tests/test-show-stack.t

bundle2: increase payload part chunk size to 32kb

Bundle2 payload parts are framed chunks. Esentially, we obtain
data in equal size chunks of size `preferedchunksize` and emit those
to a generator. That generator is fed into a compressor (which can
be the no-op compressor, which just re-emits the generator). And
the output from the compressor likely goes to a file descriptor
or socket.

What this means is that small chunk sizes create more Python objects
and Python function calls than larger chunk sizes. And as we know,
Python object and function call overhead in performance sensitive
code matters (at least with CPython).

This commit increases the bundle2 part payload chunk size from 4k
to 32k. Practically speaking, this means that the chunks we feed
into a compressor (implemented in C code) or feed directly into a
file handle or socket write() are larger. It's possible the chunks
might be larger than what the receiver can handle in one logical
operation. But at that point, we're in C code, which is much more
efficient at dealing with splitting up the chunk and making multiple
function calls than Python is.

A downside to larger chunks is that the receiver has to wait for that
much data to arrive (either raw or from a decompressor) before it
can process the chunk. But 32kb still feels like a small buffer to
have to wait for. And in many cases, the client will convert from
8 read(4096) to 1 read(32768). That's happening in Python land. So
we cut down on the number of Python objects and function calls,
making the client faster as well. I don't think there are any
significant concerns to increasing the payload chunk size to 32kb.

The impact of this change on performance significant. Using `curl`
to obtain a stream clone bundle2 payload from a server on localhost
serving the mozilla-unified repository:

before: 20.78 user; 7.71 system;  80.5 MB/s
after:  13.90 user; 3.51 system; 132 MB/s
legacy:  9.72 user; 8.16 system; 132 MB/s

bundle2 stream clone generation is still more resource intensive than
legacy stream clone (that's likely because of the use of a
util.chunkbuffer). But the throughput is the same. We might
be in territory we're this is effectively a benchmark of the
networking stack or Python's syscall throughput.

From the client perspective, `hg clone -U --stream`:

before: 33.50 user; 7.95 system; 53.3 MB/s
after:  22.82 user; 7.33 system; 72.7 MB/s
legacy: 29.96 user; 7.94 system; 58.0 MB/s

And for `hg clone --stream` with a working directory update of
~230k files:

after:  119.55 user; 26.47 system; 0:57.08 wall
legacy: 126.98 user; 26.94 system; 1:05.56 wall

So, it appears that bundle2's stream clone is now definitively faster
than legacy stream clone!

Differential Revision: https://phab.mercurial-scm.org/D1932

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > show =
  > EOF

  $ hg init repo0
  $ cd repo0

Empty repo / no checkout results in error

  $ hg show stack
  abort: stack view only available when there is a working directory
  [255]

Stack displays single draft changeset as root revision

  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 0'
  $ hg show stack
    @  9f17 commit 0

Stack displays multiple draft changesets

  $ echo 1 > foo
  $ hg commit -m 'commit 1'
  $ echo 2 > foo
  $ hg commit -m 'commit 2'
  $ echo 3 > foo
  $ hg commit -m 'commit 3'
  $ echo 4 > foo
  $ hg commit -m 'commit 4'
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
    o  181c commit 1
    o  9f17 commit 0

Public parent of draft base is displayed, separated from stack

  $ hg phase --public -r 0
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
    o  181c commit 1
   /   (stack base)
  o  9f17 commit 0

  $ hg phase --public -r 1
  $ hg show stack
    @  2737 commit 4
    o  d1a6 commit 3
    o  128c commit 2
   /   (stack base)
  o  181c commit 1

Draft descendants are shown

  $ hg -q up 2
  $ hg show stack
    o  2737 commit 4
    o  d1a6 commit 3
    @  128c commit 2
   /   (stack base)
  o  181c commit 1

  $ hg -q up 3
  $ hg show stack
    o  2737 commit 4
    @  d1a6 commit 3
    o  128c commit 2
   /   (stack base)
  o  181c commit 1

working dir on public changeset should display special message

  $ hg -q up 1
  $ hg show stack
  (empty stack; working directory parent is a published changeset)

Branch point in descendants displayed at top of graph

  $ hg -q up 3
  $ echo b > foo
  $ hg commit -m 'commit 5 (new dag branch)'
  created new head
  $ hg -q up 2
  $ hg show stack
   \ /  (multiple children)
    |
    o  d1a6 commit 3
    @  128c commit 2
   /   (stack base)
  o  181c commit 1

  $ cd ..

Base is stopped at merges

  $ hg init merge-base
  $ cd merge-base
  $ echo 0 > foo
  $ hg -q commit -A -m initial
  $ echo h1 > foo
  $ hg commit -m 'head 1'
  $ hg -q up 0
  $ echo h2 > foo
  $ hg -q commit -m 'head 2'
  $ hg phase --public -r 0:tip
  $ hg -q up 1
  $ hg merge -t :local 2
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg commit -m 'merge heads'

TODO doesn't yet handle case where wdir is a draft merge

  $ hg show stack
    @  8ee9 merge heads
   /   (stack base)
  o  5947 head 1

  $ echo d1 > foo
  $ hg commit -m 'draft 1'
  $ echo d2 > foo
  $ hg commit -m 'draft 2'

  $ hg show stack
    @  430d draft 2
    o  787b draft 1
   /   (stack base)
  o  8ee9 merge heads

  $ cd ..

Now move on to stacks when there are more commits after the base branchpoint

  $ hg init public-rebase
  $ cd public-rebase
  $ echo 0 > foo
  $ hg -q commit -A -m 'base'
  $ hg phase --public -r .
  $ echo d1 > foo
  $ hg commit -m 'draft 1'
  $ echo d2 > foo
  $ hg commit -m 'draft 2'
  $ hg -q up 0
  $ echo 1 > foo
  $ hg commit -m 'new 1'
  created new head
  $ echo 2 > foo
  $ hg commit -m 'new 2'
  $ hg -q up 2

Newer draft heads don't impact output

  $ hg show stack
    @  eaff draft 2
    o  2b21 draft 1
   /   (stack base)
  o  b66b base

Newer public heads are rendered

  $ hg phase --public -r '::tip'

  $ hg show stack
    o  baa4 new 2
   /    (2 commits ahead)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

If rebase is available, we show a hint how to rebase to that head

  $ hg --config extensions.rebase= show stack
    o  baa4 new 2
   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

Similar tests but for multiple heads

  $ hg -q up 0
  $ echo h2 > foo
  $ hg -q commit -m 'new head 2'
  $ hg phase --public -r .
  $ hg -q up 2

  $ hg show stack
    o  baa4 new 2
   /    (2 commits ahead)
  : o  9a84 new head 2
  :/    (1 commits ahead)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base

  $ hg --config extensions.rebase= show stack
    o  baa4 new 2
   /    (2 commits ahead; hg rebase --source 2b21 --dest baa4)
  : o  9a84 new head 2
  :/    (1 commits ahead; hg rebase --source 2b21 --dest 9a84)
  :
  :    (stack head)
  : @  eaff draft 2
  : o  2b21 draft 1
  :/   (stack base)
  o  b66b base