You are right on the money!
There is about 35-40% throughput killjoy "copy-from-kernel-to-userspace".
It is easy to demo if you have a bus analizer and can do accounting on the
data io less the command block overhead.
CR3's are your friend, not ...
On Thu, 9 May 2002, Lincoln Dale wrote:
> 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.
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