I see, i think i will use first way the constructor woith MMap and i
will not use setPreload api to avoid slowdowns.
yes, i was expecting a warning from eclipse in the second usage but
nothing came up.
Thanks for the clarifications.
Best regards
On 12/14/20 2:55 PM, Uwe Schindler wrote:
Hi,
Thanks Uwe, i am not insisting on to load everything into memory
but loading into memory might speed up and i would like to see how much
speedup.
but i have one more question and that is still not clear to me:
"it is much better to open index, with MMAP directory"
does this mean i should not use the constructor but instead use the open
api?
No that means, use MMapDirectory, it should fit your needs. If you have enough
memory outside of heap in your operating system that can be used by Lucene to
have all pages of the mmaped file in memory then it’s the best you can have.
FSDirectory.open() is fine as it will always use MMapDirectory on 64 bit
platforms.
in other words: which way should be preferred?
Does not matter. If you want to use setPreload() [beware of slowdowns on
opening index files for first time!!!], use constructor of MMAPDirectory,
because the FSDirectoryFactory cannot guarantee which implementation you get.
Calling a static method on a class that does not implement it, is generally
considered bad practise (Eclipse should warn you). The static
FSDirectory.open() is a factory method and should be used (on FSDircetory not
its subclass) if you don't know what you want to have and be operating system
independent. If you want MMapDirectory and its features specifically, use the
constructor.
The example is from both during indexing and searching:
/*First way: Using constructor (without setPreload) :*/
MMapDirectory dir = new MMapDirectory(Paths.get(indexDir)); // Uses
FSLockFactory.getDefault() and DEFAULT_MAX_CHUNK_SIZE which is 1GB
////if (dir.getPreload() == false)
//// dir.setPreload(Constants.PRELOAD_YES); // In-Memory Lucene Index
enabled-> *commented out*
IndexReader reader = DirectoryReader.open(dir);
...
/*Second way: Or using open (without setPreload) :*/
*Directory* dir = MMapDirectory.open(Paths.get(indexDir)); //open is
inherited from FSDirectory
////if (dir.getPreload() == false)
//// dir.setPreload(Constants.PRELOAD_YES); // In-Memory Lucene Index
enabled-> *here setPreload cannot be used*
IndexReader reader = DirectoryReader.open(dir);
IndexSearcher is = new IndexSearcher(reader);
...
Best regards
On 12/14/20 1:51 PM, Uwe Schindler wrote:
Hi,
as writer of the original bog post, here my comments:
Yes, MMapDirectory.setPreload() is the feature mentioned in my blog post is
to load everything into memory - but that does not guarantee anything!
Still, I would not recommend to use that function, because all it does is to
just touch every page of the file, so the linux kernel puts it into OS cache
- nothing more; IMHO very ineffective as it slows down openining index for a
stupid for-each-page-touch-loop. It will do this with EVERY page, if it is
later used or not! So this may take some time until it is done. Lateron,
still Lucene needs to open index files, initialize its own data
structures,...
In general it is much better to open index, with MMAP directory and execute
some "sample" queries. This will do exactly the same like the preload
function, but it is more "selective". Parts of the index which are not used
won't be touched, and on top, it will also load ALL the required index
structures to heap.
As always and as mentioned in my blog post: there's nothing that can ensure
your index will stays in memory. Please trust the kernel to do the right
thing. Why do you care at all?
If you are curious and want to have everything in memory all the time:
- use tmpfs as your filesystem (of course you will loose data when OS shuts
down)
- disable swap and/or disable swapiness
- use only as much heap as needed, keep everything of free memory for your
index outside heap.
Fake feelings of "everything in RAM" are misconceptions like:
- use RAMDirectory (deprecated): this may be a desaster as it described in
the blog post
- use ByteBuffersDirectory: a little bit better, but this brings nothing, as
the operating system kernel may still page out your index pages. They still
live in/off heap and are part of usual paging. They are just no longer
backed by a file.
Lucene does most of the stuff outside heap, live with it!
