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From Emmanuel Lecharny <elecha...@gmail.com>
Subject Re: Update about subtree problems
Date Tue, 20 Jul 2010 08:15:19 GMT
  Hi Howard,

On 7/20/10 9:29 AM, Howard Chu wrote:Some side note :
>> after having done some perf tests on the evaluator, and applied some
>> improvement, I can tell that depending on the number of subentries an
>> entry is depending on, the cost of this evaluation can goes up to 50% of
>> the search itself cost - not counting the network layer -. For instance,
>> evaluating a subtreeSpecification with a min and a max, no chop, will be
>> done up to 1 000 000 times per second on a 3 level DN (this is all
>> dependent on the DN size)
>
> IMO, the considerations here are the same as for the O(1) rename. 
> I.e., when you remove the entryDN from the entry in the DB, you have 
> to calculate the DN on the fly, and it certainly is a frequently 
> referenced datum. You make this cheap by caching the entryDN in 
> memory, and it's very clear when a cached DN must be invalidated - 
> most of the time the cached value will not change. 
The DN cache is most certainly needed for faster operations. Building a 
DN on the fly for every entry is one of the most costly operation, so if 
we can speed it up with a cache, it's a net gain. Having the DN in the 
entry OTOH is not necessary a big gain : you still have to deserialize 
it if it's not in cache, and this is also costly.

Obviously, all those considerations fell in a big dark hole if you have 
a decent entry cache, as the entries in memory already store the full 
DN... Any modification like a rename or a move will of course invalidate 
the entries in this cache.

All in all, most of the case, you don't have to do all those 
computations...

Regarding the subtree handling, it's different, as you can't spare the 
entry evaluation if the entries don't contain the reference to the 
subentry they depend upon. This evaluation can be costly, up to a point 
it's more expensive than fetching the entry itself.

The rational being the choice I made 3 years ago (and which was 
reverted) to put the DN into the entry was just to speed up any search 
by avoiding costly computation at a price of costly unfrequent 
operations like Move or Rename (MODDN).

If you have to move data in a Ldap base, User, then you have to pay the 
price !

-- 
Regards,
Cordialement,
Emmanuel L├ęcharny
www.iktek.com


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