Re: [PATCH 06/14] libata: improve ATAPI draining

[Tejun Heo <htejun@gmail.com>]

Alan Cox wrote:
> On Fri, 30 Nov 2007 01:42:46 +0900
> Tejun Heo <htejun@gmail.com> wrote:
> 
>> Alan Cox wrote:
>>>> * Limit the amount of draining to ATAPI_MAX_DRAIN (16k currently).
>>> Why 16 not 64K ?
>> Oops, forgot to answer here.  That would result in 16 sg entries for
>> draining which felt a bit too much. 
> 
> 16 sg entries but only one page of memory so its not a bit bump ?

The only added cpu cycle overhead is to setup sg entries and controller
DMA table accordingly which should be pretty small.  I was more worried
about reserving that many sg entries as it can affect bulk data
transfers.  I thought about appending draining sg's on left-over SGs.
ie. drain only min(nr_left_sgs, ATA_MAX_DRAIN_PAGES) pages but it ends
up dereferencing deep upto request_queue and things get a bit complex
because the way libata currently associates ata_device's with scsi_device's.

So, I thought 16k was a good trade off.  Not as comfortable as 64k tho,
I agree.  Would it be worth to add more complexity for this?

Thanks.

-- 
tejun