Steve Loughran updated HADOOP-14937:
    Parent Issue: HADOOP-15220  (was: HADOOP-14831)

> 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: Sub-task
>          Components: fs/s3
>    Affects Versions: 3.0.0-beta1
>            Reporter: Steven Rand
>            Assignee: Steven Rand
>            Priority: Major
>         Attachments: yjp_threads.png
> 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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