[jira] [Commented] (HAWQ-1210) Documentation regarding usage of libhdfs3 in concurrent environment
[
https://issues.apache.org/jira/browse/HAWQ-1210?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=15742603#comment-15742603
]
William Forson commented on HAWQ-1210:
--
Hi Zhanwei Wang,
Unfortunately, I don't think I will have the bandwidth to debug this further
for at least a few weeks. So far, I've been using libhdfs3 as a black-box
component (i.e. I've really only looked at {{hdfs.h}} and build logic), so I
will have to get myself up to speed on the basic organization of the codebase,
etc.
However, since there is a decent chance I will be using libhdfs3 as a
production dependency, in a multi-threaded environment, I would definitely like
to understand what is going on here. So I will try to look into this as soon as
I have the time.
Thanks!
> Documentation regarding usage of libhdfs3 in concurrent environment
> ---
>
> Key: HAWQ-1210
> URL: https://issues.apache.org/jira/browse/HAWQ-1210
> Project: Apache HAWQ
> Issue Type: Bug
> Components: libhdfs
>Reporter: William Forson
>Assignee: Lei Chang
> Attachments: hdfs_fs_concurrent_test.cpp
>
>
> Hi,
> I've been using libhdfs3 in a single-threaded environment for several months
> now, without any problems. However, as soon as I tried using the library
> concurrently from multiple threads: hello, segfaults.
> Although the source of these segfaults is annoyingly subtle, I've managed to
> isolate it to a relatively small block of my code that does nothing
> interesting aside from using libhdfs3 to download a single hdfs file.
> To be clear: I assume that the mistake here is mine -- that is, that I am
> using your library incorrectly. However, I have been unable to find any
> documentation as to how the libhdfs3 API _should_ be used in a multi-threaded
> environment. I initially interpreted this to mean, "go to town, it's all more
> or less thread-safe", but I am now questioning that interpretation.
> So, I have a question, and a request.
> Question: Are there any known, non-obvious concurrency gotchas regarding the
> usage of libhdfs3 (or whatever it's currently called)?
> Request: Could you please add some documentation, to the README and/or
> hdfs.h, regarding usage in a concurrent environment? (ideally, such notes
> would annotate individual components of the API in hdfs.h, but if the answer
> to my question above is, "No", then this could perhaps be a single sentence
> in the README which affirmatively states that the library is generally safe
> for concurrent usage without additional/explicit synchronization -- anything
> would be better than nothing :))
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[jira] [Commented] (HAWQ-1210) Documentation regarding usage of libhdfs3 in concurrent environment
[
https://issues.apache.org/jira/browse/HAWQ-1210?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=15738862#comment-15738862
]
Zhanwei Wang commented on HAWQ-1210:
Hi [~wdf]
Thanks for your report. As the design, sharing FileSystem between threads
should be safe, but sharing Input/OutputStream is not. If you find anything
against this design, it should be consider as bug.
There is concurrent read test case in current code
{{test/function/TestInputStream.cpp}}, but I think it is far away from enough.
Any contribution will be welcomed.
You may want to take a look at the current concurrent test case to find out
what we have missed.
> Documentation regarding usage of libhdfs3 in concurrent environment
> ---
>
> Key: HAWQ-1210
> URL: https://issues.apache.org/jira/browse/HAWQ-1210
> Project: Apache HAWQ
> Issue Type: Bug
> Components: libhdfs
>Reporter: William Forson
>Assignee: Lei Chang
> Attachments: hdfs_fs_concurrent_test.cpp
>
>
> Hi,
> I've been using libhdfs3 in a single-threaded environment for several months
> now, without any problems. However, as soon as I tried using the library
> concurrently from multiple threads: hello, segfaults.
> Although the source of these segfaults is annoyingly subtle, I've managed to
> isolate it to a relatively small block of my code that does nothing
> interesting aside from using libhdfs3 to download a single hdfs file.
> To be clear: I assume that the mistake here is mine -- that is, that I am
> using your library incorrectly. However, I have been unable to find any
> documentation as to how the libhdfs3 API _should_ be used in a multi-threaded
> environment. I initially interpreted this to mean, "go to town, it's all more
> or less thread-safe", but I am now questioning that interpretation.
> So, I have a question, and a request.
> Question: Are there any known, non-obvious concurrency gotchas regarding the
> usage of libhdfs3 (or whatever it's currently called)?
