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From "Ashish Singhi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13686) Fail to limit rate in RateLimiter
Date Wed, 20 May 2015 13:18:01 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13686?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14552301#comment-14552301
] 

Ashish Singhi commented on HBASE-13686:
---------------------------------------

The work of refillStrategy.refill is to give the number of resources that are refilled and
that can then be checked and added to the existing avail to form a new avail.

bq. The canExecute() should not to handle the special case of refillStrategy
All the special case of refillStrategy is handled by themselves in {{refill}} method {{canExecute}}
has only the code which will be common to all the refillStrategy after they finish calculating
the refill amount and can avoid duplicating the code.

{quote}
Assume there is a RateLimiter which limit and avail is Long.MAX_VALUE. Then consume(1). The
avail will be Long.MAX_VALUE - 1. After a long time, canExecute(1) again. This will refill
again. The delta will be much greater than 1. Then available + delta will be negative.
{quote}
This you have already answered in your first comment that **"check positive overflow" can
catch this case**. So I feel better to leave this piece of code in {{canExecute}} only and
avoid duplicating this code in each refill method as of now.

I do not see any strong reason to move this code in {{refill}} of each {{RefillStrategy}}

> Fail to limit rate in RateLimiter
> ---------------------------------
>
>                 Key: HBASE-13686
>                 URL: https://issues.apache.org/jira/browse/HBASE-13686
>             Project: HBase
>          Issue Type: Bug
>    Affects Versions: 2.0.0, 1.1.0
>            Reporter: Guanghao Zhang
>            Assignee: Ashish Singhi
>             Fix For: 2.0.0, 1.2.0, 1.1.1
>
>         Attachments: HBASE-13686.patch
>
>
> While using the patch in HBASE-11598 , I found that RateLimiter can't to limit the rate
right.
> {code} 
>  /**
>    * given the time interval, are there enough available resources to allow execution?
>    * @param now the current timestamp
>    * @param lastTs the timestamp of the last update
>    * @param amount the number of required resources
>    * @return true if there are enough available resources, otherwise false
>    */
>   public synchronized boolean canExecute(final long now, final long lastTs, final long
amount) {
>     return avail >= amount ? true : refill(now, lastTs) >= amount;
>   }
> {code}
> When avail >= amount, avail can't be refill. But in the next time to call canExecute,
lastTs maybe update. So avail will waste some time to refill. Even we use smaller rate than
the limit, the canExecute will return false. 



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