Re: [RFC] AHCI Command Completion Coalescing(CCC) proposal

[Tejun Heo <htejun@gmail.com>]

Tejun Heo wrote:
> Jeff Garzik wrote:
> 
>> I'm still not sure I follow you?
>>
>> When AHCI runs out of commands to execute, it transitions from H:Idle 
>> to Ccc:SetIS.
>>
>> IMPORTANT NOTE:  In order for CCC to be effective on AHCI, ahci.c and 
>> libata (and sata_sil24) must be updated to support queuing a list of 
>> non-NCQ commands onto the controller, and recovering from errors in 
>> the case where a command list full of non-NCQ commands is present.
> 
> I thought about it but am not really sure whether it's worth the 
> trouble.  We'll be saving on inter-command latency and interrupt 
> handling which is great but not so sure how noticeable the improvement 
> would be.  NCQ is already all around.  How about doing CCC only during 
> NCQ command phase?
> 
> Hmmm... maybe those SSDs would benefit from CCC during non-NCQ commands 
> though if they don't support NCQ, which they don't really need.
> 

If we're gonna do it.  EH needs only a few changes probably during 
autopsy and report.  Fixing up command issue path and implementing 
command exclusion (NCQ vs. non-NCQ, sil24 does it in hardware, ahci 
doesn't) will be a bit complex though.

-- 
tejun