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From "Mikhail Antonov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16973) Revisiting default value for hbase.client.scanner.caching
Date Mon, 31 Oct 2016 21:37:59 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16973?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15623504#comment-15623504
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Mikhail Antonov commented on HBASE-16973:
-----------------------------------------

Will have a look later today

> Revisiting default value for hbase.client.scanner.caching
> ---------------------------------------------------------
>
>                 Key: HBASE-16973
>                 URL: https://issues.apache.org/jira/browse/HBASE-16973
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Yu Li
>            Assignee: Yu Li
>         Attachments: Scan.next_p999.png
>
>
> We are observing below logs for a long-running scan:
> {noformat}
> 2016-10-30 08:51:41,692 WARN  [B.defaultRpcServer.handler=50,queue=12,port=16020] ipc.RpcServer:
> (responseTooSlow-LongProcessTime): {"processingtimems":24329,
> "call":"Scan(org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ScanRequest)",
> "client":"11.251.157.108:50415","scandetails":"table: ae_product_image region: ae_product_image,494:
> ,1476872321454.33171a04a683c4404717c43ea4eb8978.","param":"scanner_id: 5333521 number_of_rows:
2147483647
> close_scanner: false next_call_seq: 8 client_handles_partials: true client_handles_heartbeats:
true",
> "starttimems":1477788677363,"queuetimems":0,"class":"HRegionServer","responsesize":818,"method":"Scan"}
> {noformat}
> From which we found the "number_of_rows" is as big as {{Integer.MAX_VALUE}}
> And we also observed a long filter list on the customized scan. After checking application
code we confirmed that there's no {{Scan.setCaching}} or {{hbase.client.scanner.caching}}
setting on client side, so it turns out using the default value the caching for Scan will
be Integer.MAX_VALUE, which is really a big surprise.
> After checking code and commit history, I found it's HBASE-11544 which changes {{HConstants.DEFAULT_HBASE_CLIENT_SCANNER_CACHING}}
from 100 to Integer.MAX_VALUE, and from the release note there I could see below notation:
> {noformat}
> Scan caching default has been changed to Integer.Max_Value 
> This value works together with the new maxResultSize value from HBASE-12976 (defaults
to 2MB) 
> Results returned from server on basis of size rather than number of rows 
> Provides better use of network since row size varies amongst tables
> {noformat}
> And I'm afraid this lacks of consideration of the case of scan with filters, which may
involve many rows but only return with a small result.
> What's more, we still have below comment/code in {{Scan.java}}
> {code}
>   /*
>    * -1 means no caching
>    */
>   private int caching = -1;
> {code}
> But actually the implementation does not follow (instead of no caching, we are caching
{{Integer.MAX_VALUE}}...).
> So here I'd like to bring up two points:
> 1. Change back the default value of HConstants.DEFAULT_HBASE_CLIENT_SCANNER_CACHING to
some small value like 128
> 2. Reenforce the semantic of "no caching"



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