Remove use of mmap for random reads

Summary:
Reads via mmap on concurrent workloads are much slower than pread.
For example on a 24-core server with storage that can do 100k IOPS or more
I can get no more than 10k IOPS with mmap reads and 32+ threads.

Test Plan: db_bench benchmarks

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D5433

util/env_posix.cc

@@ -386,24 +386,13 @@ 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 (useOsBuffer && sizeof(void*) >= 8) {
-      // 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);
     }