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From Kay Ousterhout <>
Subject Re: File JIRAs for all flaky test failures
Date Tue, 28 Mar 2017 04:47:14 GMT
Following up on this with a renewed plea to file JIRAs when you see flaky
tests.  I did a quick skim of the PR builder, and there were 17 times in
the last week when a flaky test led to a Jenkins failure, and someone
re-ran the tests without filing (or updating) a JIRA (my apologies to
anyone who was incorrectly added here):

cloud-fan (4)
gatorsmile (4)
wzhfy (4)
holdenk (1)
kunalkhamar (1)
hyukjinkwon (1)
scrapcodes (1)
srowen (1)

Are you on this list?  It's not too late to look at the test that failed
and file (or update) the appropriate JIRA.

If you weren't convinced by my last email, here are some reasons to file a

(0) Flaky tests are not always broken tests -- sometimes the underlying
code is broken. (e.g., SPARK-19803
<>, SPARK-19988
<>, SPARK-19072

(1) Before a flaky test gets fixed, some human needs to file a JIRA.  The
person who sees the flaky test via the PR builder is best suited to do
this, because you already had to look at which test failed (to make sure it
wasn't related to your change) and you know the test is flaky (because
you're re-running it assuming it will succeed), at which point it takes <1
minute to file a JIRA

(2) Related to the above, existing automation is not sufficient.  Josh's
tool is very useful for debugging flaky tests [1] but it does not yet
automatically file JIRAs.  Many recent flaky tests might have shown up on
the nifty interesting recent failures dashboard, but they didn't get
noticed until they were failing for more than a week and dropped from that
dashboard.  One recent test, SPARK-19990
<>, was causing *every*
Maven build to fail for over > 1 week, but was only noticed when someone
filed a JIRA as a result of a flaky PR test.

(3) JIRAs result in helpful people fixing the tests!  If you're interested
in doing this, the flaky test label
a good place to start.  Many thanks to folks who have recently helped with
filing and fixing flaky tests: Sital Kedia, Song Jun, Xiao Li, Shubham
Chopra, Shixiong Zhu, Genmao Yu, Imran Rashid, and I'm sure many more (this
list is based on a quick JIRA skim).



[1] You can create a URL using the "suite_name" and optionally "test_name"
GET parameters in Josh's app to investigate a flaky test; e.g., to see how
often the "hive bucketing is not supported" test in ShowCreateTableSuite
has been failing:
patient -- it takes a minute and sometimes a re-load to work).

On Thu, Feb 16, 2017 at 9:22 AM, Reynold Xin <> wrote:

