[ZODB-Dev] Re: [Zope-dev] Re: 64-bit BTrees

[Jim Fulton]

Tres Seaver wrote:
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> Fred Drake wrote:
> 
>>I have a need for 64-bit BTrees (at least for IOBTree and OIBTree),
>>and I'm not the first.  I've created a feature development branch for
>>this, and checked in my initial implementation.
>>
>>I've modified the existing code to use PY_LONG_LONG instead of int for
>>the key and/or value type; there's no longer a 32-bit version in the
>>modified code.  Any Python int or long that can fit in 64 bits is
>>accepted; ValueError is raised for values that require 65 bits (or
>>more).  Keys and values that can be reported as Python ints are, and
>>longs are only returned when the value cannot be converted to a Python
>>int.
>>
>>This can have a substantial effect on memory consumption, since keys
>>and/or values now take twice the space.  There may be performance
>>issues as well, but those have not been tested.
>>
>>There are new unit tests, but more are likely needed.
>>
>>If you're interested in getting the code from Subversion, it's available at:
>>
>>    svn://svn.zope.org/repos/main/ZODB/branches/fdrake-64bits/
>>
>>Ideally, this or some variation on this could be folded back into the
>>main development for ZODB.  If this is objectionable, making 64-bit
>>btrees available would require introducing new versions of the btrees
>>(possibly named LLBTree, LOBTree, and OLBTree).
> 
> 
> I think coming up with new types is the only reasonable thing to do,
> given the prevalence of persistent BTrees out in the wild.  Changing the
> runtime behavior (footprint, performance) of those objects is probably
> not something which most users are going to want, at least not without
> carefully considering the implications.

It really depends on what the impact is.  It would be nice to get a feel
for whether this really impacts memory or performance for real applications.
This adds 4-bytes per key or value.  That isn't much, especially in a typical
Zope application.  Similarly, it's hard to say what the difference in C integer
operations will be.  I can easily imagine it being negligible (or being
significant :).

OTOH, adding a new type could be a huge PITA. We'd like to use these with existing
catalog and index code, all of which uses IIBTrees.  If the performance impacts are
modest, I'd much rather declare IIBTrees to use 64-bit rather than 32-bit integers.

I suppose an alternative would be to add a mechanism to configure IIBTrees to use
either 32-bit or 64-bit integers at run-time.

Jim

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