Re: slow eMMC write speed

Linus Walleij
2011-09-29T07:05:41Z

On Wed, Sep 28, 2011 at 10:34 PM, Praveen G K <praveen.gk@gmail.com> wrote:

> The problem is I am seeing write
> speeds of about 5MBps on a Sandisk eMMC product and I can clearly see
> the time lost when measured between sending a command and receiving a
> data irq.  I am not sure what kind of an issue this is.  5MBps feels
> really slow but can the internal housekeeping of the card take so much
> time?

It can indeed take as much time as it wants as long as it meets
the specifications, what does your datasheet say?

If you connect a signal analyzer to your MMC bus you *will*
know for sure whether this is actually caused by the card,
of if there is some kernel irq/workqueue latency involved.

If the card is the issue, what you can do to improve
performance is to look for other eMMC vendors... ;-)

> I mean, for the usual transfers it takes about 3ms to transfer
> 64kB of data, but for the 63rd and 64th transfers, it takes 250 ms.
> The thing is this is not on a file system.  I am measuring the speed
> using basic "dd" command to write directly to the block device.

Have you tried to make a deeper analysis of the card
characteristics using Arnd Bergmanns "flashbench" tool?
http://git.linaro.org/gitweb?p=people/arnd/flashbench.git;a=summary

There you will see for sure if there are some problematic
read boundaries on this specific card.

Yours,
Linus Walleij