Swap to SD cards: performance and burnout test

Sean DALY sdaly.be at gmail.com
Fri Dec 11 09:25:04 EST 2009

A few years ago with a colleague we crafted a stress test with
imagemagick on a small web server.

First we generated a series of fairly small but complex bitmap images
(as I recall we used dumped frames from a video). The concept was to
build a tiled file from the smaller images, then rotate the resulting
image 90° four times, then loop again, but changing the sequence of
the source images for the final image so as to minimize the
possibility of reading from cache.

We tuned the resulting image by hand, incrementing the number of small
images, until we located a "tipping point" beyond which the
performance difference was dramatic. We assumed that was the swap disk
threshold. We weren't sure how imagemagick managed RAM vs. disk, but
at least we had empirical evidence where the limit was.

We ran those tests in the background while stepping through a
predefined manual procedure of clicking on pages in the webserver for
a "subjective" idea of sluggishness (correlated to the iteration of
the background job).

As I recall we were able to run the system into the ground quicker by
incrementing the rotation 1° instead of 90° each time, the disks
started thrashing and performance disappeared.

I don't remember what version of imagemagick we used but this link has
some useful info:


On Mon, Dec 7, 2009 at 6:41 PM, Emiliano Pastorino
<epastorino at plan.ceibal.edu.uy> wrote:
> Hi everyone,
> We're testing how much performance you gain by swapping to an sd card.
> The first thing we need to do is to compare the performance between a
> regular XO 1.0 (no swapping) and one which swaps to an SD card.
> The other thing is to measure how long an SD lasts if used that way.
> So long, we've only compared two XO by running some activities, but we need
> some numeric results, or something measurable.
> Does anyone come out with a possible test?
> Also, we need to write and erase data as fast as possible to measure the
> lifetime of the SD card.
> Is there any tool or a way to write random data to the swap space?
> We're using a 4GB C6 SD card, assigning 512 Mb for swap.
> --
> Ing. Emiliano Pastorino
> LATU - Plan Ceibal
> Av. Italia 6201 CP: 11500, Montevideo, Uruguay
> Tel: (598 2) 601 5773 int.: 213
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