Mark page cache blocks as DONT_NEED later,

So we are sure we don't need them again.

Author: joto

include/osmium/io/detail/pbf_input_format.hpp

@@ -133,13 +133,6 @@ namespace osmium {
 
                     *m_offset_ptr += size;
 
-                    if (m_want_buffered_pages_removed && size > 100) {
-                        // The size check is there to avoid the system call
-                        // when we have only been reading a small number of
-                        // bytes, i.e. for a header.
-                        osmium::io::detail::remove_buffered_pages(m_fd, *m_offset_ptr);
-                    }
-
                     return true;
                 }
 
@@ -267,6 +260,10 @@ namespace osmium {
                         } else {
                             send_to_output_queue(data_blob_parser());
                         }
+
+                        if (m_want_buffered_pages_removed) {
+                            osmium::io::detail::remove_buffered_pages(m_fd, *m_offset_ptr);
+                        }
                     }
                 }
 

include/osmium/io/detail/read_write.hpp

@@ -272,7 +272,11 @@ namespace osmium {
 #ifdef __linux__
             inline void remove_buffered_pages(int fd, std::size_t size) noexcept {
                 constexpr const std::size_t block_size = 4096;
-                if (fd > 0 && size > 0) {
+                // Make sure to keep the last 10 blocks around, so were are
+                // definitely not removing something which might be needed
+                // again soon.
+                if (fd > 0 && size > 10 * block_size) {
+                    size -= 10 * block_size;
                     ::posix_fadvise(fd, 0, (size - 1) & ~(block_size - 1), POSIX_FADV_DONTNEED);
                 }
 #else