Enable per-request buffer allocation in RandomAccessFile

This change impacts only non-buffered I/O on Windows.
 Currently, there is a buffer per RandomAccessFile
 instance that is protected by a lock. The reason we
 maintain the buffer is non-buffered I/O requires an aligned
 buffer to work.
 XPerf traces demonstrate that we accumulate a considerable
 wait time while waiting for that lock.
 This change enables to set random access buffer size to zero
 which would indicate a per request allocation.
 We are expecting that allocation expense would be much less than
 I/O costs plus wait time due to the fact that the memory heap
 would tend to re-use page aligned allocations especially with the
 use of Jemalloc.
 This change does not affect buffer use as a read_ahead_buffer for
 compaction purposes.

include/rocksdb/options.h

@@ -1121,6 +1121,9 @@ struct DBOptions {
   // This option is currently honored only on Windows
   //
   // Default: 1 Mb
+  //
+  // Special value: 0 - means do not maintain per instance buffer. Allocate
+  //                per request buffer and avoid locking.
   size_t random_access_max_buffer_size;
 
   // This is the maximum buffer size that is used by WritableFileWriter.

port/win/env_win.cc

@@ -766,6 +766,18 @@ class WinRandomAccessFile : public RandomAccessFile {
     return read;
   }
 
+  void CalculateReadParameters(uint64_t offset, size_t bytes_requested,
+                                size_t& actual_bytes_toread,
+                                uint64_t& first_page_start) const {
+
+    const size_t alignment = buffer_.Alignment();
+
+    first_page_start = TruncateToPageBoundary(alignment, offset);
+    const uint64_t last_page_start =
+      TruncateToPageBoundary(alignment, offset + bytes_requested - 1);
+    actual_bytes_toread = (last_page_start - first_page_start) + alignment;
+  }
+
  public:
   WinRandomAccessFile(const std::string& fname, HANDLE hFile, size_t alignment,
                       const EnvOptions& options)
@@ -797,15 +809,36 @@ class WinRandomAccessFile : public RandomAccessFile {
 
   virtual Status Read(uint64_t offset, size_t n, Slice* result,
                       char* scratch) const override {
+
     Status s;
     SSIZE_T r = -1;
     size_t left = n;
     char* dest = scratch;
 
+    if (n == 0) {
+      *result = Slice(scratch, 0);
+      return s;
+    }
+
     // When in unbuffered mode we need to do the following changes:
     // - use our own aligned buffer
     // - always read at the offset of that is a multiple of alignment
     if (!use_os_buffer_) {
+
+      uint64_t first_page_start = 0;
+      size_t actual_bytes_toread = 0;
+      size_t bytes_requested = left;
+
+      if (!read_ahead_ && random_access_max_buffer_size_ == 0) {
+        CalculateReadParameters(offset, bytes_requested, actual_bytes_toread,
+          first_page_start);
+
+        assert(actual_bytes_toread > 0);
+
+        r = ReadIntoOneShotBuffer(offset, first_page_start,
+          actual_bytes_toread, left, dest);
+      } else {
+
         std::unique_lock<std::mutex> lock(buffer_mut_);
 
         // Let's see if at least some of the requested data is already
@@ -824,19 +857,16 @@ class WinRandomAccessFile : public RandomAccessFile {
         // Still some left or none was buffered
         if (left > 0) {
           // Figure out the start/end offset for reading and amount to read
-        const size_t alignment = buffer_.Alignment();
-        const size_t first_page_start =
-            TruncateToPageBoundary(alignment, offset);
+          bytes_requested = left;
 
-        size_t bytes_requested = left;
           if (read_ahead_ && bytes_requested < compaction_readahead_size_) {
             bytes_requested = compaction_readahead_size_;
           }
 
-        const size_t last_page_start =
-            TruncateToPageBoundary(alignment, offset + bytes_requested - 1);
-        const size_t actual_bytes_toread =
-            (last_page_start - first_page_start) + alignment;
+          CalculateReadParameters(offset, bytes_requested, actual_bytes_toread,
+            first_page_start);
+
+          assert(actual_bytes_toread > 0);
 
           if (buffer_.Capacity() < actual_bytes_toread) {
             // If we are in read-ahead mode or the requested size
@@ -848,17 +878,20 @@ class WinRandomAccessFile : public RandomAccessFile {
               lock.unlock();
               r = ReadIntoOneShotBuffer(offset, first_page_start,
                 actual_bytes_toread, left, dest);
-          } else {
+            }
+            else {
               buffer_.AllocateNewBuffer(actual_bytes_toread);
               r = ReadIntoInstanceBuffer(offset, first_page_start,
                 actual_bytes_toread, left, dest);
             }
-        } else {
+          }
+          else {
             buffer_.Clear();
             r = ReadIntoInstanceBuffer(offset, first_page_start,
               actual_bytes_toread, left, dest);
           }
         }
+      }
     } else {
       r = pread(hFile_, scratch, left, offset);
       if (r > 0) {