Re: Is Greenspun enough?

On Sat, 10 Dec 2005 20:15:48 -0600, rpw3@xxxxxxxx (Rob Warnock) wrote:

>Duane Rettig <duane@xxxxxxxxx> wrote:
>+---------------
>| Ulrich Hobelmann <u.hobelmann@xxxxxx> writes:
>| > George Neuner wrote:
>| >>>>> mmap()ed files are also cached, no?
>| >>>> No. Mapped files are handled by the virtual memory system and all
>| >>>> modern systems DMA pages directly to/from disk with no buffering.
>| >>> Seriously? I thought every common OS would buffer/cache most pages...
>| >> Seriously, you need to read up on MMUs, virtual memory and demand
>| >> paging systems.
>| >
>| > What I (think I) know about modern OSes is that they cache everything
>| > from disk, I suppose by associating file blocks with in-memory-blocks.
>| > Surely I could be wrong...
>+---------------
>
>Ulrich isn't wrong, George. The "global page cache" has pretty much
>completely replaced the "file block cache" in recent versions of the
>VM systems of most decent operating systems [e.g., Irix, *BSD, Linux].
>Pretty much *everything* is cached using the same mechanism [which
>does have its downsides sometimes, to be sure.].


I think this is a debate about terminology. You are arguing that the
page in RAM is a "cache" of the disk data and I am arguing that it is
not - at least not in general.

The traditional software notion of cache is that of a copy kept for
some reason ... the operative term being "copy". It is applicable to
mapped executables, but not to swappable data because the page in RAM
is not, in general, a copy of the disk data.


WRT "global page cache", AFAICT it is nothing more than a new and
somewhat misleading name for what used to be called the "page frame
table".


I think we've wasted enough bandwidth ... time to agree to disagree.

George
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