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I am using mmap call to read from a very big file using simple pointer arithmetic in C++. The problem is that when I read small chunks of data (in the order of KBs) multiple times, each read take the same amount of time as the previous one. How can I know if the disk is being accessed to fulfill my request or whether the request is being fulfilled from main memory (page cache) in calls after the first one.
The issue is the following: both reads were being performed from cache. I guess caching starts when the file is opened or mmapped, before asking for the data. To verify this, I issued:
echo 3 > /proc/sys/vm/drop_cacheswhich flushes out the cache, then, if I run two iterations for getting the same data, the first run is (in my case) 10 times slower than the second.