Use mmap() IO for reading uncompressed .blends

Instead of submitting tons of tiny IO syscalls, we can speed things up
significantly by `mmap`ing the .blend file into virtual memory and directly
accessing it.

In my local testing, this speeds up loading the Dweebs file with all its
linked files from 19sec to 10sec (on Linux).

As far as I can see, this should be supported on Linux, OSX and BSD.
For Windows, a second code path uses `CreateFileMapping` and
`MapViewOfFile` to achieve the same result.

Reviewed By: mont29, brecht

Differential Revision: https://developer.blender.org/D8246

2 files changed, 69 insertions, 3 deletions

diff --git a/source/blender/blenloader/intern/readfile.c b/source/blender/blenloader/intern/readfile.c
index b61abd4ed06..2192e9a378f 100644
--- a/source/blender/blenloader/intern/readfile.c
+++ b/source/blender/blenloader/intern/readfile.c
@@ -68,6 +68,7 @@
 #include "BLI_math.h"
 #include "BLI_memarena.h"
 #include "BLI_mempool.h"
+#include "BLI_mmap.h"
 #include "BLI_threads.h"
 
 #include "BLT_translation.h"
@@ -1179,6 +1180,53 @@ static ssize_t fd_read_from_memory(FileData *filedata,
   return readsize;
 }
 
+/* Memory-mapped file reading.
+ * By using mmap(), we can map a file so that it can be treated like normal memory,
+ * meaning that we can just read from it with memcpy() etc.
+ * This avoids system call overhead and can significantly speed up file loading.
+ */
+
+static ssize_t fd_read_from_mmap(FileData *filedata,
+                                 void *buffer,
+                                 size_t size,
+                                 bool *UNUSED(r_is_memchunck_identical))
+{
+  /* don't read more bytes than there are available in the buffer */
+  size_t readsize = MIN2(size, (size_t)(filedata->buffersize - filedata->file_offset));
+
+  if (!BLI_mmap_read(filedata->mmap_file, buffer, filedata->file_offset, readsize)) {
+    return 0;
+  }
+
+  filedata->file_offset += readsize;
+
+  return readsize;
+}
+
+static off64_t fd_seek_from_mmap(FileData *filedata, off64_t offset, int whence)
+{
+  off64_t new_pos;
+  if (whence == SEEK_CUR) {
+    new_pos = filedata->file_offset + offset;
+  }
+  else if (whence == SEEK_SET) {
+    new_pos = offset;
+  }
+  else if (whence == SEEK_END) {
+    new_pos = filedata->buffersize + offset;
+  }
+  else {
+    return -1;
+  }
+
+  if (new_pos < 0 || new_pos > filedata->buffersize) {
+    return -1;
+  }
+
+  filedata->file_offset = new_pos;
+  return filedata->file_offset;
+}
+
 /* MemFile reading. */
 
 static ssize_t fd_read_from_memfile(FileData *filedata,
@@ -1306,6 +1354,8 @@ static FileData *blo_filedata_from_file_descriptor(const char *filepath,
 {
   FileDataReadFn *read_fn = NULL;
   FileDataSeekFn *seek_fn = NULL; /* Optional. */
+  size_t buffersize = 0;
+  BLI_mmap_file *mmap_file = NULL;
 
   gzFile gzfile = (gzFile)Z_NULL;
 
@@ -1322,14 +1372,21 @@ static FileData *blo_filedata_from_file_descriptor(const char *filepath,
     return NULL;
   }
 
-  BLI_lseek(file, 0, SEEK_SET);
-
   /* Regular file. */
   if (memcmp(header, "BLENDER", sizeof(header)) == 0) {
     read_fn = fd_read_data_from_file;
     seek_fn = fd_seek_data_from_file;
+
+    mmap_file = BLI_mmap_open(file);
+    if (mmap_file != NULL) {
+      read_fn = fd_read_from_mmap;
+      seek_fn = fd_seek_from_mmap;
+      buffersize = BLI_lseek(file, 0, SEEK_END);
+    }
   }
 
+  BLI_lseek(file, 0, SEEK_SET);
+
   /* Gzip file. */
   errno = 0;
   if ((read_fn == NULL) &&
@@ -1363,6 +1420,8 @@ static FileData *blo_filedata_from_file_descriptor(const char *filepath,
 
   fd->read = read_fn;
   fd->seek = seek_fn;
+  fd->mmap_file = mmap_file;
+  fd->buffersize = buffersize;
 
   return fd;
 }
@@ -1531,6 +1590,11 @@ void blo_filedata_free(FileData *fd)
       fd->buffer = NULL;
     }
 
+    if (fd->mmap_file) {
+      BLI_mmap_free(fd->mmap_file);
+      fd->mmap_file = NULL;
+    }
+
     /* Free all BHeadN data blocks */
 #ifndef NDEBUG
     BLI_freelistN(&fd->bhead_list);
diff --git a/source/blender/blenloader/intern/readfile.h b/source/blender/blenloader/intern/readfile.h
index c724cc32051..86f05eda7b7 100644
--- a/source/blender/blenloader/intern/readfile.h
+++ b/source/blender/blenloader/intern/readfile.h
@@ -41,6 +41,7 @@ struct Object;
 struct OldNewMap;
 struct ReportList;
 struct UserDef;
+struct BLI_mmap_file;
 
 typedef struct IDNameLib_Map IDNameLib_Map;
 
@@ -83,8 +84,9 @@ typedef struct FileData {
   /** Regular file reading. */
   int filedes;
 
-  /** Variables needed for reading from memory / stream. */
+  /** Variables needed for reading from memory / stream / memory-mapped files. */
   const char *buffer;
+  struct BLI_mmap_file *mmap_file;
   /** Variables needed for reading from memfile (undo). */
   struct MemFile *memfile;
   /** Whether we are undoing (< 0) or redoing (> 0), used to choose which 'unchanged' flag to use