Matt points to a PHP script that uses shm memory to cache data.
I rather use the filesystem to cache data – disk access is an order of magnitude slower than memory accesses but the operating system is generally clever about caching filesystem reads in memory. I would presume that the algorithms used are more complex than can be practically achieved by a PHP script too.
AFAIR, Apache has problems with Shm segments too although that could be something I remember from long ago!
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buffer cache calls (e.g. fread/fwrite) are always system calls so will incur context switches, buffering, etc.
whereas process memory accesses can be local pages –
so no, the OS won’t necessary be better for FS calls..
debuging multiprocess applications is a bit tricky though,
but same could be said for filesytstems.
mmap should be comprable