Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

Steve Lord
2001-02-06T21:23:21Z

>
> On Tue, 6 Feb 2001, Marcelo Tosatti wrote:
>
> > Think about a given number of pages which are physically contiguous on
> > disk -- you dont need to cache the block number for each page, you
> > just need to cache the physical block number of the first page of the
> > "cluster".
>
> ranges are a hell of a lot more trouble to get right than page or
> block-sized objects - and typical access patterns are rarely 'ranged'. As
> long as the basic unit is not 'too small' (ie. not 512 byte, but something
> more sane, like 4096 bytes), i dont think ranging done in higher levels
> buys us anything valuable. And we do ranging at the request layer already
> ... Guess why most CPUs ended up having pages, and not "memory ranges"?
> It's simpler, thus faster in the common case and easier to debug.
>
> > Usually we need to cache only block information (for clustering), and
> > not all the other stuff which buffer_head holds.
>
> well, the other issue is that buffer_heads hold buffer-cache details as
> well. But i think it's too small right now to justify any splitup - and
> those issues are related enough to have significant allocation-merging
> effects.
>
> Ingo

Think about it from the point of view of being able to reduce the number of
times you need to talk to the allocator in a filesystem. You can talk to
the allocator about all of your readahead pages in one go, or you can do
things like allocate on flush rather than allocating page at a time (that is
a bit more complex, but not too much).

Having to talk to the allocator on a page by page basis is my pet peeve about
the current mechanisms.

Steve