Disks are slow!

Many people have noticed that GraphicsMagick performance sometimes 
goes south once it needs to use temporary files to store the image 
pixels.  Sometimes I am able to improve the situation via algorithmic 
improvements, but other times the algorithm is inherently random 
access and can't be helped.

Most desktop computers come with slow SATA disks with a long 
rotational latency and long seek time.  We are talking 12ms (or more) 
per random I/O.  Expensive SAS disks improve performance quite a lot 
but they are smaller, and (of course) expensive!

Yesterday I did a little experiment on my Windows system.  I set the 
MAGICK_TMPDIR environment variable so that GraphicsMagick temporary 
files were sent across a CIFS (Samba) share to a Unix server with lots 
of RAM.  Connectivity is gigabit ethernet so file transfer speed is 
perhaps 60MB/second.  It seems that CIFS does not issue many sync 
requests so storage in the server remained memory based.

A test of file rotate by 90 degrees showed a 30X speed improvement 
even with the (relatively) slow ethernet and Samba.

An emerging trend in computing is Solid State Disk (SSD).  While 
originally introduced for digital cameras and laptop computers, there 
are now "enterprise" SSDs available which can store 30GB or more, and 
should last quite a few years, even when used as a temporary file 
store (FLASH based SSDs wear out due to repeated writes).  The cool 
thing about these SSDs is that since they are not rotating media and 
don't have any head to seek, a small 2-1/2" form factor SATA device 
(e.g. Intel X25-E) can handle 1K to 5K writes per second.  People who 
are using GraphicsMagick to manipulate huge images should benefit 
greatly by using a high-grade SSD as a temporary file store for 
GraphicsMagick.

Of course, if your system can fit enough RAM, performance will be as 
good as it can get if the image fits in RAM and avoids any filesystem 
storage.

Bob
--
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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