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From Dan OConnor <docon...@acquiremedia.com>
Subject RE: simultaneous indexing and searching causing intermitently long searches.
Date Sat, 04 Apr 2009 21:03:41 GMT
Mike,
Thanks for the response -- we've already jumped on a couple of your suggestions.

Here is some feedback and follow ups:

We have watched GC times closely in the past. Most of the results of us trying various settings
was to make GC worse instead of better.

We didn't know about reopen() until recently so we still create a new IndexSearcher in the
background and warm it up before we put it into service. We are going to switch to ping-ponging
between to IndexSearchers. When we call reopen(), do we have to warm it up again if reopen
returns a new IndexReader?

After I sent this email, I found that the ConcurrentMergeScheduler has both a setMaxThreadCount
and a setMergeThreadPriority. Does this allow our code to tell the JVM to run merge at a lower
priority (and perhaps with more threads) than our IndexWriter and IndexSearcher threads (which
are created with normal priority)?

For warm-ups, since we sort on a couple of date fields within the document (in addition to
the straight relevance sort), I'm reading your suggestion that it is important to issue warm
up queries that date sort as well?

Thanks again for your time and effort.

Regards,
Dan


-----Original Message-----
From: Michael McCandless [mailto:lucene@mikemccandless.com] 
Sent: Saturday, April 04, 2009 6:38 AM
To: java-user@lucene.apache.org
Subject: Re: simultaneous indexing and searching causing intermitently long searches.

On Fri, Apr 3, 2009 at 10:21 PM, Dan OConnor <doconnor@acquiremedia.com> wrote:
> All,
>
> I have a several questions regarding query response time and I would appreciate any help
that can be provided.
>
> We have a system that indexes approximately 200,000 documents per day at a fairly constant
rate and holds them in a cfs-style file system directory index for 8 days. The index is approximately
50 GBs when optimized - which we do semi-monthly.
>
> We are running lucene 2.3.2 with jre 1.6. 0_10 on Centos5 on 64-bit Dell 2950s - 3GHz
dual/quad core processors with local ext3 Raid-5 15k disks (approximately 1.7TBs) The box
has 16GB and the JVM is allocated 11G (both Xms and Xmx)

With such a large heap, I would watch the GC times closely.  Turn on
verbose GC and see what's happening, when.  There have been threads
recently around how to tune the JRE's GC when using such a large heap.

> Every 15 minutes, we flush the IndexWriter and create a new IndexSearcher to expose the
newly indexed content.

Are you using reopen() to open the new reader?

> Every hour, approximately 1 hours worth of content (approximately 8,000 documents) is
deleted, we flush the IndexWriter, and create a new IndexSearcher.
>
> Q1: Given these settings, are there general rules of thumb for setting the MergeFactor,
MaxMergeDocs, MaxBufferedDocs, and RAMBufferSizeMB?

Large maxRamBufferSizeMB.  I would keep mergeFactor smallish (<=
10)... it means more frequent merges, but possibly less IO saturation.
 You should experiment...

The lack of IO prioritization from Java (and really from the OS) is a
big problem.  We have no way to tell the OS that the IO being done for
a merge is very low priority.

> We do a series of warm up searches every time we create a new IndexSearcher. Right now
we are directly calling the IndexSearcher.search() method with a query, null filter, and 10
documents to return. We run searches against all of the index fields.
>
> Q1: Are there any rules of thumb for the number or complexity of warm up searches?

The goal is to warm Lucene's internal caches (norms & field cache).
So run one search per field that's searchable (loads the norms), and
one search per sorted field (loads field cache).

A secondary goal may be to warm the OS's IO cache, though that's
trickier because you'd need to track the common and large terms that
need to be queried.  I believe Solr does this (carries over its query
cache to the warmed reader), but I'm not certain.

Note that by far the biggest bang for you bug is to switch to a solid
state device to hold your index. EG Fusion IO's devices are insanely
fast.

Note that 2.9 has some performance improvements on the warm
performance after reopen, if you use field sort.

> Q2: Is it important to "warmup" the query parser, analyzer, etc or the ranges we use
in queries or the sorting?

Not important -- only the IndexReader needs warming.

> When the system is receiving regular queries, between 1 and 5 per second for example,
the search response times are extremely fast (sub 500ms) and mostly independent of query complexity.
We see slower query responses (on the order of 2-4 seconds) for the first few queries  when
using a newly created IndexSearcher. However, the extremely fast response times return quickly
and continue.
>
> When the system has not received any search requests for a period of time, as little
as 5 seconds, the query response time for even a simple query starts climbing (5 -8 seconds)
and the longer the idle period between queries, the longer the query response time (growing
to 15-30 seconds if the idle time is 30seconds to a minute). NOTE: the system is still indexing
new content and removing old content when there are no incoming queries.

You should try to watch your process, eg with top, to see if the OS is
moving pages out in favor of populating the system's IO cache.  Watch
for page faults when you see a slow query happening. If so, there's a
linux kernel parameter called "swapiness" that you should tune to
prevent swapout (though I'm not certain if CentOS exposes it; I would
assume so).

It's also possible the slowness comes from the OS swapping out the IO
cache for those queries' posting lists, in which case an SSD device
should solve it.

> Q3: Is there a known issue where the IndexSearcher cache empties over time?

IndexSearcher doesn't free its caches, but the OS may.

> Finally, there are times when the query response times completely go off the charts -
to 100s of seconds.

Gotta watch with top to see what's happening then.

> Q4: Is it possible that this is due to segments being merged together? If so, besides
the MergeFactor, etc. settings are there ways to mitigate this?

Yes this is possible.  Turn on IndexWriter.setInfoStream to see if you
can correlate massively slow queries with ongoing merging.

Mike

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