part uploads seem to block unnecessarily in S3ABlockOutputStream

["Steven Rand (JIRA)" <ji...@apache.org>]

Steven Rand created HADOOP-14937:
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             Summary: initial part uploads seem to block unnecessarily in S3ABlockOutputStream
                 Key: HADOOP-14937
                 URL: https://issues.apache.org/jira/browse/HADOOP-14937
             Project: Hadoop Common
          Issue Type: Bug
          Components: fs/s3
    Affects Versions: 3.0.0-beta1
            Reporter: Steven Rand


From looking at a YourKit snapshot of an FsShell process running a {{hadoop fs -put file:///... s3a://...}}, it seems that the first part in the multipart upload doesn't begin to upload until n of the {{s3a-transfer-shared-pool}} threads are able to start uploading, where n is the value of {{fs.s3a.fast.upload.active.blocks}}.

To hopefully clarify a bit, the series of events that I expected to see with {{fs.s3a.fast.upload.active.blocks}} set to 4 is:

1.  An amount of data equal to {{fs.s3a.multipart.size}} is buffered into off-heap memory (I have {{fs.s3a.fast.upload.buffer = bytebuffer}}).
2. As soon as that happens, a thread begins to upload that part. Meanwhile, the main thread continues to buffer data into off-heap memory.
3. Once another part has been buffered into off-heap memory, a separate thread uploads that part, and so on.

Whereas what I think the YK snapshot shows happening is:

1. An amount of data equal to {{fs.s3a.multipart.size}} * 4 is buffered into off-heap memory.
2. Four threads start to upload one part each at the same time.

I've attached a picture of the "Threads" tab to show what I mean. Basically the times at which the first four {{s3a-transfer-shared-pool}} threads start to upload are roughly the same, whereas I would've expected them to be more staggered.

I'm actually not sure whether this is the expected behavior or not, so feel free to close if this doesn't come as a surprise to anyone.

For some context, I've been trying to get a sense for roughly which values of {{fs.s3a.multipart.size}} perform the best at different file sizes. One thing that I found confusing is that a part size of 5 MB seems to outperform a part size of 64 MB up until files that are upwards of about 500 MB in size. This seems odd, since each {{uploadPart}} call is its own HTTP request, and I would've expected the overhead of those to become costly at small part sizes. My suspicion is that with 4 concurrent part uploads and 64 MB blocks, we have to wait until 256 MB are buffered before we can start uploading, while with 5 MB blocks we can start uploading as soon as we buffer 20 MB, and that's what gives the smaller parts the advantage for smaller files.

I'm happy to submit a patch if this is in fact a problem, but wanted to check to make sure I'm not just misunderstanding something.



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