view tests/test-excessive-merge.t @ 35793:4fb2bb61597c

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

  $ hg init

  $ echo foo > a
  $ echo foo > b
  $ hg add a b

  $ hg ci -m "test"

  $ echo blah > a

  $ hg ci -m "branch a"

  $ hg co 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

  $ echo blah > b

  $ hg ci -m "branch b"
  created new head
  $ HGMERGE=true hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

  $ hg ci -m "merge b/a -> blah"

  $ hg co 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ HGMERGE=true hg merge 2
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -m "merge a/b -> blah"
  created new head

  $ hg log
  changeset:   4:2ee31f665a86
  tag:         tip
  parent:      1:96155394af80
  parent:      2:92cc4c306b19
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     merge a/b -> blah
  
  changeset:   3:e16a66a37edd
  parent:      2:92cc4c306b19
  parent:      1:96155394af80
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     merge b/a -> blah
  
  changeset:   2:92cc4c306b19
  parent:      0:5e0375449e74
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     branch b
  
  changeset:   1:96155394af80
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     branch a
  
  changeset:   0:5e0375449e74
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     test
  
  $ hg debugindex --changelog
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0      60  .....       0 5e0375449e74 000000000000 000000000000 (re)
       1        60      62  .....       1 96155394af80 5e0375449e74 000000000000 (re)
       2       122      62  .....       2 92cc4c306b19 5e0375449e74 000000000000 (re)
       3       184      69  .....       3 e16a66a37edd 92cc4c306b19 96155394af80 (re)
       4       253      69  .....       4 2ee31f665a86 96155394af80 92cc4c306b19 (re)

revision 1
  $ hg manifest --debug 1
  79d7492df40aa0fa093ec4209be78043c181f094 644   a
  2ed2a3912a0b24502043eae84ee4b279c18b90dd 644   b
revision 2
  $ hg manifest --debug 2
  2ed2a3912a0b24502043eae84ee4b279c18b90dd 644   a
  79d7492df40aa0fa093ec4209be78043c181f094 644   b
revision 3
  $ hg manifest --debug 3
  79d7492df40aa0fa093ec4209be78043c181f094 644   a
  79d7492df40aa0fa093ec4209be78043c181f094 644   b
revision 4
  $ hg manifest --debug 4
  79d7492df40aa0fa093ec4209be78043c181f094 644   a
  79d7492df40aa0fa093ec4209be78043c181f094 644   b

  $ hg debugindex a
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0       5  .....       0 2ed2a3912a0b 000000000000 000000000000 (re)
       1         5       6  .....       1 79d7492df40a 2ed2a3912a0b 000000000000 (re)

  $ hg verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  2 files, 5 changesets, 4 total revisions