DMA in B2Test board?

[Jim Gettys]

On Sat, 2007-03-10 at 22:58 +0545, Bipin Gautam wrote:
> hello,
> I experimented with various block size for file I/O. I experimented
> with vfat and ext2 filesystem in the pen drive. 

Without specifications on the Pen drive, it's hard predict what would
happen.  Write performance of file systems on flash depends on whether
the block size of the file system matches well the underlying flash
block size.

> i also experimented
> with file write within nand storage; like
> 
> date
> dd if=/dev/zero of=/testfile bs=6144 count=102400
> date

On the NAND flash:

JFFS2 does data compression.

If you are writing zeros into a file, the actual write will extremely
highly compressed, and so you are appending small writes to the log; to
rewrite blocks on the log, you have to read the blocks, and might
encounter errors.  This can have significant performance consequences,
which I think are fixed in later builds.


> 
> The maximum preformance of XO nand storage (alone) was ~1.8 mb/sec
> (jffs2) while the preformance of external usb drive is ~ 3-11 mb /sec
> (at various block size)

Please install build 299, rather than 239.  There has been work on the
NAND driver to highly optimize the error correction path in the NAND
driver, which I believe was not in build 239, but IIRC, is in build 299.

So if you had NAND with errors, the error correction path was a possible
source of CPU usage.  I believe the error correction code is now 100 or
more times faster, courtesy of a community developer whose name slips my
mind.


> 
> i thought 100% cpu use is because direct memory access(DMA) for disk
> I/O has some problems (or disabled). Also, running  XO OS[1] in
> virtual machine inside Parallels Workstation 2.2 with host OS xp sp2
> shows the same problem of 100% cpu use during disk i/o
> 
> 
> i cant give you a detailed stats. right now as my B2 OS is dead and
> i'm downloading the latest build. (maybe tomorrow... if you want to
> hear it)

Please install build 299.

> 
> with regards,
> -bipin
> [1]olpc-redhat-stream-development-build-239-20070118_1416-devel_ext
-- 
Jim Gettys
One Laptop Per Child