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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-16284) KMS Cache Miss Storm
Date Fri, 10 May 2019 13:07:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-16284?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16837257#comment-16837257
] 

Daryn Sharp commented on HADOOP-16284:
--------------------------------------

Wow, that's really bad.  IIRC, no content to map is symptomatic of java's HTTPUrlConnection
being completely broken.  It will internal retry failed POSTs or PUTs (forget which) and
the retry doesn't resend the original payload.  Of course it can't buffer the payload w/o
a high risk of OOM so not sure why sun thought the retry was a good idea.

The bug is guaranteed to happen under load if jetty's low resource monitor is enabled.  I'd
recommend completely disabling it and increasing the client side timeout to mitigate the customer
impact.

> KMS Cache Miss Storm
> --------------------
>
>                 Key: HADOOP-16284
>                 URL: https://issues.apache.org/jira/browse/HADOOP-16284
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: kms
>    Affects Versions: 2.6.0
>         Environment: CDH 5.13.1, Kerberized, Cloudera Keytrustee Server
>            Reporter: Wei-Chiu Chuang
>            Priority: Major
>         Attachments: 4 kms, no KTS patch.png
>
>
> We recently stumble upon a performance issue with KMS, where occasionally it exhibited
"No content to map" error (this cluster ran an old version that doesn't have HADOOP-14841)
and jobs crashed. *We bumped the number of KMSes from 2 to 4, and situation went even worse.*
> Later, we realized this cluster had a few hundred encryption zones and a few hundred
encryption keys. This is pretty unusual because most of the deployments known to us has at
most a dozen keys. So in terms of number of keys, this cluster is 1-2 order of magnitude
higher than any one else.
> The high number of encryption keys in creases the likelihood of key cache miss in KMS.
In Cloudera's setup, each cache miss forces KMS to sync with its backend, the Cloudera Keytrustee
Server. Plus the high number of KMSes amplifies the latency, effectively causing a [cache
miss storm|https://en.wikipedia.org/wiki/Cache_stampede].
> We were able to reproduce this issue with KMS-o-meter (HDFS-14312) - I will come up with
a better name later surely - and discovered a scalability bug in CKTS. The fix was verified
again with the tool.
> Filing this bug so the community is aware of this issue. I don't have a solution for
now in KMS. But we want to address this scalability problem in the near future because we
are seeing use cases that requires thousands of encryption keys.
> ----
> On a side note, 4 KMS doesn't work well without HADOOP-14445 (and subsequent fixes).
A MapReduce job acquires at most 3 KMS delegation tokens, and so for cases, such as distcp,
it wouldn fail to reach the 4th KMS on the remote cluster. I imagine similar issues exist
for other execution engines, but I didn't test.



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