ATA rman performance enhancement

[Nate Lawson]

Søren Schmidt wrote:
> Nate Lawson wrote:
> 
>> While doing some benchmarking of other code, I noticed that there were 
>> a lot of calls to rman_get_bustag/handle().  They weren't taking up 
>> much actual time since they're pretty lightweight but seemed to be 
>> unnecessary.
>>
>> I worked up the attached diff and benchmarked it.  There are about 
>> 1000 calls a second to the rman routines without this patch and 
>> essentially none with it.  It makes about a 1% difference in 
>> throughput under some IO loads.  It is only for non-Promise or non-SII 
>> controllers right now since I didn't extend the initialization step to 
>> more than ata-pci.c. The same approach could be used for the other 
>> INW/OUTW calls as well but they're not in the fast path.  I think it 
>> may make more of a difference with small reads.
>>
>> Feel free to test, cleanup, and commit.
> 
> 
> I had something semilar to this once back when, but since HW got lots 
> faster I couldn't measure it anymore, but maybe things has changed...
> 
> Anyhow it needs to be applied to ata-isa.c ata-card.c ata-cbus.c etc to 
> not break anything at least. 

Yes, I agree.  I limited the change only to the IDX macros that were 
used in the fast path although it could apply to all of them.

> I'll think about it and eventually do 
> something about it in ATA-mkIII if it really is mesureable again..

It is a minimal difference but with small transfers on a fast drive with 
a low-powered CPU, it's measurable.  I made the same change a while back 
in acpi_timer.c since the resource is only allocated once but accessed 
very often and in a low-latency context.

-- 
Nate