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From Andrew Miehs <>
Subject Re: Tomcat with 8 GB memory
Date Mon, 30 Jul 2007 10:10:15 GMT
Hi Peter,

On 30/07/2007, at 11:55 AM, Peter Stavrinides wrote:
> In theory yes you are right, but remember that a 64bit Integer can  
> also be calculated by a 32bit processor, but only in two CPU  
> cycles, this is where the theoretical advantage of the 64 bit  
> architecture lies.

Yes agreed. The AMD64 arch (compared to i386) has an additional  
advantage however, and that is in the number of registers. It is  
possible to store twice as many (and twice as large) temporary  
variables in the local CPU registers - and I think that this will  
make the most difference at the moment for Java apps - especially  
ones not dealing with 64bit values.

> However in reality introducing the 64bit processor also introduces  
> a number of new hardware related problems (peak bandwidth,  
> sustained bandwidth, access latency), which have to do with he  
> amount of data that two components (i.e. the CPU and RAM) can  
> exchange over a given period. The catch 22 is that with increased  
> processing power, the latency problem is getting worse (a side- 
> effect of faster CPU's is more bus cycles worth of delay for each  
> memory access) Vendors are trying to compensate in various ways,  
> for example widening the bus. So, a 64bit processor alone cannot  
> speed up your application, the 12G RAM will help, but your actual  
> bandwidth may not be, in real terms, too different from a 32bit  
> system with far less RAM because of latency problems. 64bit  
> hardware designs are improving... this is where the real promise lies.

We found a 15% increase in speed - I definitely wasn't expecting it -  
but as I said, my guess is that it is probably based on the number of  
additional cpu registers.
(And no, the increase in speed was not due to more memory).

The 12GB app that I was referring to uses this memory for caching  
(disk and database access) - and due to the fact that we can now,  
without difficulties, allocate 12GB RAM, have made in this case the  
application quicker. No - the 'extra' memory doesn't make access to  
one block of memory quicker, but it does mean than I can store a  
whole lot more without needing to go back to the network or disk.



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