[RFC] Dynamic window size on repack?

I have a CVS repository which is mostly sane, but has an approximately 20MB RTF file that has two hundred revisions or so. (Thank you, Windows help.)

Now, since this is old history, I want to make it as small as possible. The only problem is that when I use high --window values for repack, it goes along swimmingly until it gets to this file, at which point memory usage quickly rises to the point where I'm well into my swap file.

I think what I'd like is an extra option to repack to limit window memory usage. This would dynamically scale the window size down if it can't fit within the limit, then scale it back up once you're off of the nasty file. This would let me repack my repository with --window=100 and have it actually finish someday on the machines I have access to. The big file may not be as efficiently packed as possible, but I can live with that.

My question is, is this sane? Does the repack algorithm depend on having a fixed window size to work? I'd rather not look into implementing this if it's silly on the face of it.

Thanks, -bcd

On 7/8/07, Brian Downing <bdowning@lavos.net> wrote:

Sounds very sane to me.

It seems like you want something like this
(I've not referred to the code,  but there is a loop
 that could be modifed to include something like this):
  /* build list of delta candidates */
  tot = 0;
  for (i = 0; i < window; ++i ) {
    obj = objects_sorted_for_delta[here + i];
    tot += SIZE(obj);
    if ( tot > window_limit )
      break;
    /* insert obj in list of things to delta, or just try it here */
    ...
  }
  if ( i <= 1 )
    break/return;

window_limit could be set automatically like the variables for the mmap windows are (no new options). SIZE() should be defined to return the actual bytes consumed by the object (I think for this it's uncompressed and undeltified, but as I said I haven't looked at the code).

It would be better if the current list of objects in the window were a FIFO. Before each deltification attempt, add objects from the sort list until #objects > window or tot > window_limit. After each attempt, drop off the object we were trying to delta.

I like your idea and think you should look into implementing it.

On Sun, 8 Jul 2007, Brian Downing wrote:

It doesn't sound silly, and it should even be fairly easy. The window code is all in builtin-pack-objects.c (find_deltas()) and while it's currently coded for a constant-sized window, it shouldn't be too hard to free more old entries if you allocate one big one to make sure that the "array" thing doesn't grow to contain too much data.

In other words, just look at how the variables "struct unpacked *array" (the whole window array) and the "struct unpacked *n" (the "next entry" in the array using a simple circular queue using "idx") are accessed.

		Linus

On Sun, 8 Jul 2007, Linus Torvalds wrote:

Side note: a limit based on object sizes is likely a much better way to handle the window than just a "number of objects" thing ever was. Doing the size in number of objects was easier, and is fine for source code that tends to have a reasonably normal distribution of sized, but yeah, if you have a few really big objects with lots of history, then it's likely the wrong thing to do just because it can get really expensive.

		Linus