GeneralDelta

Using the generaldelta compression option.

1. Introduction

The original Mercurial compression format has a particular weakness in storing and transmitting deltas for branches that are heavily interleaved. In some instances, this can make the size of the manifest data (stored in 00manifest.d) balloon by 10x or more. The generaldelta option is an effort to mitigate that, while still maintaining Mercurial's O(1)-bounded performance.

The generaldelta feature is available in Mercurial 1.9 and later.

2. Enabling generaldelta

The generaldelta feature can be enabled for new clones with:

[format]
generaldelta = true

This will actually enable two features:

• generaldelta
• delta reordering on pulls when this is enabled on the server side

The latter feature will let clients without generaldelta enabled experience some of the disk space and bandwidth benefits.

3. Converting a repo to generaldelta

This is as simple as:

$ hg clone -U --config format.generaldelta=1 --pull project project-generaldelta

The aforementioned reordering can also marginally improve compression for generaldelta clients, which can be tried with a second pass:

$ hg clone -U --config format.generaldelta=1 --pull project-generaldelta project-generaldelta-pass2

Detailed compression statistics for the manifest can be checked with debugrevlog:

$ hg debugrevlog -m
format : 1
flags  : generaldelta

revisions     :   14932
    merges    :    1763 (11.81%)
    normal    :   13169 (88.19%)
revisions     :   14932
    full      :      61 ( 0.41%)
    deltas    :   14871 (99.59%)
revision size : 3197528
    full      :  744577 (23.29%)
    deltas    : 2452951 (76.71%)

avg chain length  : 172
compression ratio : 229

uncompressed data size (min/max/avg) : 125 / 80917 / 49156
full revision size (min/max/avg)     : 113 / 37284 / 12206
delta size (min/max/avg)             : 0 / 27029 / 164

deltas against prev  : 13770 (92.60%)
    where prev = p1  : 13707     (99.54%)
    where prev = p2  :     8     ( 0.06%)
    other            :    55     ( 0.40%)
deltas against p1    :  1097 ( 7.38%)
deltas against p2    :     4 ( 0.03%)
deltas against other :     0 ( 0.00%)

Of particular interest are the number of full revisions and the average delta size.

4. Further work

Mercurial's bundle protocol doesn't yet fully support generaldelta. This creates two barriers to making this Mercurial's default format:

• pulling from a generaldelta repo uses more server CPU as the server has to recalculate some deltas
• more than an optimal amount of bandwidth is still used due to sending old-style deltas
• cloning where cg2/gd is natively supported by client and server results in client re-encoding and not using generaldelta in requirements

We intend to eventually address this by updating Mercurial's bundle protocol with BundleFormat2, after which the generaldelta feature will be enabled by default on new clones.

5. See also

• ParentDeltaPlan

• ManifestCompressionPlan

• Call for testing: generaldelta

CategoryInternals CategoryNewFeatures