Paul Kelly wrote: > Markus: > >> What are memory-mapped files? Excuse my ignorance, I'm just a >> self-trained coder (learning by doing). > > http://en.wikipedia.org/wiki/Memory-mapped_file > A chunk of a disk file is directly mapped into memory so you can > access it using normal pointers as if it was permanently in memory, > and the OS transparently handles paging the relevant chunks of the > file in and out of memory as required. > It can be a useful and elegant way of managing access to large files a > section at a time. For files small enough to be completely cached in > memory the performance penalty over keeping the same data in memory > (without a file) should be relatively small, and will be faster than > accessing a file using conventional fopen()/fread() etc. functions. > But how much of a performance gain it would give for very large files > strongly depends on the nature of access to the file, which I know > very little about in this case. E.g. for completely random access > there might not be a lot of gain. It is completely random, the next chunk to be read/written can be anywhere in the file. > But if there was random access only within a certain section of a > file, that section could be mapped into memory and access would then > be quite fast. Does it make sense to always map a different section of the file? This mapping would then need to replace each call to fread/fwrite because of completely random access. I'm currently using the same method like for the coor file which seems to work fine so far.
Markus M _______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev