make redis purge jemalloc after flush, and enable background purging thread

jemalloc 5 doesn't immediately release memory back to the OS, instead there's a decaying
mechanism, which doesn't work when there's no traffic (no allocations).
this is most evident if there's no traffic after flushdb, the RSS will remain high.

1) enable jemalloc background purging
2) explicitly purge in flushdb

1 files changed, 2 insertions, 0 deletions

diff --git a/src/zmalloc.h b/src/zmalloc.h
index 6fb19b046..b136a910d 100644
--- a/src/zmalloc.h
+++ b/src/zmalloc.h
@@ -86,6 +86,8 @@ size_t zmalloc_used_memory(void);
 void zmalloc_set_oom_handler(void (*oom_handler)(size_t));
 size_t zmalloc_get_rss(void);
 int zmalloc_get_allocator_info(size_t *allocated, size_t *active, size_t *resident);
+void set_jemalloc_bg_thread(int enable);
+int jemalloc_purge();
 size_t zmalloc_get_private_dirty(long pid);
 size_t zmalloc_get_smap_bytes_by_field(char *field, long pid);
 size_t zmalloc_get_memory_size(void);