Re: linux-2.4.10-pre5

Alex Bligh - linux-kernel
2001-09-10T21:23:16Z

--On Monday, 10 September, 2001 11:18 PM +0200 Daniel Phillips
<phillips@bonn-fries.net> wrote:

> We lost 78.5 seconds somewhere. From the sound of the disk drives, I'd
> say we lost it to seeking, which physical readahead with a large cache
> would be able to largely eliminate in this case.

So you only get a large % of your 78.5 seconds back
from pure physical readahead if the files and metadata
are (a) ordered in monitonically increasing disk
locations w.r.t. access order, and (b) physically close (no time
wasted reading in irrelevant data), or you apply some
form of clairvoyance patch :-) An alternative benchmark
would be do dd the /entire/ disk into RAM, then
run your diff on that, and I bet you get the
opposite result.

More serious point: if we retain readahead at a
logical level, you get it for non-physical
files too (e.g. NFS) - I presume this is
the intention. If so, what advantage does
additional physical readahead give you,
given physical ordering is surely no better
(and probably worse than) logical ordering.

--
Alex Bligh