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From Boris Tyukin <bo...@boristyukin.com>
Subject Re: first and second run 2x query time difference
Date Thu, 14 Dec 2017 14:40:37 GMT
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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