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From "Robert Engels" <reng...@ix.netcom.com>
Subject RE: major searching performance improvement
Date Wed, 25 May 2005 22:08:28 GMT
I will look at separating it out. I wanted to get initial feedback before
moving on.

1. I agree that the initialValue() is the way to go. I'll make the changes.

2. I agree that creating NioFSDirectory rather than modifying FSDirectory. I
originally felt the memory mapped files would be the fastest, but it also
requires OS calls, the "caching" code is CONSIDERABLY faster, since it does
not need to do any JNI, or make OS calls.

3. I think a "simple" fix for the case you cite, is to add an additional
'max size' parameter, which controls the maximum size of the cache for each
'segment file', so using the mergeFactor, and compound files, you can easily
compute what this max would be based on available memory and expected index
size (number of files). The problem with a SoftCache and indices of that
size, is that the JVM memory consumption would still grow to the limit
before it discarded anything (which may be ideal in some cases).

As for creating a CachingDirectory that can cache any directory that should
be feasible as well, but I am not sure it would perform as well as the
direct internal cache version.

Robert

-----Original Message-----
From: Doug Cutting [mailto:cutting@apache.org]
Sent: Wednesday, May 25, 2005 4:20 PM
To: java-dev@lucene.apache.org
Subject: Re: major searching performance improvement


Robert Engels wrote:
> Attached are files that dramatically improve the searching performance
> (2x improvement on several hardware configurations!) in a multithreaded,
> high concurrency environment.

This looks like some good stuff!  Can you perhaps break it down into
independent, layered patches?  That way it would be easier to discuss
and integrate them.

> The change has 3 parts:
>
> 1) remove synchronization required in SegmentReader document. This
> required changes to FieldsReader to handle concurrent access.

This makes good sense.  Stylistically, I would prefer the cloning be
done in ThreadLocal.initialValue().  That way if another method ever
needs the input streams the cloning code need not be altered.

> 2) change FSDirectory to use a 'nio' to improve concurrency. Changed to
> use NioFile. This class has some workaround because under Windows, the
> FileChannel is not fully reentrant, and so allocates multiple handles
> per physical file - this code can be removed under non-Windows
> systems. This also required changes to InputStream to allow for reading
> at a direct offset.

Could you please explore making this a new Directory class, extending
rather than replacing FSDirectory?  That would make it easier for folks
to evaluate.  Look at MMapDirectory for an example.

Also, did you compare the performance of this to MMapDirectory?  That
already uses nio, and should thus avoid the thread contention of
FSDirectory.  However it does not scale well on 32-bit machines whose
address space limits indexes to 4GB.

Finally, for Windows-specific code, you can check
org.apache.lucene.util.Constants.WINDOWS at runtime.

> 3) move disk buffering into the Java layer to avoid the overhead of OS
> calls. The buffer percentage can be configured to store the entire index
> in memory. Running with as little as a 10% cache, the performance is
> dramatically improved. Reading larger blocks also improves the
> performance in most cases, but can actually degrade performance if doing
> very small reads. Using the cache implies that you have configured the
> JVM to have as much heap space available as the percent of index size on
> the disk. The NioFile can be easily changed to use a "soft" cache to
> avoid the potential of OutOfMemoryExceptions.

It would be nice if this functionality could be layered on any
Directory.  Did you consider making a CachingDirectory that one can wrap
around an existing Directory implementation, that keeps an LRU cache of
data?  Even 10% by default will probably break a lot of applications.
At the Internet Archive I frequently search indexes 100GB gigabyte
indexes on machines with just 1GB of RAM.  So I am leery of enabling
this by default.

Cheers,

Doug

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