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From Andrzej Bialecki>
Subject Google performance bottlenecks ;-) (Re: Lucene performance bottlenecks)
Date Fri, 09 Dec 2005 09:42:48 GMT

I made an experiment with Google, to see if they use a similar approach.

I find the results to be most interesting. I selected a query which is 
guaranteed to give large result sets, but is more complicated than a 
single term query: http com.

The total number of hits (approx) is 2,780,000,000. BTW, I find it 
curious that the last 3 or 6 digits always seem to be zeros ... there's 
some clever guesstimation involved here. The fact that Google Suggest is 
able to return results so quickly would support this suspicion.

When I ran the query for the first time, the response time was 0.29 sec. 
All subsequent queries retrieving the first 10 results are in the order 
of 0.07 sec.

This is for retrieving just the first page (first 10 results). 
Retrieving results 10-20 also takes 0.08 sec, which suggests that this 
result was cached somewhere. Starting from results 20+ the response time 
increases (linearly?), although it varies wildly between requests, 
sometimes returning quicker, sometimes taking the max time - which 
suggests that I'm hitting different servers each time. Also, if I wait 
~30 sec to 1 minute, the response times are back to the values for the 
first-time run.

start   first     repeated response
30      0.14      0.08-0.21
50      0.29      0.11-0.22
100     0.36      0.22-0.45
200     0.73      0.49-0.65
300     0.96      0.64-0.98
500     1.36      1.43-1.87
650     2.24      1.49-1.85

The last range was the maximum in this case - Google wouldn't display 
any hit above 652 (which I find curious, too - because the total number 
of hits is, well, significantly higher - and Google claims to return up 
to the first 1000 results).

My impressions from this excercise are perhaps not so surprising: Google 
is highly optimized for retrieving the first couple of results, and the 
more results you want to retrieve the worse the performance. Finally, 
you won't be able to retrieve any results above a couple hundred, quite 
often less than the claimed 1000 results threshold.

As for the exact techniques of this optimization, we'll never know for 
sure, but it seems like something similar is going on to what you 
outlined in your email. I think it would be great to try it out.


Doug Cutting wrote:

> Doug Cutting wrote:
>> Implementing something like this for Lucene would not be too 
>> difficult. The index would need to be re-sorted by document boost: 
>> documents would be re-numbered so that highly-boosted documents had 
>> low document numbers.
> In particular, one could:
> 1. Create an array of int[maxDoc], with a[i] = i.
> 2. Sort the array with order(i,j) = boost(i) - boost(j);
> 3. Implement a FilterIndexReader that re-numbers using the sorted 
> array.  So, for example, the document numbers in the TermPositions 
> will a[old.doc()].  Each term's positions will need to be read 
> entirely into memory and sorted to perform this renumbering.
> The class in the searcher package was an old 
> attempt to create something like what Suel calls "fancy postings".  It 
> creates an index with the top 10% scoring postings.  Since documents 
> are not renumbered one can intermix postings from this with the full 
> index.  So for example, one can first try searching using this index 
> for terms that occur more than, e.g., 10k times, and use the full 
> index for rarer words.  If that does not find 1000 hits then the full 
> index must be searched.  Such an approach can be combined with using a 
> pre-sorted index.
> I think the first thing to implement would be to implement something 
> like what Suel calls first-1000.  Then we need to evaluate this and 
> determine, for query log, how different the results are.
>> Then a HitCollector can simply stop searching once a given number of 
>> hits are found.
>> Doug

Best regards,
Andrzej Bialecki     <><
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