[ http://issues.apache.org/jira/browse/LUCENE-687?page=comments#action_12445683 ] Michael Busch commented on LUCENE-687: --------------------------------------
I made some performance experiments with phrase searches. For every query I measured the search time as well as the number of invocations of IndexInput.readVint(). While the search time is very specific for my test environment, the number of Vints should tell us how much I/O we safe independent from the actual hardware. Test environment: 2x Intel Xeon CPU 2.80GHz 4GB RAM 280GB SCSI HD Windows Server 2003 My index contains about 460,000 documents from different websites like www.cnn.com, www.joelonsoftware.com,..., also German sites like www.heise.de. The search times below are average values over 10 searches. Tests: - Query: "the der" This should be a hard query, because "the" appears in almost every English document whereas "der" in almost every German document. Number of hits: 15 Execution time in ms (old, new, improvement): 651.3, 472.1, 28% # if VInts (old, new, improvement): 25786474, 16007634, 38% - Query: "joel on software" "joel" only appears in a small subset of documents, "on" in all English documents, and "software" can appear in German and English documents. Number of hits: 1613 Execution time in ms (old, new, improvement): 33.0, 20.1, 39% # if VInts (old, new, improvement): 950243, 256481, 73% - Query: "much much more" This is an example where we don't save so much I/O, because it is likely that all terms appear in the most English documents. Number of hits: 76 Execution time in ms (old, new, improvement): 228.2, 215.5, 6% # if VInts (old, new, improvement): 3580288, 3067693, 14% I think these numbers look pretty good. In the future, when we hopefully have proximity scoring, every search would benefit from this improvement. (I actually have a running version of proximity scoring in my local code and verified this). Did you have some time to look into the patch yet, Yonik? Let me know if you need more information, please. > Performance improvement: Lazy skipping on proximity file > -------------------------------------------------------- > > Key: LUCENE-687 > URL: http://issues.apache.org/jira/browse/LUCENE-687 > Project: Lucene - Java > Issue Type: Improvement > Components: Index > Reporter: Michael Busch > Priority: Minor > Attachments: lazy_prox_skipping.patch > > > Hello, > I'm proposing a patch here that changes > org.apache.lucene.index.SegmentTermPositions to avoid unnecessary skips and > reads on the proximity stream. Currently a call of next() or seek(), which > causes a movement to a document in the freq file also moves the prox pointer > to the posting list of that document. But this is only necessary if actual > positions have to be retrieved for that particular document. > Consider for example a phrase query with two terms: the freq pointer for term > 1 has to move to document x to answer the question if the term occurs in that > document. But *only* if term 2 also matches document x, the positions have to > be read to figure out if term 1 and term 2 appear next to each other in > document x and thus satisfy the query. > A move to the posting list of a document can be quite expensive. It has to be > skipped to the last skip point before that document and then the documents > between the skip point and the desired document have to be scanned, which > means that the VInts of all positions of those documents have to be read and > decoded. > An improvement is to move the prox pointer lazily to a document only if > nextPosition() is called. This will become even more important in the future > when the size of the proximity file increases (e. g. by adding payloads to > the posting lists). > My patch implements this lazy skipping. All unit tests pass. > I also attach a new unit test that works as follows: > Using a RamDirectory an index is created and test docs are added. Then the > index is optimized to make sure it only has a single segment. This is > important, because IndexReader.open() returns an instance of SegmentReader if > there is only one segment in the index. The proxStream instance of > SegmentReader is package protected, so it is possible to set proxStream to a > different object. I am using a class called SeeksCountingStream that extends > IndexInput in a way that it is able to count the number of invocations of > seek(). > Then the testcase searches the index using a PhraseQuery "term1 term2". It is > known how many documents match that query and the testcase can verify that > seek() on the proxStream is not called more often than number of search hits. > Example: > Number of docs in the index: 500 > Number of docs that match the query "term1 term2": 5 > Invocations of seek on prox stream (old code): 29 > Invocations of seek on prox stream (patched version): 5 > - Michael -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
