changeset 5451:0a43875677b1

revlog: break up compression of large deltas

Python's zlib apparently makes an internal copy of strings passed to
compress(). To avoid this, compress strings 1M at a time, then join
them at the end if the result would be smaller than the original.

For initial commits of large but compressible files, this cuts peak
memory usage nearly in half.

--- a/mercurial/revlog.py	Thu Oct 11 00:46:53 2007 -0500
+++ b/mercurial/revlog.py	Thu Oct 11 00:46:54 2007 -0500
@@ -61,12 +61,27 @@
     """ generate a possibly-compressed representation of text """
     if not text:
         return ("", text)
-    if len(text) < 44:
+    l = len(text)
+    if l < 44:
         if text[0] == '\0':
             return ("", text)
         return ('u', text)
-    bin = _compress(text)
-    if len(bin) > len(text):
+    elif l > 1000000:
+        # zlib makes an internal copy, thus doubling memory usage for
+        # large files, so lets do this in pieces
+        z = zlib.compressobj()
+        p = []
+        pos = 0
+        while pos < l:
+            pos2 = pos + 2**20
+            p.append(z.compress(text[pos:pos2]))
+            pos = pos2
+        p.append(z.flush())
+        if sum(map(len, p)) < l:
+            bin = "".join(p)
+    else:
+        bin = _compress(text)
+    if len(bin) > l:
         if text[0] == '\0':
             return ("", text)
         return ('u', text)