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From "Stefania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8180) Optimize disk seek using min/max column name meta data when the LIMIT clause is used
Date Fri, 24 Jul 2015 09:29:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8180?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14640202#comment-14640202
] 

Stefania commented on CASSANDRA-8180:
-------------------------------------

We definitely need a place to archive these stress profiles, attaching them to the tickets
like I've done so far is not very efficient. CASSANDRA-8503 aims to collect important stress
profiles for regression testing, perhaps we can continue the discussion there?

In terms of this specific performance optimization, so far things aren't going too well. The
problem is that I don't know how to create a 'timeseries' profile with cassandra-stress, is
this possible [~benedict]?  We basically need to have many clustering rows per partition and
they must be ordered.

With the profile attached, at best I have been able to show that we are not worse:

http://cstar.datastax.com/tests/id/ac8c686c-31de-11e5-95c3-42010af0688f

However, when I increased the data from 1M to 5M and changed the primary key population distribution
from 1..100K to 1B, I ended up with something worse:

http://cstar.datastax.com/tests/id/11dc0080-31dd-11e5-80f3-42010af0688f

So I think there is still some work to do. I have also some weirdness in the flight recorder
profiles, which I have not attached due to their size but I can do if anyone is interested.
I fixed two hot-spots today but there must be at least one more problem. The whole approach
of a lazy wrapping iterator is a bit fragile IMO, all it takes is to call a method too soon
and the entire optimization is lost, except the overhead of the wrapper iterator and the increased
complexity in the merge iterator remains.

> Optimize disk seek using min/max column name meta data when the LIMIT clause is used
> ------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-8180
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8180
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>         Environment: Cassandra 2.0.10
>            Reporter: DOAN DuyHai
>            Assignee: Stefania
>            Priority: Minor
>             Fix For: 3.x
>
>         Attachments: 8180.yaml
>
>
> I was working on an example of sensor data table (timeseries) and face a use case where
C* does not optimize read on disk.
> {code}
> cqlsh:test> CREATE TABLE test(id int, col int, val text, PRIMARY KEY(id,col)) WITH
CLUSTERING ORDER BY (col DESC);
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 10, '10');
> ...
> >nodetool flush test test
> ...
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 20, '20');
> ...
> >nodetool flush test test
> ...
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 30, '30');
> ...
> >nodetool flush test test
> {code}
> After that, I activate request tracing:
> {code}
> cqlsh:test> SELECT * FROM test WHERE id=1 LIMIT 1;
>  activity                                                                  | timestamp
   | source    | source_elapsed
> ---------------------------------------------------------------------------+--------------+-----------+----------------
>                                                         execute_cql3_query | 23:48:46,498
| 127.0.0.1 |              0
>                             Parsing SELECT * FROM test WHERE id=1 LIMIT 1; | 23:48:46,498
| 127.0.0.1 |             74
>                                                        Preparing statement | 23:48:46,499
| 127.0.0.1 |            253
>                                   Executing single-partition query on test | 23:48:46,499
| 127.0.0.1 |            930
>                                               Acquiring sstable references | 23:48:46,499
| 127.0.0.1 |            943
>                                                Merging memtable tombstones | 23:48:46,499
| 127.0.0.1 |           1032
>                                                Key cache hit for sstable 3 | 23:48:46,500
| 127.0.0.1 |           1160
>                                Seeking to partition beginning in data file | 23:48:46,500
| 127.0.0.1 |           1173
>                                                Key cache hit for sstable 2 | 23:48:46,500
| 127.0.0.1 |           1889
>                                Seeking to partition beginning in data file | 23:48:46,500
| 127.0.0.1 |           1901
>                                                Key cache hit for sstable 1 | 23:48:46,501
| 127.0.0.1 |           2373
>                                Seeking to partition beginning in data file | 23:48:46,501
| 127.0.0.1 |           2384
>  Skipped 0/3 non-slice-intersecting sstables, included 0 due to tombstones | 23:48:46,501
| 127.0.0.1 |           2768
>                                 Merging data from memtables and 3 sstables | 23:48:46,501
| 127.0.0.1 |           2784
>                                         Read 2 live and 0 tombstoned cells | 23:48:46,501
| 127.0.0.1 |           2976
>                                                           Request complete | 23:48:46,501
| 127.0.0.1 |           3551
> {code}
> We can clearly see that C* hits 3 SSTables on disk instead of just one, although it has
the min/max column meta data to decide which SSTable contains the most recent data.
> Funny enough, if we add a clause on the clustering column to the select, this time C*
optimizes the read path:
> {code}
> cqlsh:test> SELECT * FROM test WHERE id=1 AND col > 25 LIMIT 1;
>  activity                                                                  | timestamp
   | source    | source_elapsed
> ---------------------------------------------------------------------------+--------------+-----------+----------------
>                                                         execute_cql3_query | 23:52:31,888
| 127.0.0.1 |              0
>                Parsing SELECT * FROM test WHERE id=1 AND col > 25 LIMIT 1; | 23:52:31,888
| 127.0.0.1 |             60
>                                                        Preparing statement | 23:52:31,888
| 127.0.0.1 |            277
>                                   Executing single-partition query on test | 23:52:31,889
| 127.0.0.1 |            961
>                                               Acquiring sstable references | 23:52:31,889
| 127.0.0.1 |            971
>                                                Merging memtable tombstones | 23:52:31,889
| 127.0.0.1 |           1020
>                                                Key cache hit for sstable 3 | 23:52:31,889
| 127.0.0.1 |           1108
>                                Seeking to partition beginning in data file | 23:52:31,889
| 127.0.0.1 |           1117
>  Skipped 2/3 non-slice-intersecting sstables, included 0 due to tombstones | 23:52:31,889
| 127.0.0.1 |           1611
>                                 Merging data from memtables and 1 sstables | 23:52:31,890
| 127.0.0.1 |           1624
>                                         Read 1 live and 0 tombstoned cells | 23:52:31,890
| 127.0.0.1 |           1700
>                                                           Request complete | 23:52:31,890
| 127.0.0.1 |           2140
> {code}



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