Changeset 2164

Timestamp:

Jun 19, 2007, 6:01:30 AM (15 years ago)

Author:

charles

Message:

better algorithm to decide on a good piece size. (SoftwareElves? & charles_)

File:

• trunk/libtransmission/makemeta.c (modified) (3 diffs)

trunk/libtransmission/makemeta.c

@@ -9 +9 @@
  */
 
+#include <math.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -74 +75 @@
 
 static int
-bestPieceSize( uint64_t totalSize )
-{
-    const int MiB = 1048576;
-    const int GiB = totalSize / (uint64_t)1073741824;
-
-    /* almost always best to have a piee size of 512 or 256 kb.
-       common practice seems to be to bump up to 1MB pieces at
-       at total size of around 8GiB or so */
-
-    if( GiB >= 8 )
-        return MiB;
-
-    if( GiB >= 1 )
-        return MiB / 2;
-
-    return MiB / 4;
+bestPieceSize( uint64_t totalSize, uint64_t fileCount )
+{
+    int pieceSize = 1;
+    const int minPieceSize = 65536; /* arbitrary; 2^16 */
+    const int maxPieceSize = 16777216; /* arbitrary; 2^24 */
+    const int desiredMinPiecesPerFile = 15; /* arbitrary */
+    uint64_t log;
+
+    /* start off in the range of (1024..2048] pieces
+       for "normal" torrents... */
+    log = totalSize;
+    while( log > 2048 ) {
+        log >>= 1;
+        pieceSize <<= 1;
+    }
+
+    /* special case 1: torrents with a lot of small files.
+       try to have a reasonable number of pieces per file,
+       which will reduce overhead on selective downloading
+       and increase swarm speed. */
+    while( totalSize/pieceSize < (fileCount * desiredMinPiecesPerFile) ) {
+        const int swap = pieceSize >> 1;
+        if( swap < minPieceSize )
+            break;
+        pieceSize = swap;
+    }
+
+    /* special case 2: enormous single-file torrents. our normal
+       case creates unwieldly piece sizes in that case */
+    while( pieceSize < maxPieceSize )
+        pieceSize >>= 1;
+
+    return pieceSize;
 }
 
@@ -149 +167 @@
            builderFileCompare );
 
-    ret->pieceSize = bestPieceSize( ret->totalSize );
+    ret->pieceSize = bestPieceSize( ret->totalSize, ret->fileCount );
     ret->pieceCount = (int)( ret->totalSize / ret->pieceSize);
     if( ret->totalSize % ret->pieceSize )