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From DM Smith <>
Subject Re: Token implementation
Date Tue, 20 May 2008 00:25:23 GMT

On May 19, 2008, at 4:33 PM, Michael McCandless wrote:

> DM Smith <> wrote:
>> Michael McCandless wrote:
>>> I agree the situation is not ideal, and it's confusing.
>> My problem as a user is that I have to read the code to figure out  
>> how to
>> optimally use the class. The JavaDoc is a bit wanting.
> Yeah we should fix the javadocs.
>>> This comes back to LUCENE-969.
>>> At the time, we decided to keep both String & char[] only to avoid
>>> performance cost for those analyzer chains that use String tokens
>>> exclusively.
>>> The idea was to allow Token to keep both text or char[] and  
>>> sometimes
>>> both (if they are storing the same characters, as happens if
>>> termBuffer() is called when it's a String being stored)
>> When termBuffer() is called termText is always null on return. This  
>> is the
>> invariant of initTermBuffer() which is called directly or  
>> indirectly by
>> every method that touches termBuffer.
> Sorry I meant termText().
>> It is only after calling termText() that one could have both. The  
>> only
>> advantage I see here is that calling it twice without any  
>> intervening call
>> to a termBuffer method would it return the same physical string.
> Right.
>> After calling setTermText(newText), termBuffer is set to null.
>> I presume the purpose of a filter is to get the token content, and if
>> modified, set the new content. If so, the result of the setXXX will  
>> be that
>> either termText or termBuffer will be null.
> Right, though, if there's no change, and the next filter in the chain
> calls termText(), we save constructing a new String by caching it.
>>> Then, in 3.0, we would make the change you are proposing (to only
>>> store char[] internally).  That was the plan, anyway.  Accelerating
>>> this plan (to store only char[] today) is compelling, but I worry
>>> about the performance hit to legacy analyzer chains...
>> I wonder whether it is all that significant an issue. Today, some  
>> of the
>> Lucene analyzers have been migrated to using char[], while others,  
>> notably
>> contrib, continue to use text.
>> IMHO: Prior to char[], the text was replaceable, but not  
>> modifiable. From a
>> practical perspective, Token reuse minimized the cost of  
>> construction, but
>> not much else. The performance of a Token was predictable, but the  
>> filter
>> was potentially sub-optimal. With char[] and supporting methods,  
>> the text
>> became modifiable, too.
>> When a filter calls setTermText or setTermBuffer, it does not know  
>> how the
>> consumer of the token will work. It could be that it stores it with
>> setTermText and the next filter calls termBuffer().
>> I may not understand this correctly, but it seems to me that the  
>> following
>> is plausible given a filter chain of Lucene provided filters  
>> (including contrib)
>> If we have a token filter chain of A -> B -> C, which uses next()  
>> in any
>> part of the chain, the flow of a reusable Token is stopped. A given  
>> filter
>> may cache a Token and reuse it. So consider the following scenario:
>> A overrides next(Token) and reuses the token via char[]
>> B overrides next() and has a cached Token and updates text.
>> C overrides next(Token) and reuses the token via char[].
>> First run:
>> After A is done, the termText in the supplied Token is null and  
>> termBuffer
>> has a value.
>> B provides it's own token so it is not influenced by A.
>> C is given the token that B returns, because the caller is  
>> effectively using
>> "token =", but because the token has text, a  
>> performance
>> hit is taken to put it into char[]. Both text and char[] start out  
>> the same,
>> but because char[] is changed, termText is set to null.
>> Second run:
>> A starts with a Token with a char[] because it is reusing the token  
>> from the
>> last run or because it is using a localToken from the caller. If it  
>> is a
>> localToken, then the scenario is as above. But if it is the end  
>> result of
>> the first run, then A is re-using the token that is cached in B.  
>> Since C
>> last modified it, it is char[].
>> B uses its cached Token, but it was modified by A to be char[] with  
>> null
>> text. Now B takes a performance hit as it creates a new String.
>> C is as in the first run.
>> Another scenario:
>> A, B and C are all legacy. This would only be filter chains that  
>> are not
>> provided by core Lucene as the core filter chains have been  
>> migrated. This
>> would be a performance hit.
> Why is this a performance hit?  If they are all legacy they would all
> implemented the non-reuse next() API, and would use setTermText, and
> no conversion to char[] would happen (except inside DocumentsWriter)?

