On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Matthew Wilcox wrote:
> >>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>Problem confirmed. 2.6.23.8 regularly generates segments up to 64KB
> >>>>for libata,
> >>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>Just a suspicion ... could this be slab vs slub? ie check your configs
> >>>are the same / similar between the two kernels.
> >>..
> >>
> >>Mmmm.. a good thought, that one.
> >>But I just rechecked, and both have CONFIG_SLAB=y
> >>
> >>My guess is that something got changed around when Jens
> >>reworked the block layer for 2.6.24.
> >>I'm going to dig around in there now.
> >
> >I didn't rework the block layer for 2.6.24 :-). The core block layer
> >changes since 2.6.23 are:
> >
> >- Support for empty barriers. Not a likely candidate.
> >- Shared tag queue fixes. Totally unlikely.
> >- sg chaining support. Not likely.
> >- The bio changes from Neil. Of the bunch, the most likely suspects in
> > this area, since it changes some of the code involved with merges and
> > blk_rq_map_sg().
> >- Lots of simple stuff, again very unlikely.
> >
> >Anyway, it sounds odd for this to be a block layer problem if you do see
> >occasional segments being merged. So it sounds more like the input data
> >having changed.
> >
> >Why not just bisect it?
> ..
>
> Because the early 2.6.24 series failed to boot on this machine
> due to bugs in the block layer -- so the code that caused this regression
> is probably in the stuff from before the kernels became usable here.
That would be the sg chain stuff, I don't think there's any correlation
between the two "bugs" (ie I don't get how you jump to the conclusion
that this regression is from stuff before that). Just go back as early
as you can, you could even just start with a -rc bisect.
--
Jens Axboe
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