Check env var OSMIUM_CLEAN_PAGE_CACHE_AFTER_READ

Read OSMIUM_CLEAN_PAGE_CACHE_AFTER_READ to decide whether the page cache
should be cleaned by calling fadvise DONT_NEED after reading blocks from
an OSM file. Allowed values are "no" and "yes". If set to anything else
or not set at all the implementation does what it thinks best, currently
it clears the page cache.

Author: joto

include/osmium/io/compression.hpp

@@ -93,7 +93,7 @@ namespace osmium {
             std::atomic<std::size_t> m_file_size{0};
             std::atomic<std::size_t> m_offset{0};
 
-            std::atomic_bool m_want_buffered_pages_removed{true};
+            std::atomic_bool m_want_buffered_pages_removed{false};
 
         public:
 

include/osmium/io/reader.hpp

@@ -330,6 +330,11 @@ namespace osmium {
                 std::promise<osmium::io::Header> header_promise;
                 m_header_future = header_promise.get_future();
 
+                const auto cpc = osmium::config::clean_page_cache_after_read();
+                if (cpc >= 0) {
+                    m_decompressor->set_want_buffered_pages_removed(true);
+                }
+
                 const int fd_for_parser = m_decompressor->is_real() ? -1 : m_fd;
                 m_thread = osmium::thread::thread_handler{parser_thread, std::ref(*m_pool), fd_for_parser, std::ref(m_creator),
                                                           std::ref(m_input_queue), std::ref(m_osmdata_queue),

include/osmium/util/config.hpp

@@ -103,6 +103,19 @@ namespace osmium {
             return value;
         }
 
+        inline int8_t clean_page_cache_after_read() noexcept {
+            const char* env = osmium::detail::getenv_wrapper("OSMIUM_CLEAN_PAGE_CACHE_AFTER_READ");
+            if (env) {
+                if (!strcasecmp(env, "yes")) {
+                    return 1;
+                }
+                if (!strcasecmp(env, "no")) {
+                    return -1;
+                }
+            }
+            return 0;
+        }
+
     } // namespace config
 
 } // namespace osmium