XO-1.5 slow disk writes

[John Watlington]

We already tried running the SD interface at half speed to reduce
power dissipation.   It runs at full speed by default.

wad

On Nov 17, 2009, at 12:49 PM, Peter Robinson wrote:

> Hi Daniel,
>
>> Today I tried to figure out why running "sync" often takes 5-10  
>> seconds
>> or longer. This slows down suspend, where all data is synced to disk.
>> In all cases that I looked at, the amount of data being synced was  
>> tiny.
>> One example: in one run it had 160kB data to sync and it took 7.7
>> seconds. (blktrace is very handy for figuring this out)
>>
>> I traced this all the way to the SDHCI driver. These writes are
>> typically small and scattered, and our hardware (or the card itself)
>> takes a long time to process them. Many of the 1024 byte writes take
>> 500-600ms. All other disk I/O is halted during these times. The  
>> delay is
>> purely after submitting the SDHCI write command, and waiting for the
>> completion interrupt to arrive.
>>
>> I then wrote a C application to reproduce the exact set of writes  
>> from a
>> 20-second sync that I logged (using random data, but the same  
>> sectors,
>> sizes and ordering) and reproduced the issue that way.
>>
>> I also moved the card over to my Dell laptop, ran the same program  
>> and
>> saw the same (terrible) results.
>>
>> All info here:
>> http://dev.laptop.org/ticket/9688
>>
>>
>> So, I have a feeling that at least with today's generation of miniSD
>> cards we're going to be stuck with bad write performance,  
>> particularly
>> for random-style access like this.
>>
>> One experiment that would be interesting to run would be to try  
>> this on
>> one of the PhoenixBIOS boards, and then try it with the exact same SD
>> card on a regular XO-1.5. Just in case...
>
> You might want to look at some of the SD/MMC frequency patches that he
> applies to the kernel for the Nokia Internet tablet devices. I think
> the default kernel defaults to a frequency half or even less than
> half. The issue that might be encountered is that it doesn't always
> work well on low quality SD cards but it would be at least worth
> experimenting with it.
>
> His blog is here
> http://intr.overt.org/blog/
>
> The patches and a readme about them are here for the Nokia device (I
> suspect they're just about the same everywhere).
> http://intr.overt.org/5.2008.43-mmc-kernel/
>
> Hope that can be of some help, or at least head you in the right  
> direction.
>
> Cheers,
> Peter
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