Re: Reprise: 'in out' parameters for functions

["Robert I. Eachus" <rieachus@comcast.net>]

Wojtek Narczynski wrote:

> This is not easy but solveable. For example ADABAS (and forks)
> relational database is organized as one large B* tree. It can be
> edited concurrently, it rebalances, and does not deadlock.
> 
> One more example: Java heap, millions of objects, 64 CPUs, thousands
> of threads. Would you like to have one lock for the whole data
> strucutre?

You are missing the point.  The answer is no, one lock per storage pool. 
  How many storage pools do you have?  At least one per CPU, more 
usually two or three due to different storage management protocols.  But 
the major insight is that you either update a copy of an object and then 
  swap it in atomically as Randy said, or (my preference) the lock for 
the data structure controls the data structure and a separate per object 
lock controls the object updates.  (Except changing index keys, which 
has to be done Randy's way.)

The simplest data structure locking protocol is usually by far the most 
efficient.  Spending a dozen times as many CPU cycles on lock management 
to double or triple the effective concurrency is a joke, and you will 
find that holding complex schemes to only a dozen times as long is 
difficult.  Factor in memory caches, and anything else is crazy.

What is important, in real cases, is memory management.  Having any part 
of a data structure paged to disk can kill performance.  (The data 
structure now, not the actual data.)  This is why I tend to implement 
the tree or whatever with a pointer (excuse me access value) designating 
  the data.  That way the next data structure entry you need will be in 
main memory, and is much more likely to be in cache.

(If you also figure from this that I am one of those people who designs 
large objects and not that many of them, you are right.  Complex data 
dictionaries are all right for the theoretical part of design, but when 
it comes to practice, you want to merge data nodes to the maximum extent 
possible.)

-- 

                                           Robert I. Eachus

"The terrorist enemy holds no territory, defends no population, is 
unconstrained by rules of warfare, and respects no law of morality. Such 
an enemy cannot be deterred, contained, appeased or negotiated with. It 
can only be destroyed--and that, ladies and gentlemen, is the business 
at hand."  -- Dick Cheney