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From "Tyler Hobbs (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6683) BADNESS_THRESHOLD does not working correctly with DynamicEndpointSnitch
Date Wed, 12 Feb 2014 18:19:20 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13899375#comment-13899375
] 

Tyler Hobbs commented on CASSANDRA-6683:
----------------------------------------

bq. I'm not sure what you mean exactly, we always end up calling it, we just don't check badness
when it's set to zero.

I was a little confused on my last comment, but let me try again. Right now we only call {{sortByProximityWithScore()}}
if {{BADNESS_THRESHOLD != 0}} and two neighbors in the list returned by the subsnitch differ
by BADNESS_THRESHOLD.  I think it would make more sense (and fix Kirill's case) to always
call {{sortByProximityWithScore()}} and then compare that ordering against the subsnitch list.
 Something like this:

{noformat}
defaultOrder = subsnitch.sort(address, addresses);
scoredOrder = sortByProximityWithScore(address, addresses);  // make this return a new list
instead of sorting in place
for (int i = 0; i < defaultOrder.size(); i++)
{
    if (scores.get(defaultOrder.get(i)) > scores.get(scoredOrder.get(i)) * (1 + BADNESS_THRESHOLD))
        return scoredOrder;
}
return defaultOrder;
{noformat}

bq. Possible, but it'd be a lot of work, because it would change the snitch interface and
we'd still need the old call because not all uses of it have a consistency level available.

It looks like there aren't too many callers, so it shouldn't be that much work.  I would just
make the arg optional and default it to the length of {{addresses}}.

> BADNESS_THRESHOLD does not working correctly with DynamicEndpointSnitch
> -----------------------------------------------------------------------
>
>                 Key: CASSANDRA-6683
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6683
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>         Environment: Linux 3.8.0-33-generic
>            Reporter: Kirill Bogdanov
>              Labels: snitch
>             Fix For: 2.0.6
>
>
> There is a problem in *DynamicEndpointSnitch.java* in sortByProximityWithBadness()
> Before calling sortByProximityWithScore we comparing each nodes score ratios to the badness
threshold.
> {code}
> if ((first - next) / first >  BADNESS_THRESHOLD)
>             {
>                 sortByProximityWithScore(address, addresses);
>                 return;
>             }
> {code}
> This is not always the correct comparison because *first* score can be less than *next*
 score and in that case we will compare a negative number with positive.
> The solution is to compute absolute value of the ratio:
> {code}
> if (Math.abs((first - next) / first) > BADNESS_THRESHOLD)
> {code}
> This issue causing an incorrect sorting of DCs based on their performance and affects
performance of the snitch.
> Thanks.
>  



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