Re: Is Parallel Programming Hard, And, If So, What Can You Do About It?

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)