wireproto: compress data from a generator
Currently, the "getbundle" wire protocol command obtains a generator of
data, converts it to a util.chunkbuffer, then converts it back to a
generator via the protocol's groupchunks() implementation. For the SSH
protocol, groupchunks() simply reads 4kb chunks then write()s the
data to a file descriptor. For the HTTP protocol, groupchunks() reads
32kb chunks, feeds those into a zlib compressor, emits compressed data
as it is available, and that is sent to the WSGI layer, where it is
likely turned into HTTP chunked transfer chunks as is or further
buffered and turned into a larger chunk.
For both the SSH and HTTP protocols, there is inefficiency from using
util.chunkbuffer.
For SSH, emitting consistent 4kb chunks sounds nice. However, the file
descriptor it is writing to is almost certainly buffered. That means
that a Python .write() probably doesn't translate into exactly what is
written to the I/O layer.
For HTTP, we're going through an intermediate layer to zlib compress
data. So all util.chunkbuffer is doing is ensuring that the chunks we
feed into the zlib compressor are of uniform size. This means more CPU
time in Python buffering and emitting chunks in util.chunkbuffer but
fewer function calls to zlib.
This patch introduces and implements a new wire protocol abstract
method: compresschunks(). It is like groupchunks() except it operates
on a generator instead of something with a .read(). The SSH
implementation simply proxies chunks. The HTTP implementation uses
zlib compression.
To avoid duplicate code, the HTTP groupchunks() has been reimplemented
in terms of compresschunks().
To prove this all works, the "getbundle" wire protocol command has been
switched to compresschunks(). This removes the util.chunkbuffer from
that command. Now, data essentially streams straight from the
changegroup emitter to the wire, possibly through a zlib compressor.
Generators all the way, baby.
There were slim to no performance changes on the server as measured
with the mozilla-central repository. This is likely because CPU
time is dominated by reading revlogs, producing the changegroup, and
zlib compressing the output stream. Still, this brings us a little
closer to our ideal of using generators everywhere.
$ 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