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From Jean-Daniel Cryans <jdcry...@apache.org>
Subject Re: first and second run 2x query time difference
Date Sat, 16 Dec 2017 21:05:54 GMT
I'm more thinking in terms of the startup IO having some impact on the
co-located services, but we really need to know what "went down" means.

On Sat, Dec 16, 2017 at 12:50 PM, Boris Tyukin <boris@boristyukin.com>
wrote:

> yep it is really weird since Kudu does not use neither one. I'll get with
> him on Monday to gather more details
>
> On Sat, Dec 16, 2017 at 3:28 PM, Jean-Daniel Cryans <jdcryans@apache.org>
> wrote:
>
>> Hi Boris,
>>
>> How exactly did HDFS and ZK go down? A Kudu restart is fairly
>> IO-intensive but I don't know how that can cause things like DataNodes to
>> fail.
>>
>> J-D
>>
>> On Sat, Dec 16, 2017 at 11:45 AM, Boris Tyukin <boris@boristyukin.com>
>> wrote:
>>
>>> well our admin had fun two days - it was the first time we restarted
>>> Kudu on our DEV cluster and it did not go well. He is still troubleshooting
>>> what happened but after Kudu restart zookeeper and HDFS went down after 3-4
>>> minutes. If we disable Kudu, all is well. No error in Kudu logs...I will
>>> have more details next week so not asking for help as I do not know all the
>>> details. What is obvious thought is that it has to do something with Kudu :)
>>>
>>> On Thu, Dec 14, 2017 at 9:40 AM, Boris Tyukin <boris@boristyukin.com>
>>> wrote:
>>>
>>>> thanks for your suggestions, J-D, I am sure you are right more often
>>>> than that! :))
>>>>
>>>> I will report back with our results. So far I am really impressed with
>>>> Kudu - we have been benchmarking ingest and egress throughput and our
>>>> typical queries runtime. The biggest pain so far is lack of support for
>>>> decimals
>>>>
>>>> On Wed, Dec 13, 2017 at 5:07 PM, Jean-Daniel Cryans <
>>>> jdcryans@apache.org> wrote:
>>>>
>>>>> On Wed, Dec 13, 2017 at 11:30 AM, Boris Tyukin <boris@boristyukin.com>
>>>>> wrote:
>>>>>
>>>>>> thanks J-D! we are going to try that and see how it impacts the
>>>>>> runtime.
>>>>>>
>>>>>> is there any way to load this metadata upfront? a lot of our queries
>>>>>> are adhoc in nature but they will be hitting the same tables with
different
>>>>>> predicates and join patterns though.
>>>>>>
>>>>>
>>>>> You could use Impala to compute all the stats of all the tables after
>>>>> each Kudu restart. Actually, do try that, restart Kudu then compute stats
>>>>> and see how fast it scans.
>>>>>
>>>>>
>>>>>>
>>>>>> I am curious why this metadata does not survive restarts though.
We
>>>>>> are going to run our benchmarks again and this time restart Kudu
and Impala.
>>>>>>
>>>>>
>>>>> It's in the tserver memory, it can't survive a restart.
>>>>>
>>>>>
>>>>>>
>>>>>> I just ran another query first time which hits 2 large tables and
>>>>>> these tables have been scanned by the previous query and this time
I do not
>>>>>> see any difference in query time before the first and second time
- I guess
>>>>>> this confirms your statement about " first time ever scanning the
>>>>>> table since a Kudu restart" and collecting metadata.
>>>>>>
>>>>>
>>>>> Maybe, I've been known to be right once or twice a year :)
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>> On Wed, Dec 13, 2017 at 11:18 AM, Jean-Daniel Cryans <
>>>>>> jdcryans@apache.org> wrote:
>>>>>>
>>>>>>> Hi Boris,
>>>>>>>
>>>>>>> Given that we don't have much data we can use here, I'll have
to
>>>>>>> extrapolate. As an aside though, this is yet another example
where we need
>>>>>>> more Kudu-side metrics in the query profile.
>>>>>>>
>>>>>>> So, Kudu lazily loads a bunch of metadata and that can really
affect
>>>>>>> scan times. If this was your first time ever scanning the table
since a
>>>>>>> Kudu restart, it's very possible that that's where that time
