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From "Nadav Har'El (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-695) Improve BufferedIndexInput.readBytes() performance
Date Thu, 26 Oct 2006 14:33:56 GMT
    [ http://issues.apache.org/jira/browse/LUCENE-695?page=comments#action_12444903 ] 
            
Nadav Har'El commented on LUCENE-695:
-------------------------------------

> If "given" a null array? Is this ever done in Lucene? Which should be fixed, the testcase
or the code? 

I don't know - readBytes() documentation doesn't explictly say what should happen if it is
asked to read zero bytes: is it simply supposed to do nothing (and in this case it doesn't
matter which array you give it - could even be null), or should it still expect the array
to be non-null? I don't know if any code in Lucene itself assumes that it can work when given
a null array and a 0 count - I doubt it. But one test does assume this, so I simply added
an extra "if" to check for the 0 count, and when that happens, avoid calling System.arraycopy()
(which even when the count is 0, expects the array to be non-null, for some reason).

> Improve BufferedIndexInput.readBytes() performance
> --------------------------------------------------
>
>                 Key: LUCENE-695
>                 URL: http://issues.apache.org/jira/browse/LUCENE-695
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Store
>    Affects Versions: 2.0.0
>            Reporter: Nadav Har'El
>            Priority: Minor
>         Attachments: readbytes.patch, readbytes.patch
>
>
> During a profiling session, I discovered that BufferedIndexInput.readBytes(),
> the function which reads a bunch of bytes from an index, is very inefficient
> in many cases. It is efficient for one or two bytes, and also efficient
> for a very large number of bytes (e.g., when the norms are read all at once);
> But for anything in between (e.g., 100 bytes), it is a performance disaster.
> It can easily be improved, though, and below I include a patch to do that.
> The basic problem in the existing code was that if you ask it to read 100
> bytes, readBytes() simply calls readByte() 100 times in a loop, which means
> we check byte after byte if the buffer has another character, instead of just
> checking once how many bytes we have left, and copy them all at once.
> My version, attached below, copies these 100 bytes if they are available at
> bulk (using System.arraycopy), and if less than 100 are available, whatever
> is available gets copied, and then the rest. (as before, when a very large
> number of bytes is requested, it is read directly into the final buffer).
> In my profiling, this fix caused amazing performance
> improvement: previously, BufferedIndexInput.readBytes() took as much as 25%
> of the run time, and after the fix, this was down to 1% of the run time! However, my
scenario is *not* the typical Lucene code, but rather a version of Lucene with added payloads,
and these payloads average at 100 bytes, where the original readBytes() did worst. I expect
that my fix will have less of an impact on "vanilla" Lucene, but it still can have an impact
because it is used for things like reading fields. (I am not aware of a standard Lucene benchmark,
so I can't provide benchmarks on a more typical case).
> In addition to the change to readBytes(), my attached patch also adds a new
> unit test to BufferedIndexInput (which previously did not have a unit test).
> This test simulates a "file" which contains a predictable series of bytes, and
> then tries to read from it with readByte() and readButes() with various
> sizes (many thousands of combinations are tried) and see that exactly the
> expected bytes are read. This test is independent of my new readBytes()
> inplementation, and can be used to check the old implementation as well.
> By the way, it's interesting that BufferedIndexOutput.writeBytes was already efficient,
and wasn't simply a loop of writeByte(). Only the reading code was inefficient. I wonder why
this happened.

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