Parent Topic: Improving Performance

Reducing Disk Thrashing

One way to improve the effectiveness of the input cache is to increase its size. By default the input cache is 4MB, about four million bytes. It can be increased using the ``Memory'' control on the ``Disk to Disk Registration'' panel. For instance setting the memory size to ``10.0'' would start GCPWorks with a 10MB cache. Increasing the size of the cache is often useful, but only if there is enough memory (RAM) on the computer to satisfy the request. On a system with 16MB of memory and only one user it is likely that 10MB or so is already used by the computer. Increasing the cache size above 6MB is likely to adversely affect performance. However, on a workstation with 64MB of memory, it may be useful to specify a cache as large as 40MB.

If increasing the size of the cache does not help enough, it can also be useful to reduce the number of channels registered at once. For instance, if seven bands of a full Landsat TM scene (6000x6000) are registered at once with a 4MB cache, then only about 100 scanlines of the input image can be held at once (6000x100x7=4200000); however, if only one band is registered, then 700 scanlines can be held (6000x700x1 = 4200000). If only one band at a time is registered, the polynomial transform and resampling calculations have to be performed once for each band; so this is a tradeoff of I/O against CPU usage. It is also wise to ensure that the input file is stored in band interleaved form if it is registered one band at a time.

Since the input image caching tends to break down when rotating by a large angle, it is helpful to scan input images in such an orientation that little rotation is needed (north up). This is not possible with data received in digital form.