view tests/test-bookmarks-merge.t @ 30442:41a8106789ca

util: implement zstd compression engine

Now that zstd is vendored and being built (in some configurations), we
can implement a compression engine for zstd!

The zstd engine is a little different from existing engines. Because
it may not always be present, we have to defer load the module in case
importing it fails. We facilitate this via a cached property that holds
a reference to the module or None. The "available" method is
implemented to reflect reality.

The zstd engine declares its ability to handle bundles using the
"zstd" human name and the "ZS" internal name. The latter was chosen
because internal names are 2 characters (by only convention I think)
and "ZS" seems reasonable.

The engine, like others, supports specifying the compression level.
However, there are no consumers of this API that yet pass in that
argument. I have plans to change that, so stay tuned.

Since all we need to do to support bundle generation with a new
compression engine is implement and register the compression engine,
bundle generation with zstd "just works!" Tests demonstrating this
have been added.

How does performance of zstd for bundle generation compare? On the
mozilla-unified repo, `hg bundle --all -t <engine>-v2` yields the
following on my i7-6700K on Linux:

engine        CPU time     bundle size   vs orig size   throughput
none            97.0s     4,054,405,584     100.0%       41.8 MB/s
bzip2 (l=9)    393.6s       975,343,098      24.0%       10.3 MB/s
gzip (l=6)     184.0s     1,140,533,074      28.1%       22.0 MB/s
zstd (l=1)     108.2s     1,119,434,718      27.6%       37.5 MB/s
zstd (l=2)     111.3s     1,078,328,002      26.6%       36.4 MB/s
zstd (l=3)     113.7s     1,011,823,727      25.0%       35.7 MB/s
zstd (l=4)     116.0s     1,008,965,888      24.9%       35.0 MB/s
zstd (l=5)     121.0s       977,203,148      24.1%       33.5 MB/s
zstd (l=6)     131.7s       927,360,198      22.9%       30.8 MB/s
zstd (l=7)     139.0s       912,808,505      22.5%       29.2 MB/s
zstd (l=12)    198.1s       854,527,714      21.1%       20.5 MB/s
zstd (l=18)    681.6s       789,750,690      19.5%        5.9 MB/s

On compression, zstd for bundle generation delivers:

* better compression than gzip with significantly less CPU utilization
* better than bzip2 compression ratios while still being significantly
  faster than gzip
* ability to aggressively tune compression level to achieve
  significantly smaller bundles

That last point is important. With clone bundles, a server can
pre-generate a bundle file, upload it to a static file server, and
redirect clients to transparently download it during clone. The server
could choose to produce a zstd bundle with the highest compression
settings possible. This would take a very long time - a magnitude
longer than a typical zstd bundle generation - but the result would
be hundreds of megabytes smaller! For the clone volume we do at
Mozilla, this could translate to petabytes of bandwidth savings
per year and faster clones (due to smaller transfer size).

I don't have detailed numbers to report on decompression. However,
zstd decompression is fast: >1 GB/s output throughput on this machine,
even through the Python bindings. And it can do that regardless of the
compression level of the input. By the time you have enough data to
worry about overhead of decompression, you have plenty of other things
to worry about performance wise.

zstd is wins all around. I can't wait to implement support for it
on the wire protocol and in revlogs.

# init

  $ hg init
  $ echo a > a
  $ hg add a
  $ hg commit -m'a'
  $ echo b > b
  $ hg add b
  $ hg commit -m'b'
  $ hg up -C 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo c > c
  $ hg add c
  $ hg commit -m'c'
  created new head

# test merging of diverged bookmarks
  $ hg bookmark -r 1 "c@diverge"
  $ hg bookmark -r 1 b
  $ hg bookmark c
  $ hg bookmarks
     b                         1:d2ae7f538514
   * c                         2:d36c0562f908
     c@diverge                 1:d2ae7f538514
  $ hg merge "c@diverge"
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m'merge'
  $ hg bookmarks
     b                         1:d2ae7f538514
   * c                         3:b8f96cf4688b

  $ hg up -C 3
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (leaving bookmark c)
  $ echo d > d
  $ hg add d
  $ hg commit -m'd'

  $ hg up -C 3
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo e > e
  $ hg add e
  $ hg commit -m'e'
  created new head
  $ hg up -C 5
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg bookmark e
  $ hg bookmarks
     b                         1:d2ae7f538514
     c                         3:b8f96cf4688b
   * e                         5:26bee9c5bcf3

# the picked side is bookmarked

  $ hg up -C 4
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (leaving bookmark e)
  $ hg merge
  abort: heads are bookmarked - please merge with an explicit rev
  (run 'hg heads' to see all heads)
  [255]

# our revision is bookmarked

  $ hg up -C e
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (activating bookmark e)
  $ hg merge
  abort: no matching bookmark to merge - please merge with an explicit rev or bookmark
  (run 'hg heads' to see all heads)
  [255]

# merge bookmark heads

  $ hg up -C 4
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (leaving bookmark e)
  $ echo f > f
  $ hg commit -Am "f"
  adding f
  $ hg bookmarks -r 4 "e@diverged"
  $ hg up -q -C "e@diverged"
  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg parents
  changeset:   4:a0546fcfe0fb
  bookmark:    e@diverged
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     d
  
  changeset:   5:26bee9c5bcf3
  bookmark:    e
  parent:      3:b8f96cf4688b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     e
  
  $ hg up -C e
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (activating bookmark e)
  $ hg bookmarks
     b                         1:d2ae7f538514
     c                         3:b8f96cf4688b
   * e                         5:26bee9c5bcf3
     e@diverged                4:a0546fcfe0fb
  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m'merge'
  $ hg bookmarks
     b                         1:d2ae7f538514
     c                         3:b8f96cf4688b
   * e                         7:ca784329f0ba

# test warning when all heads are inactive bookmarks

  $ hg up -C 6
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  (leaving bookmark e)
  $ echo g > g
  $ hg commit -Am 'g'
  adding g
  $ hg bookmark -i g
  $ hg bookmarks
     b                         1:d2ae7f538514
     c                         3:b8f96cf4688b
     e                         7:ca784329f0ba
     g                         8:04dd21731d95
  $ hg heads
  changeset:   8:04dd21731d95
  bookmark:    g
  tag:         tip
  parent:      6:be381d1126a0
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     g
  
  changeset:   7:ca784329f0ba
  bookmark:    e
  parent:      5:26bee9c5bcf3
  parent:      4:a0546fcfe0fb
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     merge
  
  $ hg merge
  abort: heads are bookmarked - please merge with an explicit rev
  (run 'hg heads' to see all heads)
  [255]