There are really not that many hoops you need to jump through to be able to periodically optimize down to 10 segments or so.  I've used lucene at plenty of other places before LinkedIn, and rarely (since 2.3's indexing speed blew through the roof) have I had to worry about setting the merge factor too high, and even when I do, you simply index into another directory while you're optimizing the original (which is now kept read-only while keeping an in-memory delete set).  It's not that hard, and it does wonders for your performance.

Sure, plenty of lucene installations can't optimize, and while many of those could do with some much-needed refactoring to allow them the possibility of doing that (otherwise, you get what happened at my old company before I worked there - there was never any optimize, high merge factor, and commit after every document [ouch!], and eventually query latency went through the roof and the system just fell over), I understand that not everyone is going to do that. 

But even in these installations, I'm still saying that you've narrowed the field down to a very tiny number if you add up all the requirements for multiPQ to be painful for them (seriously: when is 40MB going to hurt a system that's designed to handle 100QPS per box?  Or when does 4MB hurt one designed to handle 10QPS?)

  -jake

On Tue, Nov 3, 2009 at 12:40 PM, Mark Miller <markrmiller@gmail.com> wrote:
Not *ever* being able to optimize is a common case, without jumping a lot of hoops. There are many systems that need to be on nearly 24/7 - an optimize on a large index can take many hours - usually an unknown number. Linkedin and it's use cases are not the only consumers of lucene.


- Mark

http://www.lucidimagination.com (mobile)

On Nov 3, 2009, at 10:51 AM, "Jake Mannix (JIRA)" <jira@apache.org> wrote:


  [ https://issues.apache.org/jira/browse/LUCENE-1997?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12773114#action_12773114 ]

Jake Mannix commented on LUCENE-1997:
-------------------------------------

bq. Since each approach has distinct advantages, why not offer both ("simple" and "expert") comparator extensions APIs?

+1 from me on this one, as long as the simpler one is around.  I'll bet we'll find that we regret keeping the "expert" one by 3.2 or so though, but I'll take any compromise which gets the simpler API in there.

bq. Don't forget that this is multiplied by however many queries are currently in flight.

Sure, so if you're running with 100 queries per second on a single shard (pretty fast!), with 100 segments, and you want to do sorting by value on the top 1000 values (how far down the long tail of extreme cases are we at now?  Do librarians hit their search servers with 100 QPS and have indices poorly built with hundreds of segments and can't take downtime to *ever* optimize?), we're now talking about 40MB.

*Forty megabytes*.  On a beefy machine which is supposed to be handling 100QPS across an index big enough to need 100 segments.  How much heap would such a machine already be allocating?  4GB?  6?  More?

We're talking about less than 1% of the heap is being used by the multiPQ approach in comparison to singlePQ.

Explore performance of multi-PQ vs single-PQ sorting API
--------------------------------------------------------

              Key: LUCENE-1997
              URL: https://issues.apache.org/jira/browse/LUCENE-1997
          Project: Lucene - Java
       Issue Type: Improvement
       Components: Search
 Affects Versions: 2.9
         Reporter: Michael McCandless
         Assignee: Michael McCandless
      Attachments: LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch, LUCENE-1997.patch


Spinoff from recent "lucene 2.9 sorting algorithm" thread on java-dev,
where a simpler (non-segment-based) comparator API is proposed that
gathers results into multiple PQs (one per segment) and then merges
them in the end.
I started from John's multi-PQ code and worked it into
contrib/benchmark so that we could run perf tests.  Then I generified
the Python script I use for running search benchmarks (in
contrib/benchmark/sortBench.py).
The script first creates indexes with 1M docs (based on
SortableSingleDocSource, and based on wikipedia, if available).  Then
it runs various combinations:
 * Index with 20 balanced segments vs index with the "normal" log
  segment size
 * Queries with different numbers of hits (only for wikipedia index)
 * Different top N
 * Different sorts (by title, for wikipedia, and by random string,
  random int, and country for the random index)
For each test, 7 search rounds are run and the best QPS is kept.  The
script runs singlePQ then multiPQ, and records the resulting best QPS
for each and produces table (in Jira format) as output.

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