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From Ken Krugler <kkrugler_li...@transpac.com>
Subject Re: Analyzing performance and memory consumption for boolean queries
Date Tue, 23 Jun 2009 22:15:40 GMT
Hi Chris,

Others on this list will be able to provide much better optimization 
suggestions, but based on my experience with some large indexes...

1. For search time to vary from < 1 second => 20 seconds, the only 
two things I've seen are:

* Serious JVM garbage collection problems.
* You're in Linux swap hell.

We tracked similar issued down by creating a testbed that let us run 
a set of real-world queries, such that we could trigger these types 
of problems when we had appropriate instrumentation on and recording.

2. 250M/4 = 60M docs/index. The old rule of thumb was 10M docs/index 
as a reasonable size. You might just need more hardware.

3. We had better luck running more JVMs per system, versus one JVM 
with lots of memory. E.g. run 3 32-bit JVMs with 1.5GB/JVM. Though 
this assumes you've got one drive/JVM, to avoid disk contention.

4. I'm assuming, since you don't care about scoring, that you've 
turned of field norms. There are other optimizations you can do to 
speed up query-style searches (find all docs that match X) when you 
don't care about scores, but others on the list are much better 
qualified to provide input in this area.

5. It seems like storing content outside the index helps with 
performance, though I can't say for certain what the impact might be. 
E.g. only store a single unique ID field in the index, and use that 
to access the content (say, from a MapFile) when you're processing 
the matched entries.

6. Having most of the index loaded into the OS cache was the biggest 
single performance win. So if you've got 3 GB of unused memory on a 
server, limiting the size of the index to some low multiple of 3GB 
would be a good target.

-- Ken

>Our query performance is surprisingly inconsistent, and I'm trying to figure
>out why.  I've realized that I need to better understand what's going on
>internally in Lucene when we're searching.  I'd be grateful for any answers
>(including pointers to existing docs, if any).
>
>Our situation is this: We have roughly 250 million docs spread across four
>indexes.  Each doc has about a dozen fields, all stored and most indexed.
>(They're the usual document things like author, date, title, contents,
>etc.)  Queries differ in complexity but always have at least a few terms in
>boolean combination, up to some larger queries with dozens or even hundreds
>of terms combined with ands, ors, nots, and parens.  There's no sorting,
>even by relevance: we just want to know what matches.  Query performance is
>often sub-second, but not infrequently it can take over 20 seconds (we the
>time-limited hit collector, so anything over 20 seconds is stopped).
>Obviously the more complex queries are slower on average, but a given query
>can sometimes be much slower or much faster.
>
>My assumption is that we're having memory problems or disk utilization
>problems or both.  Our app has a 5gb JVM heap on an 8gb server with no other
>user processes running, so we shouldn't be paging and should have some room
>for Linux disk cache.  The server is lightly loaded and concurrent queries
>are the exception rather than the norm.  Two of the four indexes are updated
>a few times a day via rsync and subsequently closed and re-opened, but poor
>query performance doesn't seem to be correlated with these times.
>
>So, getting to some specific questions:
>
>1) How is the inverted index for a given field structured in terms of what's
>in memory and what's on disk?  Is it dynamic, based on available memory, or
>tuneable, or fixed?  Is there a rule of thumb that could be used to estimate
>how much memory is required per indexed field, based on the number of terms
>and documents?  Likewise, is there a rule of thumb to estimate how many disk
>accesses are required to retrieve the hits for that field?  (I'm thinking,
>by perhaps false analogy, of how a database maintains a b-tree structure
>that may reside partially in RAM cache and partially in disk pages.)
>
>2) When boolean queries are searched, is it as simple as iterating the hits
>for each ANDed or ORed term and applying the appropriate logical operators
>to the results?  For example, is searching for "foo AND bar" pretty much the
>same resource-wise as doing two separate searches, and therefore should the
>query performance be a linear function of the number the number of search
>terms?  Or is there some other caching and/or decision logic (perhaps kind
>of like a database's query optimizer) at work here that makes the I/O and
>RAM requirements more difficult to model from the query?  (Remember that
>we're not doing any sorting.)
>
>I'm hoping that with some of this knowledge, I'll be able to better model
>the RAM and I/O usage of the indexes and queries, and thus eventually
>understand why things are slow or fast.
>
>Thanks,
>Chris


-- 
Ken Krugler
+1 530-210-6378

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