> Request: Could you please add some documentation, to the README and/or
> hdfs.h, regarding usage in a concurrent environment? (ideally, such notes
> would annotate individual components of the API in hdfs.h, but if the answer
> to my question above is, "No", then this could perhaps be a single sentence
> in the README which affirmatively states that the library is generally safe
> for concurrent usage without additional/explicit synchronization -- anything
> would be better than nothing :))
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[jira] [Commented] (HAWQ-1210) Documentation regarding usage of libhdfs3 in concurrent environment
[
https://issues.apache.org/jira/browse/HAWQ-1210?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=15738813#comment-15738813
]
William Forson commented on HAWQ-1210:
--
Finally: I should note that I have some convincing (ish) evidence that using
libhdfs3 concurrently in a multi-threaded environment has caused memory
corruption. The manifestation is quite subtle -- so far, it has always been a
segfault in unrelated library code. However, I have managed to eliminate these
segfaults completely (AFAICT thus far, pending more exhaustive testing) by
guarding an {{hdfsRead}} invocation (which appears in only one place in my
code) with a mutex.
Additionally, although the introduction of this mutex does not eliminate
libhdfs3-related errors from the valgrind DRD output altogether, it does
significantly reduce the number of errors reported (e.g. by a factor of 5-10).
> Documentation regarding usage of libhdfs3 in concurrent environment
> ---
>
> Key: HAWQ-1210
> URL: https://issues.apache.org/jira/browse/HAWQ-1210
> Project: Apache HAWQ
> Issue Type: Bug
> Components: libhdfs
>Reporter: William Forson
>Assignee: Lei Chang
> Attachments: hdfs_fs_concurrent_test.cpp
>
>
> Hi,
> I've been using libhdfs3 in a single-threaded environment for several months
> now, without any problems. However, as soon as I tried using the library
> concurrently from multiple threads: hello, segfaults.
> Although the source of these segfaults is annoyingly subtle, I've managed to
> isolate it to a relatively small block of my code that does nothing
> interesting aside from using libhdfs3 to download a single hdfs file.
> To be clear: I assume that the mistake here is mine -- that is, that I am
> using your library incorrectly. However, I have been unable to find any
> documentation as to how the libhdfs3 API _should_ be used in a multi-threaded
> environment. I initially interpreted this to mean, "go to town, it's all more
> or less thread-safe", but I am now questioning that interpretation.
> So, I have a question, and a request.
> Question: Are there any known, non-obvious concurrency gotchas regarding the
> usage of libhdfs3 (or whatever it's currently called)?
> Request: Could you please add some documentation, to the README and/or
> hdfs.h, regarding usage in a concurrent environment? (ideally, such notes
> would annotate individual components of the API in hdfs.h, but if the answer
> to my question above is, "No", then this could perhaps be a single sentence
> in the README which affirmatively states that the library is generally safe
> for concurrent usage without additional/explicit synchronization -- anything
> would be better than nothing :))
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[jira] [Commented] (HAWQ-1210) Documentation regarding usage of libhdfs3 in concurrent environment
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https://issues.apache.org/jira/browse/HAWQ-1210?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel=15736565#comment-15736565
]
William Forson commented on HAWQ-1210:
--
btw, to make that question _a bit_ more specific, I am particularly interested
in knowing how {{hdfsFS}} handles returned by
[this|https://github.com/apache/incubator-hawq/blob/master/depends/libhdfs3/src/client/hdfs.h#L151]
function should be used. For instance:
a) is {{hdfsFS}} construction expensive or cheap?
b) can a single {{hdfsFS}} handle be safely used for concurrent {{hdfsRead}}
operations?
c) can distinct {{hdfsFS}} handles be safely used for concurrent {{hdfsRead}}
operations (i.e. if each handle is only being used for a single read operation
at any given time)?
> Documentation regarding usage of libhdfs3 in concurrent environment
> ---
>
> Key: HAWQ-1210
> URL: https://issues.apache.org/jira/browse/HAWQ-1210
> Project: Apache HAWQ
> Issue Type: Bug
> Components: libhdfs
>Reporter: William Forson
>Assignee: Lei Chang
>
> Hi,
> I've been using libhdfs3 in a single-threaded environment for several months
> now, without any problems. However, as soon as I tried using the library
> concurrently from multiple threads: hello, segfaults.
> Although the source of these segfaults is annoyingly subtle, I've managed to
> isolate it to a relatively small block of my code that does nothing
> interesting aside from using libhdfs3 to download a single hdfs file.
> To be clear: I assume that the mistake here is mine -- that is, that I am
> using your library incorrectly. However, I have been unable to find any
> documentation as to how the libhdfs3 API _should_ be used in a multi-threaded
> environment. I initially interpreted this to mean, "go to town, it's all more
> or less thread-safe", but I am now questioning that interpretation.
> So, I have a question, and a request.
> Question: Are there any known, non-obvious concurrency gotchas regarding the
> usage of libhdfs3 (or whatever it's currently called)?
> Request: Could you please add some documentation, to the README and/or
> hdfs.h, regarding usage in a concurrent environment? (ideally, such notes
> would annotate individual components of the API in hdfs.h, but if the answer
> to my question above is, "No", then this could perhaps be a single sentence
> in the README which affirmatively states that the library is generally safe
> for concurrent usage without additional/explicit synchronization -- anything
> would be better than nothing :))
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