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From Doron Yaacoby <>
Subject RE: In memory Lucene configuration
Date Mon, 16 Jul 2012 06:32:41 GMT
Another interesting fact I just found out.
Up until now I measured query execution time via my application. Meaning, the application
would log each query it sends to Lucene and the time it takes to run it. The nature of my
application is that there will be a variable number of lucene queries per second (2-3 usually,
but could be more or less), so there isn't constant 'pressure' on Lucene.
I now created a new test which runs the same queries but independently from my application.
 This achieved much better results: MMap implementation ~17ms, and RAMDirectory ~19ms. Moreover,
the results are now reproducible, meaning there aren't any spikes in the query times. When
running through my application scenario, I got the occasional spike, where a query took 2-3
seconds. In the MMap case, I guess it could be that the OS sees some caches as unused for
a while and reclaims them? I can't really explain this phenomenon in the RAMDirectory case.

I'm currently trying to recreate this by sleeping a random time before each query, but still
without success. Will update...

-----Original Message-----
From: Doron Yaacoby [] 
Sent: 15 July 2012 13:40
Subject: RE: In memory Lucene configuration

Thanks for the quick input!
I ran a few more tests with your suggested configuration (-Xmx1G -Xms1G with MMapDirectory).
At the third time I ran the same test I finally got an improvement - an average of ~30ms per
query, although it's still not as fast as I need it to be. 
The test contains about 2200 different queries (well, some are repeated twice or thrice),
and includes search time and doc loading (reading the two fields I mentioned). The queries
are all straight boolean conjunctions, and yes, I am dropping the first few queries when calculating

BTW, didn't mention before that I'm using Lucene 3.5 and Java 1.7.

-----Original Message-----
From: Simon Willnauer [] 
Sent: 15 July 2012 11:56
Subject: Re: In memory Lucene configuration

hey there,

On Sun, Jul 15, 2012 at 10:41 AM, Doron Yaacoby <> wrote:
> Hi, I have the following situation:
> I have two pretty large indices. One consists of about 1 billion documents (takes ~6GB
on disk) and the other has about 2 billion documents (~10GB on disk). The documents are very
short (4-5 terms each in the text field, and one numeric field with a long value). This is
a read only index - I'm only going to read from it and never write. There is only one segment
in each index (At least there should be, I called forceMerge(1) on them).
> Search latency is the most important thing to me. I need it to be blazing fast, ~20ms
per query. Queries are always of the type +term1 +term2 +term3, and I'm asking for 10 results
from each index (searching is done simultaneously on both indices).
> I have a fast server (12 cores@3GHz each) with 32Gb RAM (running Linux) and I can keep
both indices in-memory when using a RAMDirectory. This didn't achieve the expected result
(average query time = ~43ms). I'm seeing latency spikes, where the same query is sometimes
answered in 10ms, but in a different occasion takes 2-3 seconds. I'm guessing this is due
to GC (as explained here<>).
Using a warmed up MMapDirectory didn't help; the average query time was a bit slower. I tried
using InstantiatedIndex, but it has a huge memory consumption, I couldn't even load the smaller
6GB index.

its very hard to believe that you can't get this returning results faster though. I'd definitely
recommend you MMapDirectory here or NIO should do too. When you measure this do you measure
a large number of different queries or just a handful? Do you discard the first queries until
caches are warmed up? What are you measuring, pure search time including doc loading?
If you use MMapDir how much memory do you grant to your JVM? I'd recommend you to sum up the
term dictionary file size (.tii) and the norm file size (nrm) and give the JVM something like
3x the size as Xmx and Xms provided you don't need any more memory elsewhere. A guess from
the given index is that Xmx1G Xms1G should do the job and let the Filesystem use the rest
(that is important for lucene if you use MMap / NIOFS)

Your queries are straight boolean conjunctions or do you use positions ie phrase queries or

> Any ideas about what could be the ideal configuration for me?
> Thanks.

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