mfi(4) IO performance regression, post 8.1

[Alexander Motin]

Hi.

On 20.07.2012 22:38, Adrian Chadd wrote:
> I'm worried that this won't be the only source of "freebsd is slower
> than linux" issues.
>
> What can we add to the timer path to make identifying and root causing
> this issue easy? I'd just like to be absolutely sure that we're not
> only doing the best job possible, but we can provide some tools and
> statistics to the user/administrator so as to make debugging much
> easier.

The only instrument to diagnose this problem without provided input I 
could propose is hwpmc profiling. It should be able to show that we are 
spending much time in those timer routines. If we guessed somehow that 
reason is in slow ACPI timer, it is easy to write respective benchmark, 
but we can't write tests for everything, and even if we could, users 
won't be able to run/analyze output of them without some level of knowledge.

I've spent much time profiling that on hardware I have, but the only way 
to be sure in general case I see is more testing and feedbacks. For this 
specific area I am using very simple test, that effectively depends on 
interrupt latency and CPUs wakeup times: `dd if=/dev/ada0 of=/dev/null 
bs=512`. Depending on device, controller and other factors, gives me 
about 20-30K IOPS.

If you have some ideas what and how could we test automatically -- welcome.

-- 
Alexander Motin