Re: java.util.Hashtable.size()/isEmpty()

[Artur Biesiadowski]

Etienne M. Gagnon wrote:
> Artur Biesiadowski wrote:
>
> > First - removing as much synchronization as possible, while staying 
> > compatible is certainly a good thing. Synchronizing is _really_ costly - 
>
>
> Have you heard of "thin locks" and other VM level optimizations that 
> reduces "significantly" the cost of locks?  Do not compute the cost of 
> locks based on their implementation in Sun's old JDK, or based on an 
> implementation that does not implement state-of-the-art techniques.

By your definition, 1.3 hotspot is not a state-of-art implementation... 
Ok, I have tried also with ibm, and cost was smaller - something like 
2.5 times slower.

Do you know any state-of-art jvms ? Hotspot, ibm and orp have quite 
costly locks :)

> > As for the Hashtable size(). Is there any point where it matters ?

> Yes.  The "synchronize" will force the "current processor" to update its 
> data cache.  Without the synchronize keyword, you processor could read 
> an obsolete value from its local cache, even though all other methods 
> are synchronized.  e.g:

I DO understand why synchronization is needed in general case. I was 
asking about scenario in which _Hashtable.size()_ synchronization would 
make a difference. Except using it as different way of 
synchronized(anything) {} - but this is not in spec (I mean that 
Hashtable.size() javadoc does not require us to create a memory barrier 
as a side effect).

In other mail I've explained why it doesn't matter with clear() (and why 
it would matter if clear would decrease size one by one). I'm looking 
exactly for such example - some test which can possibly show that size() 
is unsynchronized. Remember that you cannot really use words 'later' and 
'before' comparing threads, because before synchronization happens, 
temporal relation between them is 'fuzzy'.


Artur

P.S.
Sorry for defending my ideas so strong, but this is quite important 
point - it can affect implementation of many other classes and it would 
be good to create some kind of consensus.