Re: mmc blkqueue is empty even if there are pending reads in do_generic_file_read()

[Per Forlin <per.forlin@linaro.org>]

On 3 May 2011 22:02, Arnd Bergmann <arnd@arndb.de> wrote:
> On Tuesday 03 May 2011 20:54:43 Per Forlin wrote:
>> >> page_not_up_to_date:
>> >> /* Get exclusive access to the page ... */
>> >> error = lock_page_killable(page);
>> > I looked at the code in do_generic_file_read(). lock_page_killable
>> > waits until the current read ahead is completed.
>> > Is it possible to configure the read ahead to push multiple read
>> > request to the block device queue?add
>
> I believe sleeping in __lock_page_killable is the best possible scenario.
> Most cards I've seen work best when you use at least 64KB reads, so it will
> be faster to wait there than to read smaller units.
>
>> When I first looked at this I used dd if=/dev/mmcblk0 of=/dev/null bs=1M count=4
>> If bs is larger than read ahead, this will make the execution loop in
>> do_generic_file_read() reading 512 until 1M is read. The second time
>> in this loop it will wait on lock_page_killable.
>>
>> If bs=16k the execution wont stuck at lock_page_killable.
>
> submitting small 512 byte read requests is a real problem when the
> underlying page size is 16 KB. If your interpretation is right,
> we should probably find a way to make it read larger chunks
> on flash media.
Sorry a typo. I missed out a "k" :)
It reads 512k until 1M.

>
>        Arnd
>
Per

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