From 1d549eddbeeccf7d108310466de5f63af04b004c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Wed, 29 Aug 2018 16:21:17 +0200
Subject: Mark free blocks in pooled carriers as unused (MADV_FREE)

This lets the OS reclaim the physical memory associated with these
blocks which reduces the impact of long-lived awkward allocations.
A small allocated block will still keep a huge carrier alive, but
the unused part of the carrier will now be available to the OS.

Co-authored-by: Dmytro Lytovchenko <dmytro.lytovchenko@erlang-solutions.com>
---
 erts/emulator/sys/common/erl_mmap.h | 57 +++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

(limited to 'erts/emulator/sys/common')

diff --git a/erts/emulator/sys/common/erl_mmap.h b/erts/emulator/sys/common/erl_mmap.h
index 539daea419..e1ff0fe80a 100644
--- a/erts/emulator/sys/common/erl_mmap.h
+++ b/erts/emulator/sys/common/erl_mmap.h
@@ -176,4 +176,61 @@ void hard_dbg_remove_mseg(void* seg, UWord sz);
 
 #endif /* HAVE_ERTS_MMAP */
 
+/* Marks the given memory region as unused without freeing it, letting the OS
+ * reclaim its physical memory with the promise that we'll get it back (without
+ * its contents) the next time it's accessed. */
+ERTS_GLB_INLINE void erts_mem_discard(void *p, UWord size);
+
+#if ERTS_GLB_INLINE_INCL_FUNC_DEF
+
+#ifdef VALGRIND
+    #include <valgrind/memcheck.h>
+
+    ERTS_GLB_INLINE void erts_mem_discard(void *ptr, UWord size) {
+        VALGRIND_MAKE_MEM_UNDEFINED(ptr, size);
+    }
+#elif defined(DEBUG)
+    /* Try to provoke crashes by filling the discard region with garbage. It's
+     * extremely hard to find bugs where we've discarded too much, as the
+     * region often retains its old contents if it's accessed before the OS
+     * reclaims it. */
+    ERTS_GLB_INLINE void erts_mem_discard(void *ptr, UWord size) {
+        static const char pattern[] = "DISCARDED";
+        char *data;
+        int i;
+
+        for(i = 0, data = ptr; i < size; i++) {
+            data[i] = pattern[i % sizeof(pattern)];
+        }
+    }
+#elif defined(HAVE_SYS_MMAN_H)
+    #include <sys/mman.h>
+
+    ERTS_GLB_INLINE void erts_mem_discard(void *ptr, UWord size) {
+    #ifdef MADV_FREE
+        /* This is preferred as it doesn't necessarily free the pages right
+         * away, which is a bit faster than MADV_DONTNEED. */
+        madvise(ptr, size, MADV_FREE);
+    #else
+        madvise(ptr, size, MADV_DONTNEED);
+    #endif
+    }
+#elif defined(_WIN32)
+    #include <winbase.h>
+
+    /* MEM_RESET is defined on all supported versions of Windows, and has the
+     * same semantics as MADV_FREE. */
+    ERTS_GLB_INLINE void erts_mem_discard(void *ptr, UWord size) {
+        VirtualAlloc(ptr, size, MEM_RESET, PAGE_READWRITE);
+    }
+#else
+    /* Dummy implementation. */
+    ERTS_GLB_INLINE void erts_mem_discard(void *ptr, UWord size) {
+        (void)ptr;
+        (void)size;
+    }
+#endif
+
+#endif /* ERTS_GLB_INLINE_INCL_FUNC_DEF */
+
 #endif /* ERL_MMAP_H__ */
-- 
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