Uwe
-----
Uwe Schindler
Achterdiek 19, D-28357 Bremen
https://urldefense.com/v3/__https://www.thetaphi.de__;!!GqivPVa7Brio!Ll3PR
4BZgqmgJNQ7MrnsXr27zNYgjsyXlMh9h6awmbZgSNW-
yVLBCDuFHTogNnw9_Q$
eMail: u...@thetaphi.de
-----Original Message-----
From: baris.ka...@oracle.com <baris.ka...@oracle.com>
Sent: Sunday, December 13, 2020 10:18 PM
To: java-user@lucene.apache.org
Cc: BARIS KAZAR <baris.ka...@oracle.com>
Subject: MMapDirectory vs In Memory Lucene Index (i.e.,
ByteBuffersDirectory)
Hi,-
it would be nice to create a Lucene index in files and then effectively
load it
into memory once (since i use in read-only mode). I am looking into if
this is
doable in Lucene.
i wish there were an option to load whole Lucene index into memory:
Both of below urls have links to the blog url where i quoted a very nice
section:
https://urldefense.com/v3/__https://lucene.apache.org/core/8_5_0/core/org/a
pache/lucene/store/MMapDi__;!!GqivPVa7Brio!Ll3PR4BZgqmgJNQ7MrnsXr27z
NYgjsyXlMh9h6awmbZgSNW-yVLBCDuFHTrcPLQ6cQ$
rectory.html
https://urldefense.com/v3/__https://lucene.apache.org/core/8_5_2/core/org/a
pache/lucene/store/MMapDi__;!!GqivPVa7Brio!Ll3PR4BZgqmgJNQ7MrnsXr27z
NYgjsyXlMh9h6awmbZgSNW-yVLBCDuFHToSKhCY-w$
rectory.html
This following blog mentions about such option
to run in the memory: (see the underlined sentence below)
https://urldefense.com/v3/__https://blog.thetaphi.de/2012/07/use-lucenes-
mmapdirectory-on-
__;!!GqivPVa7Brio!Ll3PR4BZgqmgJNQ7MrnsXr27zNYgjsyXlMh9h6awmbZgSNW
-yVLBCDuFHTpvqnQhbA$
64bit.html?m=1
MMapDirectory will not load the whole index into physical memory. Why
should it do this? We just ask the operating system to map the file into
address
space for easy access, by no means we are requesting more. Java and the
O/S
optionally provide the option to try loading the whole file into RAM (if
enough
is available), but Lucene does not use that option (we may add this
possibility
in a later version).
My question is: is there such an option?
is the method setPreLoad for this purpose:
to load all Lucene lndex into memory?
I would like to use MMapDirectory and set my
JVM heap to 16G or a bit less (since my index is
around this much).
The Lucene 8.5.2 (8.5.0 as well) javadocs say:
public void setPreload(boolean preload)
Set to true to ask mapped pages to be loaded into physical memory on init.
The
behavior is best-effort and operating system dependent.
For example Lucene 4.0.0 does not have setPreLoad method.
https://urldefense.com/v3/__https://lucene.apache.org/core/4_0_0/core/org/a
pache/lucene/store/MMapDi__;!!GqivPVa7Brio!Ll3PR4BZgqmgJNQ7MrnsXr27z
NYgjsyXlMh9h6awmbZgSNW-yVLBCDuFHTp_iadIDA$
rectory.html
Happy Holidays
Best regards
Ps. i know there is also BytesBuffersDirectory class for in memory Lucene
but
this requires creating Lucene Index on the fly.
This is great for only such kind of Lucene indexes that can be created
quickly on
the fly.
Ekaterina has a nice article on this BytesBuffersDirectory class:
https://urldefense.com/v3/__https://medium.com/@ekaterinamihailova/in-
memory-search-and-
__;!!GqivPVa7Brio!Ll3PR4BZgqmgJNQ7MrnsXr27zNYgjsyXlMh9h6awmbZgSNW
-yVLBCDuFHTry-H8S-g$
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