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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-753) Use NIO positional read to avoid synchronization in FSIndexInput
Date Thu, 17 Jul 2008 10:23:32 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12614290#action_12614290
] 

Michael McCandless commented on LUCENE-753:
-------------------------------------------


{quote}
In our implementation the synchronization/lack of concurrency has been a big issue for us.
On several occasions we've had to remove new features that perform searches from frequently
hit pages, because threads build up waiting for synchronized access to the underlying files.
 It is possible that I would still have issue even with my patch, considering from my tests
that I'm only increasing throughput by 300%, but it would be easier for me to tune and scale
my application since resource utilization and contention would be visible from the OS level.

{quote}

Can you describe your test -- OS, JRE version, size/type of your index, number of cores, amount
of RAM, type of IO system, etc?  It's awesome that you see 300% gain in search throughput.
 Is your index largely cached in the OS's IO cache, or not?

{quote}
My vote is that the benefits outway the complexity, especially considering it's an out-of-the
box solutions that works well for all platforms and single threaded as well as multi-threaded
envirnments. If it's helpful, I can spend the time to implement some of the missing feature(s)
of the pool that will be needed for it to be an acceptable solution (i.e, shared access once
a file has been deleted, and perhaps a time-based closing mechanism).
{quote}

If we can see sizable concurreny gains, reliably & across platforms, I agree we should
pursue this approach.  One particular frustration is: if you optimize your index, thinking
this gains you better search performance, you're actually making things far worse as far as
concurrency is concerned because now you are down to a single immense file.  I think we do
need to fix this situation.

On your patch, I think in addition to shared-access on a now-deleted file, we should add a
global control on the "budget" of number of open files (right now I think your patch has a
fixed cap per-filename).  Probably the budget should be expressed as a multiplier off the
minimum number of open files, rather than a fixed cap, so that an index with many segments
is allowed to use more.  Ideally over time the pool works out such that for small files in
the index (small segments) since there is very little contention they only hold 1 descriptor
open, but for large files many descriptors are opened.

I created a separate test (will post a patch & details to this issue) to explore using
SeparateFile inside FSDirectory, but unfortunately I see mixed results on both the cached
& uncached cases.  I'll post details separately.

One issue with your patch is it's using Java 5 only classes (Lucene is still on 1.4); once
you downgrade to 1.4 I wonder if the added synchronization will become costly.

I like how your approach is to pull a RandomAccessFile from the pool only when a read is taking
place -- this automatically takes care of creating new descriptors when there truly is contention.
 But one concern I have is that this defeats the OS's IO system's read-ahead optimization
since from the OS's perspective the file descriptors are getting shuffled.  I'm not sure if
this really matters much in Lucene, because many things (reading stored fields & term
vectors) are likely not helped much by read-ahead, but for example a simple TermQuery on a
large term should in theory benefit from read-ahead.  You could gain this back with a simple
thread affinity, such that the same thread gets the same file descriptor it got last time,
if it's available.  But that added complexity may offset any gains.


> Use NIO positional read to avoid synchronization in FSIndexInput
> ----------------------------------------------------------------
>
>                 Key: LUCENE-753
>                 URL: https://issues.apache.org/jira/browse/LUCENE-753
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Store
>            Reporter: Yonik Seeley
>         Attachments: FileReadTest.java, FileReadTest.java, FileReadTest.java, FileReadTest.java,
FileReadTest.java, FileReadTest.java, FileReadTest.java, FSIndexInput.patch, FSIndexInput.patch,
lucene-753.patch, lucene-753.patch
>
>
> As suggested by Doug, we could use NIO pread to avoid synchronization on the underlying
file.
> This could mitigate any MT performance drop caused by reducing the number of files in
the index format.

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