On Wed, Aug 18, 2010 at 07:07:34PM -0700, Darrick J. Wong wrote:

Oddly, I ran the entire suite of tests against a larger set of machines, and
with Tejun's RFC patchset I didn't see nearly as much of an improvement.  I
have been trying to put together a new tree based on "replace barrier with
sequenced flush" and Christoph's "explicitly flush/FUA" patch sets, though I've
gotten lost in the weeds. :(

Tejun's patches don't allow concurrent cache flushes to happen, while
my patch did.  Tejun said there are drivers that can't handly empty
flushes with a bio attached, making this nessecary.   

Tejun, any idea what drivers that would be?

I also experienced some sort of crash with Tejun's relaxed barrier patch on one
of my systems.  I was hitting the BUG_ON in drivers/scsi/scsi_lib.c, line 1115.

My kernel source doesn't have a BUG_ON line there, but only one two
lines above.  A req->nr_phys_segments that's zero sounds a bit like
empty flush requests, I'll need to look into it again.