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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-6933) Optimise Read Comparison Costs in collectTimeOrderedData
Date Wed, 02 Apr 2014 22:45:19 GMT
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[ https://issues.apache.org/jira/browse/CASSANDRA-6933?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13958264#comment-13958264
]

Jonathan Ellis edited comment on CASSANDRA-6933 at 4/2/14 10:44 PM:
--------------------------------------------------------------------

I wrote a quick script and Benedict is right, assuming uniform distribution does help us pick
better bsearch "pivots" than just "half the remaining elements."

{code}
import bisect
import random

error_bisecting = 0
error_range = 0

for _ in xrange(10000):
universe = range(100)
sample = sorted(random.sample(universe, 10))

last_index = 0
r = None
for n in sample:
i = bisect.bisect(universe, n, last_index + 1)
guess_bisecting = (len(universe) - (last_index + 1)) / 2
guess_range = last_index + r if r else guess_bisecting
error_bisecting += abs(i - guess_bisecting)
error_range += abs(i - guess_range)
r = i - last_index
last_index = i

print error_range - error_bisecting
{code}

was (Author: jbellis):
I wrote a quick script and Benedict is right, assuming uniform distribution does help us pick
better bsearch "pivots" than just "half the remaining elements."

{code}
import bisect
import random

error_bisecting = 0
error_range = 0

for _ in xrange(10000):
universe = range(100)
sample = sorted(random.sample(universe, 10))

last_index = 0
r = None
for n in sample:
i = bisect.bisect(universe, n, last_index + 1)
guess_bisecting = (len(universe) - (last_index + 1)) / 2
guess_range = last_index + r if r else guess_bisecting
error_bisecting += abs(i - guess_bisecting)
error_range = abs(i - guess_range)
r = i - last_index
last_index = i

print error_range - error_bisecting
{code}

> Optimise Read Comparison Costs in collectTimeOrderedData
> --------------------------------------------------------
>
>                 Key: CASSANDRA-6933
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6933
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>            Priority: Minor
>              Labels: performance
>             Fix For: 2.1
>
>         Attachments: 6933-v3.txt, 6933-v4.txt
>
>
> Introduce a new SearchIterator construct, which can be obtained from a ColumnFamily,
which permits efficiently iterating a subset of the cells in ascending order. Essentially,
it saves the previously visited position and searches from there, but also tries to avoid
searching the whole remaining space if possible.

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