Re: CVS commit: [tls-maxphys] src/sys/dev

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Tue, Oct 09, 2012 at 04:54:41PM -0400, Thor Lancelot Simon wrote:
> On Tue, Oct 09, 2012 at 10:15:06PM +0200, Manuel Bouyer wrote:
> > 
> > Now, iostat -x shows that sequential read of a large file (either with cat
> > or dd bs=1m) is done in 32k transfers at the disk level. So I guess 
> > something
> > is not working properly here ...
> 
> I saw that too, though I managed to get 64K transfers.  I have been meaning
> to test with mmap, since that is more like the code path through the upper
> layers that produces the attempt to page in an entire executable.
> 
> I've looked all through every layer I can identify and I do not see where
> the seemingly still remaining 64K restriction comes from.  I thought it
> was FFS, but in fact *every vestige* of the maxcontig or "maximum extent
> size" use seems to have been excised from our FFS code.  So it's a mystery
> to me.

There is still a reference to MAXPHYS in ufs_bmaparray(), which, if I got
it right, will limit the *runp in VOP_BMAP(), which will limit iobytes
in genfs_do_io().

but that doens't explain why I see 32k xfers. And, to make things worse,
I see 64k if I start the big file read with debug enabled, so it looks
timing-dependant :(

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--