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From "Yu Li (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16973) Revisiting default value for hbase.client.scanner.caching
Date Mon, 31 Oct 2016 07:19:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16973?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15621488#comment-15621488
] 

Yu Li commented on HBASE-16973:
-------------------------------

Thanks for chiming in [~yangzhe1991] and mentioning timeLimit.

bq. For your case, if you want scanner.next only blocking several seconds, you can set hbase.client.scanner.timeout.period
to 10000 at client. The time limit will be 5000
Yes but not before HBASE-15593 right (btw good work at 15593:-))? Or say in current released
versions, this time limit is controlled by server side settings and in our case {{hbase.client.scanner.timeout.period}}
is set to 180000 or say 3min...

Still I'd like to focus on the discussion about default value of {{hbase.client.scanner.caching}}.
Setting it to Integer.MAX_VALUE requires user to understand quite some details of our implementations
and I'm wondering whether this is proper.

> 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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