Strange performance issue

Ulrich Eckhardt eckhardt at satorlaser.com
Tue Oct 6 12:27:52 CEST 2009


Dan Stromberg wrote:
> My new version formats an SD card and preallocates some file space in
> about 3 minutes with "Optimize Performance" selected, and in about 30
> minutes with "Optimize for Quick Removal" selected.  Needless to say, I
> don't like the 27 minute penalty much.

For performance, the driver will probably use delayed write operations,
buffering etc. For quick removal, it will probably make all operations
atomic, i.e. perform a write operation and not return until the data has
really reached the SD card.

> But the old version of the program formats and preallocates in 3 minutes
> irrespective of whether the drive is in "optimize performance" or
> "optimize for quick removal".

Somehow, I guess the new version does many small writes while the old one
doesn't. When optimizing for quick removal, those operations add up,
otherwise their overhead is negligible.

> one_meg = 'a'*2**20

That's one mib, not one meg. ;)

> file = open('remove-me', 'w')

Try adding the 'b' flag, too. I wouldn't expect it to affect the IO speed,
but it is one factor that adds up. Otherwise, I looked over your program
and couldn't find anything that would explain a problem. Just wondering,
what transfer speed do you get with the two versions? What is the
theoretical maximum for the SD card?

Uli

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