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
Create repo with old manifest
$ cat << EOF >> $HGRCPATH
> [format]
> usegeneraldelta=yes
> EOF
$ hg init existing
$ cd existing
$ echo footext > foo
$ hg add foo
$ hg commit -m initial
We're using v1, so no manifestv2 entry is in requires yet.
$ grep manifestv2 .hg/requires
[1]
Let's clone this with manifestv2 enabled to switch to the new format for
future commits.
$ cd ..
$ hg clone --pull existing new --config experimental.manifestv2=1
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 0fc9a4fafa44
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd new
Check that entry was added to .hg/requires.
$ grep manifestv2 .hg/requires
manifestv2
Make a new commit.
$ echo newfootext > foo
$ hg commit -m new
Check that the manifest actually switched to v2.
$ hg debugdata -m 0
foo\x0021e958b1dca695a60ee2e9cf151753204ee0f9e9 (esc)
$ hg debugdata -m 1
\x00 (esc)
\x00foo\x00 (esc)
I\xab\x7f\xb8(\x83\xcas\x15\x9d\xc2\xd3\xd3:5\x08\xbad5_ (esc)
Check that manifestv2 is used if the requirement is present, even if it's
disabled in the config.
$ echo newerfootext > foo
$ hg --config experimental.manifestv2=False commit -m newer
$ hg debugdata -m 2
\x00 (esc)
\x00foo\x00 (esc)
\xa6\xb1\xfb\xef]\x91\xa1\x19`\xf3.#\x90S\xf8\x06 \xe2\x19\x00 (esc)
Check that we can still read v1 manifests.
$ hg files -r 0
foo
$ cd ..
Check that entry is added to .hg/requires on repo creation
$ hg --config experimental.manifestv2=True init repo
$ cd repo
$ grep manifestv2 .hg/requires
manifestv2
Set up simple repo
$ echo a > file1
$ echo b > file2
$ echo c > file3
$ hg ci -Aqm 'initial'
$ echo d > file2
$ hg ci -m 'modify file2'
Check that 'hg verify', which uses manifest.readdelta(), works
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
3 files, 2 changesets, 4 total revisions
Check that manifest revlog is smaller than for v1
$ hg debugindex -m
rev offset length delta linkrev nodeid p1 p2
0 0 81 -1 0 57361477c778 000000000000 000000000000
1 81 33 0 1 aeaab5a2ef74 57361477c778 000000000000