Do not cache readahead-pages in the OS cache.

Summary:
When posix_fadvise(offset, offset) is usedm it frees up only those
pages in that specified range. But the filesystem could have done some
read-aheads and those get cached in the OS cache.

Do not cache readahead-pages in the OS cache.

Test Plan: run db_bench benchmark.

Reviewers: vamsi, heyongqiang

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D5379

util/env_posix.cc

@@ -94,7 +94,9 @@ class PosixRandomAccessFile: public RandomAccessFile {
       s = IOError(filename_, errno);
     }
     if (!useOsBuffer) {
-      posix_fadvise(fd_, offset, n, POSIX_FADV_DONTNEED); // free OS pages
+      // we need to fadvise away the entire range of pages because
+      // we do not want readahead pages to be cached.
+      posix_fadvise(fd_, 0, 0, POSIX_FADV_DONTNEED); // free OS pages
     }
     return s;
   }