> Josh's tool should give enough signal there already. I don't think we need
> some manual process to document them. If you want to work on those that'd
> be great. I bet you will get a lot of love because all developers hate
> flaky tests.
> On Thu, Feb 16, 2017 at 6:19 PM, Saikat Kanjilal <>
> wrote:
>> I am specifically suggesting documenting a list of the the flaky tests
>> and fixing them, that's all.  To organize the effort I suggested tackling
>> this by module.  Your second sentence is what I was trying to gauge from
>> the community before putting anymore effort into this.
>> ------------------------------
>> *From:* Sean Owen <>
>> *Sent:* Thursday, February 16, 2017 8:45 AM
>> *To:* Saikat Kanjilal;
>> *Subject:* Re: File JIRAs for all flaky test failures
>> I'm not sure what you're specifically suggesting. Of course flaky tests
>> are bad and they should be fixed, and people do. Yes, some are pretty hard
>> to fix because they are rarely reproducible if at all. If you want to fix,
>> fix; there's nothing more to it.
>> I don't perceive flaky tests to be a significant problem. It has gone
>> from bad to occasional over the past year in my anecdotal experience.
>> On Thu, Feb 16, 2017 at 4:26 PM Saikat Kanjilal <>
>> wrote:
>>> I'd just like to follow up again on this thread, should we devote some
>>> energy to fixing unit tests based on module, there wasn't much interest in
>>> this last time but given the nature of this thread I'd be willing to deep
>>> dive into this again with some help.
>>> ------------------------------
>>> *From:* Saikat Kanjilal <>
>>> *Sent:* Wednesday, February 15, 2017 6:12 PM
>>> *To:* Josh Rosen
>>> *Cc:* Armin Braun; Kay Ousterhout;
>>> *Subject:* Re: File JIRAs for all flaky test failures
>>> The issue was not with a lack of tooling, I used the url you are
>>> describing below to drill down to the exact test failure/stack trace, the
>>> problem was that my builds would work like a charm locally but fail with
>>> these errors on Jenkins, this was the whole challenge in fixing the unit
>>> tests, it was rare (if ever) where I would be able to replicate test
>>> failures locally.
>>> Sent from my iPhone
>>> On Feb 15, 2017, at 5:40 PM, Josh Rosen <>
>>> wrote:
>>> A useful tool for investigating test flakiness is my Jenkins Test
>>> Explorer service, running at
>>> This has some useful timeline views for debugging flaky builds. For
>>> instance, at
>>> maven-hadoop-2.6 (may be slow to load) you can see this chart:
>>> Here, each column represents a test
>>> run and each row represents a test which failed at least once over the
>>> displayed time period.
>>> In that linked example screenshot you'll notice that a few columns have
>>> grey squares indicating that tests were skipped but lack any red squares to
>>> indicate test failures. This usually indicates that the build failed due to
>>> a problem other than an individual test failure. For example, I clicked
>>> into one of those builds and found that one test suite failed in test setup
>>> because the previous suite had not properly cleaned up its SparkContext
>>> (I'll file a JIRA for this).
>>> You can click through the interface to drill down to reports on
>>> individual builds, tests, suites, etc. As an example of an individual
>>> test's detail page,
>>> name=org.apache.spark.rdd.LocalCheckpointSuite&test_name
>>> =missing+checkpoint+block+fails+with+informative+message shows the
>>> patterns of flakiness in a streaming checkpoint test.
>>> Finally, there's an experimental "interesting new test failures" report
>>> which tries to surface tests which have started failing very recently:
>>> Specifically, entries
>>> in this feed are test failures which a) occurred in the last week, b) were
>>> not part of a build which had 20 or more failed tests, and c) were not
>>> observed to fail in during the previous week (i.e. no failures from [2
>>> weeks ago, 1 week ago)), and d) which represent the first time that the
>>> test failed this week (i.e. a test case will appear at most once in the
>>> results list). I've also exposed this as an RSS feed at
>>> On Wed, Feb 15, 2017 at 12:51 PM Saikat Kanjilal <>
>>> wrote:
>>> I would recommend we just open JIRA's for unit tests based on module
>>> (core/ml/sql etc) and we fix this one module at a time, this at least keeps
>>> the number of unit tests needing fixing down to a manageable number.
>>> ------------------------------
>>> *From:* Armin Braun <>
>>> *Sent:* Wednesday, February 15, 2017 12:48 PM
>>> *To:* Saikat Kanjilal
>>> *Cc:* Kay Ousterhout;
>>> *Subject:* Re: File JIRAs for all flaky test failures
>>> I think one thing that is contributing to this a lot too is the general
>>> issue of the tests taking up a lot of file descriptors (10k+ if I run them
>>> on a standard Debian machine).
>>> There are a few suits that contribute to this in particular like
>>> `org.apache.spark.ExecutorAllocationManagerSuite` which, like a few
>>> others, appears to consume a lot of fds.
>>> Wouldn't it make sense to open JIRAs about those and actively try to
>>> reduce the resource consumption of these tests?
>>> Seems to me these can cause a lot of unpredictable behavior (making the
>>> reason for flaky tests hard to identify especially when there's timeouts
>>> etc. involved) + they make it prohibitively expensive for many to test
>>> locally imo.
>>> On Wed, Feb 15, 2017 at 9:24 PM, Saikat Kanjilal <>
>>> wrote:
>>> I was working on something to address this a while ago
>>> but the difficulty in
>>> testing locally made things a lot more complicated to fix for each of the
>>> unit tests, should we resurface this JIRA again, I would whole heartedly
>>> agree with the flakiness assessment of the unit tests.
>>> [SPARK-9487] Use the same num. worker threads in Scala ...
>>> <>
>>> In Python we use `local[4]` for unit tests, while in Scala/Java we use
>>> `local[2]` and `local` for some unit tests in SQL, MLLib, and other
>>> components. If the ...
>>> ------------------------------
>>> *From:* Kay Ousterhout <>
>>> *Sent:* Wednesday, February 15, 2017 12:10 PM
>>> *To:*
>>> *Subject:* File JIRAs for all flaky test failures
>>> Hi all,
>>> I've noticed the Spark tests getting increasingly flaky -- it seems more
>>> common than not now that the tests need to be re-run at least once on PRs
>>> before they pass.  This is both annoying and problematic because it makes
>>> it harder to tell when a PR is introducing new flakiness.
>>> To try to clean this up, I'd propose filing a JIRA *every time* Jenkins
>>> fails on a PR (for a reason unrelated to the PR).  Just provide a quick
>>> description of the failure -- e.g., "Flaky test: DagSchedulerSuite" or
>>> "Tests failed because 250m timeout expired", a link to the failed build,
>>> and include the "Tests" component.  If there's already a JIRA for the
>>> issue, just comment with a link to the latest failure.  I know folks don't
>>> always have time to track down why a test failed, but this it at least
>>> helpful to someone else who, later on, is trying to diagnose when the issue
>>> started to find the problematic code / test.
>>> If this seems like too high overhead, feel free to suggest alternative
>>> ways to make the tests less flaky!
>>> -Kay

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