I was thinking faster than I was typing. I meant to say "not be a  
performance hit".

>> A last scenario:
>> A, B and C are all char[]. This would not take a performance hit.
>> It seems to me that in a mixed chain, that there will always be a
>> performance hit.
> Right, though since we cache the String we should save on multiple
> calls to termText().  And in a non-mixed chain (all new or all old) I
> think there wouldn't be a hit.
>> But my guess is that maintaining termBuffer once used would
>> be a good thing.
> Interesting...
>> So, a modified suggestion to maintain the performance but improve  
>> the first
>> scenario. Do you see any problem with the following?
>> If termBuffer is used in a token, then it is maintained and never  
>> set to  null.
>> Note also that resizeTermBuffer(size) maintains the termBuffer.  
>> That is, it
>> copies the text when the array is grown. When it is known that it  
>> is going
>> to be slammed this is unnecessary. One can implement a helper  
>> function that
>> merely grows the array. There are a couple of places this
>> Thus setTokenText would be something like:
>> public void setTermText(String text) {
>>  termText = text;
>>  if (termBuffer != null) {
>>    growTermBuffer(termText.length());
>>    termText.getChars(0, termText.length(), termBuffer, 0);
>>  }
>> }
>> A possible implementation to grow the array would be:
>> private void growTermBuffer(int newSize)
>> {
>> // determine the best size
>> if (newSize < MIN_BUFFER_SIZE) {
>>  newSize = MIN_BUFFER_SIZE;
>> }
>> // if the buffer exists and is too small, then determine a better  
>> size.
>> // this is the current doubling algorithm. it could be better.
>> int tbLength = termBuffer == null ? 0 : termBuffer.length;
>> if (tbLength > 0 && newSize > tbLength) {
>>  int size = tbLength;
>>  while (size < newSize) {
>>     size *= 2;
>>  }
>>   newSize = size;
>> }
>> // Check to see if the buffer needs to be resized
>> if (newSize > tbLength)
>> {
>>  termBuffer = new char[newSize];
>> }
>> }
> That looks good.  Though, if Token is 100% re-used (full chain is
> "new") then I would expect growing the char[] to be very low cost
> (happens very few times).

Without peeking at your Python sample below ;) the doubling algorithm  
is costly as it over allocates too much. But it has the nice behavior  
that reallocations are fewer.

But, now peeking ahead, a less aggressive allocation policy would  
cause more allocations and the array copy becomes more important.  
Though not significantly. But in a mixed environment with some  
sharing, it could be magnified.

I didn't dig into it, but in the core, are reusable Tokens per  
document or per DocumentWriter. If it is the former then the copy  
becomes more significant. If it is the latter, then it is almost moot,  
as you point out.

>> More below....
>>> More responses below:
>>> DM Smith <> wrote:
>>>> I think the Token implementation as it stands can use some  
>>>> improvement and
>>>> I'd be willing to do it. I'd like some input, though. Especially  
>>>> because it
>>>> is core to Lucene.
>>>> I've been working on eliminating deprecations from my user code  
>>>> and I ran
>>>> across Token.getText() as being deprecated.
>>>> This is not about my code, but the code in Token.
>>>> In Token, it allows one of two representations of the actual  
>>>> token, either
>>>> an immutable String, or a mutable char[]. One can flip back and  
>>>> forth
>>>> between these all to easily.
>>>> termText() is deprecated so termBuffer() is suggested as a  
>>>> replacement.
>>>> Calling termBuffer() will potentially copy the text out of the  
>>>> String
>>>> termText and into the newly created buffer and return it.
>>>> Calling setTermText(str), which is not deprecated, will drop the  
>>>> buffer and
>>>> save the str in termText.
>>>> It appears that the invariant that is trying to be established is
>>>> either termText or termBuffer holds the content, but not both.
>>>> However, termBuffer() potentially violates this by loading termText
>>>> with the termBuffer, but not nulling out the buffer.
>>> Actually, both are allowed to be set, as long as they are the same.
>>> So termBuffer() is allowed to leave both non-null.
>> I'm not sure of the advantage.
> Covered above (mixed chains).
>>>> Also, in my code, I am not manipulating char[] so getting the  
>>>> buffer, I need
>>>> to immediately convert it to a string to process it. And then  
>>>> when I'm done,
>>>> I have a String possibly of some other length. To stuff it back  
>>>> into the
>>>> termBuffer, requires a call to:
>>>> setTermBuffer(s.toCharArray(), o, s.length())
>>> It would be better to call Token.resizeTermBuffer(...), then
>>> s.getChars into the Token's term buffer (saves a buffer copy).
>> Many thanks. I saw the comment, but missed getChars. This is  
>> better, but
>> even better would be growTermBuffer (above) since resizeTermBuffer
>> potentially has an unnecessary copy.
> True, but remember growing should be very rare in 100% reuse (new)  
> case.

