anon (anon@anon.com) on 1/6/09 wrote:
---------------------------
>Joern Engel (joern@logfs.org) on 1/6/09 wrote:
>>
>>Except that certain system software defines constants like L1_CACHE_BYTES. I
>>wonder what those are all about. ;)
>
>This is a totally different issue. This is a property of the host CPU which the
>software is running on (the embedded software in the flash controller
>could define a similar constant).

Oh, yes, you are right of course. It is not a property of DRAM, but
one of the DRAM interface. And said interface has migrated into the
CPU, it is no longer part of the chipset. Don't hold it against me,
but that does look like an interface that has changed over the
years.

Software had a fairly small part in the change. It only had to learn
about cache line sizes and alignment. The bulk of it (and I agree it
is a lot) has been between DRAM, CPU and the now-gone pieces of
chipset.

>If you can propose a way to hide the details of cacheline size from software in
>a way that results in a better end result, we're all ears. Byte-sized cachelines? ;)

Well, contrary to my predictions, cache line sizes have actually
shrunk over the last generation, going back from 128B to 64B. Took me
by surprise and I am delighted. However, I see fundamental technical
reasons why cachelines will always exist and always be much bigger
than a single byte. So as you noticed, we can stand on our heads and
wiggle our toes all day, software will run faster if we use
L1_CACHE_BYTES and align our data.

Going back to the main subject, the interface between flash and the
CPU, I likewise claim that things will run faster if we know erase
sizes and align data. And more robustly if we do wear leveling
ourselves.

There is one decisive difference between DRAM and flash, though. If
the market goes along with Linus, there will effectively be a
specialized filesystem on each SSD. And things no longer run faster
when properly aligned _on top of that filesystem_. They would only
run faster if the filesystem could be bypassed somehow. DRAM is just
too low-level for anyone to put a filesystem between it and the CPU.