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

[Jeff Garzik <jgarzik@pobox.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?

Agreed with all these observations.

In general, the non-NCQ case shares characteristics with host-queue 
controllers like sx8.  The benefit is nowhere near true command queueing 
like NCQ, but the benefits (which you list) are real.

Think of this as a long term requirement.  libata _should_ eventually 
support this, simply because the gains are there to be had.  Several 
SATA controllers support queueing of non-NCQ commands.

In the short term, only turning on CCC for NCQ ports makes a lot of sense.


> 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.

My Gigabyte i-Ram reliably corrupts data within seconds, regardless of 
SATA controller :/

	Jeff