Re: Block layer / MMC: Order of segments in SG-list

[Joel A Fernandes]

Hi Jens,

Thanks for your email.

On Mon, Jun 10, 2013 at 2:15 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On Sun, Jun 09 2013, Joel A Fernandes wrote:
>> Hi,
>> So I tried dumping addresses of an SG list in omap_hsmmc driver before
>> it is passed to DMA.
>>
>> I found some interesting traces occasionally such as the below SG list
>> of length 4.
>>
>> [    6.758716] (0) length=4096, sg virt addr=c1318000, sg phy addr=81318000
>> [    6.765863] (1) length=4096, sg virt addr=c1317000, sg phy addr=81317000
>> [    6.773011] (2) length=4096, sg virt addr=c1316000, sg phy addr=81316000
>> [    6.780087] (3) length=4096, sg virt addr=c1315000, sg phy addr=81315000
>>
>> What is interesting is these chunks are really physically contiguous
>> but in reverse order in the list. I think a smarter ordering can
>> actually improve through put considerably and save precious DMA
>> resources by not having to allocate slots for parts of contiguous
>> chunk of physical memory.
>>
>> Is there any particular reason why this might be the case? I traced to
>> find that the SG list is actually prepared by mmc_queue_map_sg ->
>> blk_rq_map_sg
>
> mmc or the block layer can't do much about the memory it is handed. The
> sg mappings just reflect the fact that they happened to be in reverse,
> so to speak. You are right in that having those pages in the right order
> and being able to merge the segments is a win. Unless you are heavily SG
> entry starved or your DMA controller has a high per-sg-entry overhead,
> it's usually not a big deal.

We have currently set limits in DMA for maximum of 16 slots, some
times I noticed all 16 allocated to contiguous pages but in reverse.
:)

> That said, you should investigate WHY they are in that order :-)

Sure , I am thinking of tracing this soon to root cause the page allocations.
I am wondering if we can just reorder the SG and write in the reverse
order for such detected cases, but yeah like you said no match to
allocating them in the right order in the first place.

I appreciate your response to my post, thanks!

Regards,
Joel
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