User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 119
Read time comparisons for machine speeds
Processor Speed
Image Loading Method
266MHz 500MHz
Disk 35.85ms 18.84ms
Cache 0.36ms 0.23ms
Figure 25 illustrates the results of running the flashing checkerboard test on a 500MHz computer
with a 2x AGP video card. Note the durations for the loads from disk versus loads from the
cache. The left side of the figure shows the rates for disk-loads, with disk-loads taking
approximately 19ms. The computer could display up to a rate of 36ms per display, but could not
manage rates of 24 or 12ms (images 834-1070). In contrast, with disk-caching (i.e., cache loads)
taking only 0.2ms, the computer could display at rates of one image every 24 and 12ms, keeping
up with the requirements of the task.
Disk Reads versus Cache Reads
High Rate Image Presentation
(500 MHz Processor, 85 Hz Refresh)
0
20
40
60
80
100
120
1
151 301 451 601 751 901
1051 1201 1351 1501 1651 1801 1951 2101
Sequential Image
Milliseconds
Measured Duration
Target Duration
Onset Delay
Load Time
Disk Read Cache Read
18.84ms Disk Read
0.23ms Cache
Figure 25. Fast presentation when loading images from disk versus a user defined image
cache. With disk reading, the maximum rate is 36ms. With caching, it is reduced to a single
refresh at 12ms.
The drawback with disk caching is that it may take up significant amounts of memory. The table
below shows the memory use in megabytes (per image) for different types of images.
MB cache required by screen resolution & color depth
Color Depth 640x480 800x600 1024x768 1280x1024
8-bit
0.3 0.5 0.8 1.3
16-bit
0.6 0.9 1.5 2.5
24-bit
0.9 1.4 2.3 3.8
As the experiment uses more memory, the operating system is more likely to move virtual
memory to disk, so you cannot be certain that all the images will be available when needed.
However, if there are no other programs running, and you have substantial memory, most of the
images will nearly always come from memory at a speed much faster than can be obtained with a
straight disk access.