Re: Why is only one command issue per time in AHCI driver?

[Robert Hancock <hancockrwd@gmail.com>]

On 01/13/2011 07:06 PM, 羅秉鈞 wrote:
> Hi,
>
> Yes, what I see is every time scsi_request_fn is called, we can only pop
> up one request to be dispatched. So, How can we make block layer to
> pass more than one commands to us? So that we can fill up more
> commands into the Command List before write Px.CI. I think it might
> shorten the command interval time.
>
> Even I enable the NCQ, I observed that there is only one command to be
> executed per time and tag 0 is used.

That sounds like the I/O pattern doesn't result in any concurrent 
transfers being executed. What kind of workload is this running?

>
> Is there any possible to change the libata and ahci.c to be able to fire up
> more than one commands at one time and report complete multiple to
> upper layer(SCSI and block layer).
>
> Another question, at scsi_adjust_queue_depth() function, why sdev->ordered_tags
> and sdev->simple_tags value is only 0 or 1 regardless of queue_depth? What
> does it mean?
>
> Thank you in advance for your explanation.
>
>
>
> On Fri, Jan 14, 2011 at 1:46 AM, Jeff Garzik<jeff@garzik.org>  wrote:
>> On 01/13/2011 05:52 AM, 羅秉鈞 wrote:
>>>
>>> Hello,
>>>
>>> Inside ahci_qc_issue() function, I see that driver always write one bit
>>> to Px.CI register even enable the NCQ.
>>>
>>> AHCI allows to fire 32 commands at one time and AHCI driver also
>>> claims itself able to queue up to 31 commands.
>>>
>>> Inside ahci.c :
>>> static struct scsi_host_template ahci_sht = {
>>>         ATA_NCQ_SHT(DRV_NAME),
>>>         .can_queue              = AHCI_MAX_CMDS - 1,
>>> ............
>>>
>>> Why upper layer(Block or SCSI layer) does not fill up as many as possible
>>> commands into Command List before sending the commands out
>>> (Write to Px.CI register)?
>>
>> The driver successfully executes up to 31 commands at a time, in paralle.
>>
>> We submit one command at a time to hardware because the block layer passes
>> commands to us one at a time.
>>
>>         Jeff
>>
>>
>>
>>
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