mdbx: add mdbx_is_readahead_reasonable().

Change-Id: Iac090a9050d6b046b62b2c79cefbc70cc62c9144

mdbx.h

@@ -965,6 +965,10 @@ LIBMDBX_API char *mdbx_dump_val(const MDBX_val *key, char *const buf,
  * may help random read performance when the DB is larger than RAM and system
  * RAM is full.
  *
+ * NOTE: The mdbx_is_readahead_reasonable() function allows to quickly find out
+ *       whether to use readahead or not based on the size of the data and the
+ *       amount of available memory.
+ *
  * This flag affects only at environment opening and can't be changed after. */
 #define MDBX_NORDAHEAD 0x800000u
 
@@ -2014,6 +2018,20 @@ LIBMDBX_API int mdbx_env_set_geometry(MDBX_env *env, intptr_t size_lower,
                                       intptr_t pagesize);
 LIBMDBX_API int mdbx_env_set_mapsize(MDBX_env *env, size_t size);
 
+/* Find out whether to use readahead or not, based on the given database size
+ * and the amount of available memory.
+ *
+ * [in] volume      The expected database size in bytes.
+ * [in] redundancy  Additional reserve or overload in case of negative value.
+ *
+ * Returns:
+ *  - MDBX_RESULT_TRUE    = readahead is reasonable.
+ *  - MDBX_RESULT_FALSE   = readahead is NOT reasonable, i.e. MDBX_NORDAHEAD
+ *                          is useful to open environment by mdbx_env_open().
+ *  - Otherwise the error code. */
+LIBMDBX_API int mdbx_is_readahead_reasonable(size_t volume,
+                                             intptr_t redundancy);
+
 /* The minimal database page size in bytes. */
 #define MDBX_MIN_PAGESIZE 512
 __inline intptr_t mdbx_limits_pgsize_min(void) { return MDBX_MIN_PAGESIZE; }

src/elements/core.c

@@ -7516,6 +7516,112 @@ static int __cold mdbx_setup_lck(MDBX_env *env, char *lck_pathname,
   return lck_seize_rc;
 }
 
