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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 Tue, 01 Jul 2008 10:05:45 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12609514#action_12609514
] 

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


OK it's looking like SeparateFile is the best overall choice...  it
matches the best performance on Unix platforms and is very much the
lead on Windows.

It's somewhat surprising to me that after all this time, with these
new IO APIs, the most naive approach (using a separate
RandomAccessFile per thread) still yields the best performance.  In
fact, opening multiple IndexReaders to gain concurrency is doing just
this.

Of course this is a synthetic benchmark.  Actual IO with Lucene is
somewhat different.  EG it's a mix of serial (when iterating through a
term's docs with no skipping) and somewhat random access (when
retrieving term vectors or stored fields), and presumably a mix of
hits & misses to the OS's IO cache.  So until we try this out with a
real index and real queries we won't know for sure.

{quote}
The question would be, what would the algorithm for allocating
RandomAccessFiles to which file look like?
{quote}

Ideally it would be based roughly on contention.  EG a massive CFS
file in your index should have a separate file per-thread, if there
are not too many threads, whereas tiny CFS files in the index likely
could share / synchronize on a single file

I think it would have thread affinity (if the same thread wants the
same file we give back the same RandomAccessFile that thread last
used, if it's available).

{quote}
When would a new file open, when would a file be closed?
{quote}

I think this should be reference counting.  The first time Lucene
calls FSDirectory.openInput on a given name, we must for-real open the
file (Lucene relies on OS protecting open files).  Further opens on
that file incRef it.  Closes decRef it and when the reference count
gets to 0 we close it for real.

{quote}
If it is based on usage would it be based on the rate of calls to
readInternal?
{quote}

Fortunately, Lucene tends to call IndexInput.clone() when it wants to
actively make use of a file.

So I think the pool could work something like this: FSIndexInput.clone
would "check out" a file from the pool.  The pool decides at that
point to either return a SharedFile (which has locking per-read, like
we do now), or a PrivateFile (which has no locking because you are the
only thread currently using that file), based on some measure of
contention plus some configuration of the limit of allowed open files.

One problem with this approach is I'm not sure clones are always
closed, since they are currently very lightweight and can rely on GC
to reclaim them.

An alternative approach would be to sync() on every block (1024 bytes
default now) read, find a file to use, and use it, but I fear that
will have poor performance.

In fact, if we build this pool, we can again try all these alternative
IO APIs, maybe even leaving that choice to the Lucene user as
"advanced tuning".

> 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
>
>
> 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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