rev-list: add --disk-usage option for calculating disk usage

It can sometimes be useful to see which refs are contributing to the
overall repository size (e.g., does some branch have a bunch of objects
not found elsewhere in history, which indicates that deleting it would
shrink the size of a clone).

You can find that out by generating a list of objects, getting their
sizes from cat-file, and then summing them, like:

    git rev-list --objects --no-object-names main..branch
    git cat-file --batch-check='%(objectsize:disk)' |
    perl -lne '$total += $_; END { print $total }'

Though note that the caveats from git-cat-file(1) apply here. We "blame"
base objects more than their deltas, even though the relationship could
easily be flipped. Still, it can be a useful rough measure.

But one problem is that it's slow to run. Teaching rev-list to sum up
the sizes can be much faster for two reasons:

  1. It skips all of the piping of object names and sizes.

  2. If bitmaps are in use, for objects that are in the
     bitmapped packfile we can skip the oid_object_info()
     lookup entirely, and just ask the revindex for the
     on-disk size.

This patch implements a --disk-usage option which produces the same
answer in a fraction of the time. Here are some timings using a clone of
torvalds/linux:

  [rev-list piped to cat-file, no bitmaps]
  $ time git rev-list --objects --no-object-names --all |
    git cat-file --buffer --batch-check='%(objectsize:disk)' |
    perl -lne '$total += $_; END { print $total }'
  1459938510
  real	0m29.635s
  user	0m38.003s
  sys	0m1.093s

  [internal, no bitmaps]
  $ time git rev-list --disk-usage --objects --all
  1459938510
  real	0m31.262s
  user	0m30.885s
  sys	0m0.376s

Even though the wall-clock time is slightly worse due to parallelism,
notice the CPU savings between the two. We saved 21% of the CPU just by
avoiding the pipes.

But the real win is with bitmaps. If we use them without the new option:

  [rev-list piped to cat-file, bitmaps]
  $ time git rev-list --objects --no-object-names --all --use-bitmap-index |
    git cat-file --batch-check='%(objectsize:disk)' |
    perl -lne '$total += $_; END { print $total }'
  1459938510
  real	0m6.244s
  user	0m8.452s
  sys	0m0.311s

then we're faster to generate the list of objects, but we still spend a
lot of time piping and looking things up. But if we do both together:

  [internal, bitmaps]
  $ time git rev-list --disk-usage --objects --all --use-bitmap-index
  1459938510
  real	0m0.219s
  user	0m0.169s
  sys	0m0.049s

then we get the same answer much faster.

For "--all", that answer will correspond closely to "du objects/pack",
of course. But we're actually checking reachability here, so we're still
fast when we ask for more interesting things:

  $ time git rev-list --disk-usage --use-bitmap-index v5.0..v5.10
  374798628
  real	0m0.429s
  user	0m0.356s
  sys	0m0.072s

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 96cc89d157..1238bfd915 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -227,6 +227,15 @@ ifdef::git-rev-list[]
 	test the exit status to see if a range of objects is fully
 	connected (or not).  It is faster than redirecting stdout
 	to `/dev/null` as the output does not have to be formatted.
+
+--disk-usage::
+	Suppress normal output; instead, print the sum of the bytes used
+	for on-disk storage by the selected commits or objects. This is
+	equivalent to piping the output into `git cat-file
+	--batch-check='%(objectsize:disk)'`, except that it runs much
+	faster (especially with `--use-bitmap-index`). See the `CAVEATS`
+	section in linkgit:git-cat-file[1] for the limitations of what
+	"on-disk storage" means.
 endif::git-rev-list[]
 
 --cherry-mark::