+__cold int mdbx_is_readahead_reasonable(size_t volume, intptr_t redundancy) {
+  if (volume <= 1024 * 1024 * 4ul)
+    return MDBX_RESULT_TRUE;
+
+  const size_t pagesize = mdbx_syspagesize();
+  if (unlikely(!mdbx_is_power2(pagesize) || pagesize < MIN_PAGESIZE))
+    return MDBX_INCOMPATIBLE;
+
+#if defined(_WIN32) || defined(_WIN64)
+  MEMORYSTATUSEX info;
+  memset(&info, 0, sizeof(info));
+  info.dwLength = sizeof(info);
+  if (!GlobalMemoryStatusEx(&info))
+    return GetLastError();
+#endif
+  const int log2page = mdbx_log2(pagesize);
+
+#if defined(_WIN32) || defined(_WIN64)
+  const intptr_t total_ram_pages = (intptr_t)(info.ullTotalPhys >> log2page);
+#elif defined(_SC_PHYS_PAGES)
+  const intptr_t total_ram_pages = sysconf(_SC_PHYS_PAGES);
+  if (total_ram_pages == -1)
+    return errno;
+#elif defined(_SC_AIX_REALMEM)
+  const intptr_t total_ram_Kb = sysconf(_SC_AIX_REALMEM);
+  if (total_ram_Kb == -1)
+    return errno;
+  const intptr_t total_ram_pages = (total_ram_Kb << 10) >> log2page;
+#elif defined(HW_USERMEM) || defined(HW_PHYSMEM64) || defined(HW_MEMSIZE) ||   \
+    defined(HW_PHYSMEM)
+  size_t ram;
+  size_t len = sizeof(ram);
+  static const int mib[2] = {
+    CTL_HW,
+#if defined(HW_USERMEM)
+    HW_USERMEM
+#elif defined(HW_PHYSMEM64)
+    HW_PHYSMEM64
+#elif defined(HW_MEMSIZE)
+    HW_MEMSIZE
+#else
+    HW_PHYSMEM
+#endif
+  };
+  if (sysctl(mib, ARRAY_LENGTH(mib), &ram, &len, NULL, 0) != 0)
+    return errno;
+  if (len != sizeof(ram))
+    return MDBX_ENOSYS;
+  const intptr_t total_ram_pages = (intptr_t)(ram >> log2page);
+#else
+#error "FIXME: Get User-accessible or physical RAM"
+#endif
+  if (total_ram_pages < 1)
+    return MDBX_ENOSYS;
+
+  const intptr_t volume_pages = (volume + pagesize - 1) >> log2page;
+  const intptr_t redundancy_pages =
+      (redundancy < 0) ? -(intptr_t)((-redundancy + pagesize - 1) >> log2page)
+                       : (intptr_t)(redundancy + pagesize - 1) >> log2page;
+  if (volume_pages >= total_ram_pages ||
+      volume_pages + redundancy_pages >= total_ram_pages)
+    return MDBX_RESULT_FALSE;
+
+#if defined(_WIN32) || defined(_WIN64)
+  const intptr_t avail_ram_pages = (intptr_t)(info.ullAvailPhys >> log2page);
+#elif defined(_SC_AVPHYS_PAGES)
+  const intptr_t avail_ram_pages = sysconf(_SC_AVPHYS_PAGES);
+  if (avail_ram_pages == -1)
+    return errno;
+#elif defined(__MACH__)
+  mach_msg_type_number_t count = HOST_VM_INFO_COUNT;
+  vm_statistics_data_t vmstat;
+  mach_port_t mport = mach_host_self();
+  kern_return_t kerr = host_statistics(mach_host_self(), HOST_VM_INFO,
+                                       (host_info_t)&vmstat, &count);
+  mach_port_deallocate(mach_task_self(), mport);
+  if (unlikely(kerr != KERN_SUCCESS))
+    return MDBX_ENOSYS;
+  const intptr_t avail_ram_pages = vmstat.free_count;
+#elif defined(VM_TOTAL) || defined(VM_METER)
+  struct vmtotal info;
+  size_t len = sizeof(info);
+  static const int mib[2] = {
+    CTL_VM,
+#if defined(VM_TOTAL)
+    VM_TOTAL
+#elif defined(VM_METER)
+    VM_METER
+#endif
+  };
+  if (sysctl(mib, ARRAY_LENGTH(mib), &info, &len, NULL, 0) != 0)
+    return errno;
+  if (len != sizeof(info))
+    return MDBX_ENOSYS;
+  const intptr_t avail_ram_pages = info.t_free;
+#else
+#error "FIXME: Get Available RAM"
+#endif
+  if (avail_ram_pages < 1)
+    return MDBX_ENOSYS;
+
+  return (volume_pages + redundancy_pages >= avail_ram_pages)
+             ? MDBX_RESULT_FALSE
+             : MDBX_RESULT_TRUE;
+}
+
 /* Only a subset of the mdbx_env flags can be changed
  * at runtime. Changing other flags requires closing the
  * environment and re-opening it with the new flags. */

src/elements/osal.h

@@ -74,6 +74,14 @@
     defined(__BSD__) || defined(__NETBSD__) || defined(__bsdi__) ||            \
     defined(__DragonFly__) || defined(__APPLE__) || defined(__MACH__)
 #include <sys/cdefs.h>
+#include <sys/sysctl.h>
+#include <sys/types.h>
+#if defined(__FreeBSD__) || defined(__DragonFly__)
+#include <vm/vm_param.h>
+#elif defined(__OpenBSD__) || defined(__NetBSD__)
+#include <uvm/uvm_param.h>
+#endif
+#include <sys/vmmeter.h>
 #else
 #include <malloc.h>
 #ifndef _POSIX_C_SOURCE
@@ -89,7 +97,15 @@
 #include <malloc/malloc.h>
 #endif /* MacOS */
 
+#if defined(__MACH__)
+#include <mach/host_info.h>
+#include <mach/mach_host.h>
+#include <mach/mach_port.h>
+#undef P_DIRTY
+#endif
+
 #if defined(__linux__) || defined(__gnu_linux__)
+#include <linux/sysctl.h>
 #include <sys/sendfile.h>
 #endif /* Linux */