Add an option to reduce pagefault when available in Linux

This option instructs the kernel to synchronously load the entire mapped
region into active memory by specifying `MAP_POPULATE` in `mmap`. It will
cause read-ahead on that memory, and then the subsequent accesses to the
memory can proceed without page faults, improving some performance.

include/mimalloc.h

@@ -311,6 +311,9 @@ typedef enum mi_option_e {
   mi_option_page_reset,
   mi_option_abandoned_page_reset,
   mi_option_segment_reset,
+#if defined(__linux__)
+  mi_option_prefault,
+#endif
   mi_option_eager_commit_delay,
   mi_option_reset_delay,
   mi_option_use_numa_nodes,

readme.md

@@ -302,6 +302,10 @@ or via environment variables:
    `MIMALLOC_EAGER_COMMIT_DELAY=N` (`N` is 1 by default) to delay the initial `N` segments (of 4MiB)
    of a thread to not allocate in the huge OS pages; this prevents threads that are short lived
    and allocate just a little to take up space in the huge OS page area (which cannot be reset).
+-  `MIMALLOC_PREFAULT=1`: (Linux only) enable memory prefaulting when available. This option instructs the kernel to synchronously 
+   load the entire mapped region into active memory by specifying `MAP_POPULATE` in `mmap`. It will cause read-ahead on that memory, 
+   and then the subsequent accesses to the memory can proceed without page faults, improving some performance, but might also reduce 
+   some available memory. In `mimalloc_test_stress`, it is known to reduce about 95% page-faults.
 
 Use caution when using `fork` in combination with either large or huge OS pages: on a fork, the OS uses copy-on-write
 for all pages in the original process including the huge OS pages. When any memory is now written in that area, the

src/options.c

@@ -81,6 +81,9 @@ static mi_option_desc_t options[_mi_option_last] =
   { 1, UNINIT, MI_OPTION(page_reset) },          // reset page memory on free
   { 0, UNINIT, MI_OPTION(abandoned_page_reset) },// reset free page memory when a thread terminates
   { 0, UNINIT, MI_OPTION(segment_reset) },       // reset segment memory on free (needs eager commit)
+#if defined(__linux__)
+  { 0, UNINIT, MI_OPTION(prefault) },
+#endif
 #if defined(__NetBSD__)
   { 0, UNINIT, MI_OPTION(eager_commit_delay) },  // the first N segments per thread are not eagerly committed
 #else

src/os.c

@@ -362,6 +362,10 @@ static void* mi_unix_mmap(void* addr, size_t size, size_t try_alignment, int pro
   #define MAP_NORESERVE  0
   #endif
   int flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE;
+  #if defined(__linux__)
+  if (mi_option_get(mi_option_prefault))
+    flags = flags | MAP_POPULATE;
+  #endif
   int fd = -1;
   #if defined(MAP_ALIGNED)  // BSD
   if (try_alignment > 0) {
@@ -392,6 +396,9 @@ static void* mi_unix_mmap(void* addr, size_t size, size_t try_alignment, int pro
     }
     else {
       int lflags = flags & ~MAP_NORESERVE;  // using NORESERVE on huge pages seems to fail on Linux
+      #if defined(__linux__)
+      lflags = lflags & ~MAP_POPULATE; // don't use MAP_POPULATE on huge pages
+      #endif
       int lfd = fd;
       #ifdef MAP_ALIGNED_SUPER
       lflags |= MAP_ALIGNED_SUPER;