was spent.
>>>>>>> There's also the page cache in the OS that might now be populated.
You
>>>>>>> could do something like "sync; echo 3 > /proc/sys/vm/drop_caches"
on all
>>>>>>> the machines and run the query 2 times again, without restarting
Kudu, to
>>>>>>> understand the effect of the page cache itself. There's currently
now way
>>>>>>> to purge the cached metadata in Kudu though.
>>>>>>>
>>>>>>> Hope this helps a bit,
>>>>>>>
>>>>>>> J-D
>>>>>>>
>>>>>>> On Wed, Dec 13, 2017 at 8:07 AM, Boris Tyukin <boris@boristyukin.com
>>>>>>> > wrote:
>>>>>>>
>>>>>>>> Hi guys,
>>>>>>>>
>>>>>>>> I am doing some benchmarks with Kudu and Impala/Parquet and
hope to
>>>>>>>> share it soon but there is one thing that bugs me. This is
perhaps Impala
>>>>>>>> question but since I am using Kudu with Impala I am going
to try and ask
>>>>>>>> anyway.
>>>>>>>>
>>>>>>>> One of my queries takes 120 seconds to run the very first
time. It
>>>>>>>> joins one large 5B row table with a bunch of smaller tables
and then stores
>>>>>>>> result in Impala/parquet (not Kudu).
>>>>>>>>
>>>>>>>> Now if I run it second and third time, it only takes 60 seconds.
>>>>>>>> Can someone explain why? Is there any settings to decrease
this gap?
>>>>>>>>
>>>>>>>> I've compared query profiles in CM and the only thing that
was very
>>>>>>>> different is scan against Kudu table (the large one):
>>>>>>>>
>>>>>>>> ***************************
>>>>>>>> first time:
>>>>>>>> ***************************
>>>>>>>> KUDU_SCAN_NODE (id=0) (47.68s)
>>>>>>>> <https://lkmaorabd103.multihosp.net:7183/cmf/impala/queryDetails?queryId=5143f7165be82819%3Ae00a103500000000&serviceName=impala#>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>    - BytesRead: *0 B*
>>>>>>>>    - InactiveTotalTime: *0ns*
>>>>>>>>    - KuduRemoteScanTokens: *0*
>>>>>>>>    - NumScannerThreadsStarted: *20*
>>>>>>>>    - PeakMemoryUsage: *35.8 MiB*
>>>>>>>>    - RowsRead: *693,502,241*
>>>>>>>>    - RowsReturned: *693,502,241*
>>>>>>>>    - RowsReturnedRate: *14643448 per second*
>>>>>>>>    - ScanRangesComplete: *20*
>>>>>>>>    - ScannerThreadsInvoluntaryContextSwitches: *1,341*
>>>>>>>>    - ScannerThreadsTotalWallClockTime: *36.2m*
>>>>>>>>       - MaterializeTupleTime(*): *47.57s*
>>>>>>>>       - ScannerThreadsSysTime: *31.42s*
>>>>>>>>       - ScannerThreadsUserTime: *1.7m*
>>>>>>>>    - ScannerThreadsVoluntaryContextSwitches: *96,855*
>>>>>>>>    - TotalKuduScanRoundTrips: *52,308*
>>>>>>>>    - TotalReadThroughput: *0 B/s*
>>>>>>>>    - TotalTime: *47.68s*
>>>>>>>>
>>>>>>>>
>>>>>>>> ***************************
>>>>>>>> second time:
>>>>>>>> ***************************
>>>>>>>> KUDU_SCAN_NODE (id=0) (4.28s)
>>>>>>>> <https://lkmaorabd103.multihosp.net:7183/cmf/impala/queryDetails?queryId=53497a308f860837%3A243772e000000000&serviceName=impala#>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>    - BytesRead: *0 B*
>>>>>>>>    - InactiveTotalTime: *0ns*
>>>>>>>>    - KuduRemoteScanTokens: *0*
>>>>>>>>    - NumScannerThreadsStarted: *20*
>>>>>>>>    - PeakMemoryUsage: *37.9 MiB*
>>>>>>>>    - RowsRead: *693,502,241*
>>>>>>>>    - RowsReturned: *693,502,241*
>>>>>>>>    - RowsReturnedRate: *173481534 per second*
>>>>>>>>    - ScanRangesComplete: *20*
>>>>>>>>    - ScannerThreadsInvoluntaryContextSwitches: *1,451*
>>>>>>>>    - ScannerThreadsTotalWallClockTime: *19.5m*
>>>>>>>>       - MaterializeTupleTime(*): *4.20s*
>>>>>>>>       - ScannerThreadsSysTime: *38.22s*
>>>>>>>>       - ScannerThreadsUserTime: *1.7m*
>>>>>>>>    - ScannerThreadsVoluntaryContextSwitches: *480,870*
>>>>>>>>    - TotalKuduScanRoundTrips: *52,142*
>>>>>>>>    - TotalReadThroughput: *0 B/s*
>>>>>>>>    - TotalTime: *4.28s*
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

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