add a global var leveldb::useMmapRead to enable mmap Summary:

Summary:
as subject. this can be used for benchmarking.
If we want it for some cases, we can do more changes to make this part of the option.

Test Plan: db_test

Reviewers: dhruba

CC: MarkCallaghan

Differential Revision: https://reviews.facebook.net/D5451

db/db_bench.cc

@@ -170,6 +170,7 @@ static leveldb::Env* FLAGS_env = leveldb::Env::Default();
 
 extern bool useOsBuffer;
 extern bool useFsReadAhead;
+extern bool useMmapRead;
 
 namespace leveldb {
 
@@ -1182,6 +1183,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--bufferedio=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       useOsBuffer = n;
+    } else if (sscanf(argv[i], "--mmap_read=%d%c", &n, &junk) == 1 &&
+               (n == 0 || n == 1)) {
+      useMmapRead = n;
     } else if (sscanf(argv[i], "--readhead=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       useFsReadAhead = n;

util/env_posix.cc

@@ -28,6 +28,7 @@
 
 bool useOsBuffer = 1;     // cache data in OS buffers 
 bool useFsReadAhead = 1;  // allow filesystem to do readaheads
+bool useMmapRead = 0;
 
 namespace leveldb {
 
@@ -386,13 +387,26 @@ class PosixEnv : public Env {
 
   virtual Status NewRandomAccessFile(const std::string& fname,
                                      RandomAccessFile** result) {
-    // Use of mmap for random reads has been removed because it
-    // kills performance when storage is fast.
     *result = NULL;
     Status s;
     int fd = open(fname.c_str(), O_RDONLY);
     if (fd < 0) {
       s = IOError(fname, errno);
+    } else if (useMmapRead && sizeof(void*) >= 8) {
+      // Use of mmap for random reads has been removed because it
+      // kills performance when storage is fast.
+      // Use mmap when virtual address-space is plentiful.
+      uint64_t size;
+      s = GetFileSize(fname, &size);
+      if (s.ok()) {
+        void* base = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
+        if (base != MAP_FAILED) {
+          *result = new PosixMmapReadableFile(fname, base, size);
+        } else {
+          s = IOError(fname, errno);
+        }
+      }
+      close(fd);
     } else {
       *result = new PosixRandomAccessFile(fname, fd);
     }