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From: Stephen Fuld <sfuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: Is Parallel Programming Hard, And, If So, What Can You Do About
 It?
Date: Tue, 13 May 2025 17:49:17 -0700
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On 5/13/2025 5:18 PM, Lawrence D'Oliveiro wrote:
> On Tue, 13 May 2025 08:33:15 -0700, Stephen Fuld wrote:
> 
>> No one is arguing that host based file caches are bad.  It is simply the
>> fact that there are situations where drive caches are a useful
>> addition ...
> 
> You can tell that’s wrong because the drive cache is slower than the OS
> filesystem cache. 

I don't think that proves anything.  I gave you an example of where the 
drive cache speeded up the sequence of two reads to where it was faster 
than two reads into the file cache.


Putting a slower cache in series with a faster one is a
> waste of time ... unless the slower cache is much larger.

See my counter example posted earlier.


> This is why, for example, we typically have 3 levels of RAM cache between
> the CPU and main memory these days. There is a factor of about 100:1 in
> relative speeds, so to bridge the gap we need multiple caches of various
> intermediate speeds, and you will notice their sizes are inversely related
> to their speeds.
> 
> A drive cache can never be as big as main RAM on a modern PC. That’s why
> the drive cache is useless.

You haven't refuted my example, and besides the comparison you give here 
is not meaningful because you can't use all the main ram as a file cache.




-- 
  - Stephen Fuld
(e-mail address disguised to prevent spam)