Re: Degraded I/O performance, since 2.5.41

[Dave Hansen <haveblue@us.ibm.com>]

Linus Torvalds wrote:
 > In article <3DA4D34F.3070106@us.ibm.com> you write:
 >
 >>When I run a certain large webserver benchmark, I prefer to warm the
 >>pagecache up with the file set first, to cheat a little :)  I grep
 >>through 20 different 500MB file sets in parallel to do this.  It is 
 >>a _lot_ slower in the BK snapshot than in plain 2.5.41.
 >
 > Can you pinpont where this started happening? There aren't that many
 > changes since 2.5.41 that I'd consider suspicious, and it would 
help > to try to pinpoint it a bit.

Using stupid brute force, I narrowed it down to 12 patches.  The only
one that touches generic SCSI code is the queue length one.
1.704.1.2, in my tree from dledford@redhat.com.  Backing it out fixed
my problem.  CC'ing him...

I would guess that it's just making stupid, er.... suboptimal,
decisions about the length for my hardware.  With a controller as
"smart" as the ServeRAID, I would imagine the best approach is to just
throw as many requests at it at one time as you can, then let the
controller sort it out.  Doug, I noticed that you changed a bunch of
code to the aic7xxx_old driver, is this just a lack of optimization in
the ServeRAID driver?

-- 
Dave Hansen
haveblue@us.ibm.com