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From siddharth teotia <>
Subject Re: Memory usage
Date Thu, 07 Nov 2019 15:50:16 GMT
Hi Michael

Thanks a lot for your response. Couple of more questions

(1) During indexing, is there any knob to tell the writer to use off-heap
for buffering. I didn't find anything in the docs so probably the answer is
no. Just confirming..

(2) In my experiments, I have gone upto ingesting 5 million documents into
the lucene index and the number of segments created was 1. The writer was
committed and closed after ingesting all the documents and after that there
is no need for us to index more. So essentially it is an immutable index.
Basically I wanted to find the threshold for creating a new segment. Is
that pretty high? Or if the writer is reopened, then the next set of
documents will go into the next segment and so on? The reason for doing
this is to find the total number of files (per index) that will be opened
during querying. So far since it was a single segment, only that segment's
cfs file was opened.


On Thu, Nov 7, 2019, 6:39 AM Michael McCandless <>

> Hi Siddharth,
> Your understanding of MMapDirectory is correct -- only give your JVM
> enough heap to not spend too much CPU on GC, and then let the OS use all
> available remaining RAM to cache hot pages from your index.
> There are some structures Lucene loads into JVM heap, but even those are
> being moved off-heap (accessed via Directory) recently such as FSTs used
> for the terms index, and BKD index (for dimensional points).  I'm not sure
> exactly which structures are still in heap ... maybe the live documents
> bitset?
> During indexing, the recently indexed documents are buffered in JVM heap,
> up until the IndexWriterConfig.setRAMBufferSizeMB and then they will be
> written to the Directory as new segments.
> Mike McCandless
> On Wed, Nov 6, 2019 at 11:27 PM siddharth teotia <
>> wrote:
>> Hi All
>> I have some questions about the memory usage. I would really appreciate if
>> someone can help answer these.
>> I understand from the docs that during reading/querying, Lucene uses
>> MMapDirectory (assuming it is supported on the platform). So the Java heap
>> overhead in this case will purely come from the objects that are
>> allocated/instantiated on the query path to process the query and build
>> results etc.  But the whole index itself will not be loaded into memory
>> because we memory mapped the file. Is my understanding correct? In this
>> case, we are better off not increasing the Java heap and keep as much
>> as possible available for the file system cache for mmap to do its job
>> efficiently.
>> However, are there any portions of index structures that are completely
>> loaded in memory regardless of whether it is MMapDirectory or not? If so,
>> are they loaded in Java heap or do we use off-heap (direct buffers) in
>> such cases?
>> Secondly, on the write path I think even though the writer opens a
>> MMapDirectory, the writes are gathered/buffered in memory upto a flush
>> threshold controlled by IndexWriterConfig. Is this buffering done in Java
>> heap or direct memory?
>> Thanks a lot for help
>> Siddharth

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