I'm trying to optimize handling of large datasets using mmap. A dataset is in the gigabyte range. The idea was to mmap the whole file into memory, allowing multiple processes to work on the dataset concurrently (read-only). It isn't working as expected though.
As a simple test I simply mmap the file (using perl's Sys::Mmap module, using the "mmap" sub which I believe maps directly to the underlying C function) and have the process sleep. When doing this, the code spends more than a minute before it returns from the mmap call, despite this test doing nothing - not even a read - from the mmap'ed file.
Guessing, I though maybe linux required the whole file to be read when first mmap'ed, so after the file had been mapped in the first process (while it was sleeping), I invoked a simple test in another process which tried to read the first few megabytes of the file.
Suprisingly, it seems the second process also spends a lot of time before returning from the mmap call, about the same time as mmap'ing the file the first time.
I've made sure that MAP_SHARED is being used and that the process that mapped the file the first time is still active (that it has not terminated, and that the mmap hasn't been unmapped).
I expected a mmapped file would allow me to give multiple worker processes effective random access to the large file, but if every mmap call requires reading the whole file first, it's a bit harder. I haven't tested using long-running processes to see if access is fast after the first delay, but I expected using MAP_SHARED and another separate process would be sufficient.
My theory was that mmap would return more or less immediately, and that linux would load the blocks more or less on-demand, but the behaviour I am seeing is the opposite, indicating it requires reading through the whole file on each call to mmap.
Any idea what I'm doing wrong, or if I've completely misunderstood how mmap is supposed to work?