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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-9840) Large scans and BlockCache evictions problems
Date Fri, 25 Oct 2013 17:28:32 GMT

    [ https://issues.apache.org/jira/browse/HBASE-9840?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13805482#comment-13805482
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Jean-Daniel Cryans commented on HBASE-9840:
-------------------------------------------

bq. singleFactor = 0.25, multiFactor = 0.5 , memoryFactor = 0.25 - are all hard-coded. They
must be configurable.

Meh, +0, see the following.

bq. Default values are probably not optimal at all. If it is attempt to mimic LRU2Q cache
than optimal split for first insert is closer to 0.75 (I think I read it on Facebook engineering)

>From its javadoc:

{quote}
Each priority will retain close to its maximum size, however, if any priority is not using
its entire chunk the others are able to grow beyond their chunk size.
{quote}

So if you aren't using the in-memory part, you effectively dedicate 75% of your BC to the
multi priority.

bq. Eviction does not follow LRU2Q at all

2Q suffers from different problems, mainly the tuning of Kin and Kout.

The "best" page replacement policy is one that takes into account age and frequency of access,
but we can't use ARC.

> Large scans and BlockCache evictions problems
> ---------------------------------------------
>
>                 Key: HBASE-9840
>                 URL: https://issues.apache.org/jira/browse/HBASE-9840
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Lars Hofhansl
>
> I just ran into a scenario that baffled me first, but after some reflection makes sense.
I ran a very large scan that filled up most of the block cache with my scan's data. I ran
that scan a few times.
> That I ran a much smaller scan, and this scan will never get all its blocks cached if
it does not fit entirely into the remaining BlockCache; regardless how I often I run it!
> The reason is that the blocks of the first large scan were all promoted. Since the 2nd
scan did not fully fit into the cache all blocks are round-robin evicted as I rerun the scan.
Thus those blocks will never get accessed more than once before they get evicted again.
> Since promoted blocks are not demoted the large scan's block will never be evicted unless
we have another small enough scan/get that can promote its blocks.
> Not sure what the proper solution is, but it seems only a LRU cache that can expire blocks
over time would solve this.
> Granted, this is a pretty special case.
> Edit: My usual spelling digressions.



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