I have test results now. Without mmap(), just normal disk access, a local hard disk is producing results just a little slower than loading into memory. ie the overhead of read() instead of memory pointers is not dramatic, and the OS (Linux) appears to have cached the file into memory as expected. This small performance difference, which could theoretically be completely eliminated with mmap(), is not really important to us, but maybe we will activate it anyway. I normally don't have any dealings with networks, and was not aware that the disk was on a network, and that network disks are apparently ridiculously slow (about 6 times slower in the latest test), and probably bypassing the OS's caching ability. I'm not sure what options exist if they insist on using network drives instead of local drives. I was thinking that we could have a softlink in /dev/shm pointing to the network drive and maybe that would encourage Linux to cache it. But I'm expecting the problem to be passed on to some network people to admit that networks are fundamentally ridiculous and that applications that require performance should not do network access.