In working on
LUCENE-843, I noticed that two buffer sizes have a
substantial impact on overall indexing performance.
First is BufferedIndexOutput.BUFFER_SIZE (also used by
BufferedIndexInput). Second is CompoundFileWriter's buffer used to
actually build the compound file. Both are now 1 KB (1024 bytes).
I ran the same indexing test I'm using for
LUCENE-843. I'm indexing
~5,500 byte plain text docs derived from the Europarl corpus
(English). I index 200,000 docs with compound file enabled and term
vector positions & offsets stored plus stored fields. I flush
documents at 16 MB RAM usage, and I set maxBufferedDocs carefully to
LUCENE-845. The resulting index is 1.7 GB. The index is not
optimized in the end and I left mergeFactor @ 10.
I ran the tests on a quad-core OS X 10 machine with 4-drive RAID 0 IO
At 1 KB (current Lucene trunk) it takes 622 sec to build the index; if
I increase both buffers to 8 KB it takes 554 sec to build the index,
which is an 11% overall gain!
I will run more tests to see if there is a natural knee in the curve
(buffer size above which we don't really gain much more performance).
I'm guessing we should leave BufferedIndexInput's default BUFFER_SIZE
at 1024, at least for now. During searching there can be quite a few
of this class instantiated, and likely a larger buffer size for the
freq/prox streams could actually hurt search performance for those
searches that use skipping.
The CompoundFileWriter buffer is created only briefly, so I think we
can use a fairly large (32 KB?) buffer there. And there should not be
too many BufferedIndexOutputs alive at once so I think a large-ish
buffer (16 KB?) should be OK.