Re: [PATCH] 2.5.14 IDE 56

[Lincoln Dale]

At 01:42 PM 8/05/2002 +0100, Alan Cox wrote:
>The SCSI layer is significant overhead even in 2.5.

i did some benchmarking on a high-end dual P3 Xeon (Serverworks chipset ) 
with QLogic 2300 (2gbit/s) 64/66 Fibre Channel controllers.

using the '/dev/sgX' interface to issue scsi reads/writes allowed me to hit 
the magical limit of 200mbyte/sec throughput.  (basically just about 
linerate).  (simultaneous "sg_read if=/dev/sgX mmap=1 bs=512 count=35M"; 
sg_read from the sg-tools package)

doing the same test thru the block-layer was basically capped at around 
135mbyte/sec.  (simultaneous "dd if=/dev/sdX of=/dev/null bs=512 count=35M").

whether the bottleneck was copy-from-kernel-to-userspace (ie. exhaustion of 
Front-Side-Bus / memory bandwidth) or related to block-layer overhead and 
scsi layer overheads, i haven't yet validated, but at a ~35% performance 
difference is relatively significant nontheless.

cpu utlization on the sg interface was under 10%.  using 'dd' on the sd 
interface, both gigahertz P3 Xeons had 0% idle time.

cheers,

lincoln.