Reduce memory usage of renderd

The core issue seems to be the memory management combined with the memory usage pattern of renderd.

The glib-provided heap implementation somehow fails to give back memory to the system, but instead keeps on growing the data segment until the system decides to kill the process.

The jemalloc implementation puts allocations into arenas and once they are freed (and some time has passed), they are given back to the system and seem to reduce the RSS size of the process.

This should address/alleviate openstreetmap#181

CMakeLists.txt

@@ -47,6 +47,13 @@ include(GNUInstallDirs)
 find_package(ICU REQUIRED uc)
 find_package(Threads REQUIRED)
 
+# Switch to jemalloc (http://jemalloc.net) as the malloc/free
+# implementation in renderd.
+# Should address https://github.com/openstreetmap/mod_tile/issues/181
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(JEMALLOC jemalloc)
+pkg_search_module(JEMALLOC REQUIRED jemalloc)
+
 find_package(APR REQUIRED)
 find_package(GLIB 2.50 REQUIRED)
 find_package(HTTPD 2.4 REQUIRED)

src/CMakeLists.txt

@@ -183,7 +183,9 @@ set(renderd_SRCS
   renderd.c
   request_queue.c
 )
+# JEMALLOC libraries should come *before* the GLIB libraries.
 set(renderd_LIBS
+  ${JEMALLOC_LIBRARIES}
   ${ICU_LIBRARIES}
   ${LIBMAPNIK_LIBRARIES}
   ${RENDER_LIBRARIES}