I'm presuming then that the DocumentWriter is the holder of the one  
reusable Token.

>>>> I was looking at this in light of TokenFilter's next(Token)  
>>>> method and how
>>>> it was being used. In looking at the contrib filters, they have  
>>>> not been
>>>> modified. Further, most of them, if they work with the content  
>>>> analysis and
>>>> generation, do their work in strings. Some of these appear to be  
>>>> good
>>>> candidates for using char[] rather than strings, such as the  
>>>> NGram filter.
>>>> But others look like they'd just as well remain with String  
>>>> manipulation.
>>> It would be great to upgrade all contrib filters to use the re-use  
>>> APIs.
>> I'll be happy to work toward that end. I think it affects my  
>> performance
>> today.
> That would be great :)
>>>> I'd like to suggest that internally, that Token be changed to  
>>>> only use
>>>> char[] termBuffer and eliminate termText.
>>> The question is what performance cost we are incurring eg on the
>>> contrib (& other) sources/filters?  Every time setTermText is  
>>> called,
>>> we copy out the chars (instead of holding a reference to the  
>>> String).
>>> Every time getText() is called we create a new String(...) from the
>>> char[].  I think it's potentially a high cost, and so maybe we  
>>> should
>>> wait until 3.0 when we drop the deprecated APIs?
>> See above. I'll concede that. But I think that once termBuffer is  
>> used
>> because of a mixed chain, it should be retained.
> This may cause problems if a core Tokenizer is used to produce the
> tokens, but then a big chain of old (non-reuse) filters is used after
> that?

If the reusable token is per DocumentWriter and the chain is per  
Document, then it is an advantage to retain it. he buffer is there for  
the next Document, likely big enough that it does not need to be  
allocated again.

>>>> And also, that termText be restored as not deprecated.
>>> It made me nervous keeping this method because it looks like it  
>>> should
>>> be cheap to call, and in the past it was very cheap to call.  But,
>>> maybe we could keep it, if we mark very very clearly in the javadocs
>>> the performance cost you incur by using this method (it makes a new
>>> String() every time)?
>>>> But, in TokenFilter, next() should be deprecated, IMHO.
>>> I think this is a good idea.  After all if people don't want to  
>>> bother
>>> using the passed in Token, they are still allowed to return a new
>>> one.
>> Should the constructors in Token that take a String be deprecated,  
>> too? The
>> comments seem to suggest that.
> Actually I think they should.  We should strongly encourage the re-use
> APIs.
>>>> I have also used a better algorithm than doubling for resizing an
>>>> array. I'd have to hunt for it.
>>> That would be great!
>> I'm looking, but still haven't found it. :)
>> I'm looking into digging into this one.
> Python has an interesting approach, for its list type.  Here's a
> snippet from listobject.c from Python's sources:
> 	/* This over-allocates proportional to the list size, making room
> 	 * for additional growth.  The over-allocation is mild, but is
> 	 * enough to give linear-time amortized behavior over a long
> 	 * sequence of appends() in the presence of a poorly-performing
> 	 * system realloc().
> 	 * The growth pattern is:  0, 4, 8, 16, 25, 35, 46, 58, 72, 88, ...
> 	 */
> 	new_allocated = (newsize >> 3) + (newsize < 9 ? 3 : 6) + newsize;

This is a good one. It is computationally simple and it is not too  

> I'm sure Perl has something interesting too :)  But I think this is
> probably overkill for us ...
>> Based on the outcome of this discussion, would this be one Jira  
>> issue?
>> Reopening LUCENE-969? A separate issue for the contrib changes?
> I would say open a new issue?

OK. I'm on vacation for the next week and I don't know what kind of  
connectivity I'll have. So it might wait a bit.

>> Since this would not be an API change, would there need to be  
>> changes to
>> test cases? If so, what would you suggest?
> I think the existing test cases are likely sufficient though I always
> love adding new ones ;)

Me too ;)

-- DM

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