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From Lance Norskog <goks...@gmail.com>
Subject Re: RAMDirectory doesn't win over FSDirectory all the time, why?
Date Fri, 17 Jun 2011 01:16:01 GMT
The RAMDirectory uses Java memory, an FSDirectory does not. Holding
Java memory makes garbage collection work harder. The operating system
is very very good at managing disk buffers, and does a better job
using spare memory than Java does.

For real-world sites, RAMDirectory is almost always useless. Maybe the
Instantiated index stuff is more what you want?

Lance

On Tue, Jun 7, 2011 at 2:52 AM, zhoucheng2008 <zhoucheng2008@gmail.com> wrote:
> Makes sense. Thanks
>
> -----Original Message-----
> From: Toke Eskildsen [mailto:te@statsbiblioteket.dk]
> Sent: Tuesday, June 07, 2011 4:28 PM
> To: java-user@lucene.apache.org
> Subject: Re: RAMDirectory doesn't win over FSDirectory all the time, why?
>
> On Mon, 2011-06-06 at 15:29 +0200, zhoucheng2008 wrote:
>> I read the lucene in action book and just tested the
>> FSversusRAMDirectoryTest.java with the following uncommented:
>> [...]Here is the output:
>>
>> RAMDirectory Time: 805 ms
>>
>> FSDirectory Time : 728 ms
>
> This is the code, right?
> http://java.codefetch.com/example/in/LuceneInAction/src/lia/indexing/FSversusRAMDirectoryTest.java
>
> The test is problematic as the same two tests run sequentially.
>
> If you change
>  long ramTiming = timeIndexWriter(ramDir);
>  long fsTiming = timeIndexWriter(fsDir);
> to
>  long fsTiming = timeIndexWriter(fsDir);
>  long ramTiming = timeIndexWriter(ramDir);
> my guess is that RAMDirectory will be faster. For a better
> comparison, perform each test in separate runs (make a test
> class just for RAMDirectory and one just for FSDirectory,
> then run them one at a time, each in its own JVM).
>
> One big problem when comparing RAMDirectory to file-access
> is caching. What you measure with a test might not be what
> you see in production, as the production index might be
> large compared to RAM available for file caching.
>
>
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>



-- 
Lance Norskog
goksron@gmail.com

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