view mercurial/cacheutil.py @ 35787:a84dbc87dae9

exchange: send bundle2 stream clones uncompressed

Stream clones don't compress well. And compression undermines
a point of stream clones which is to trade significant CPU
reductions by increasing size.

Building upon our introduction of metadata to communicate bundle
information back to callers of exchange.getbundlechunks(), we add
an attribute to the bundler that communicates whether the bundle is
best left uncompressed. We return this attribute as part of the bundle
metadata. And the wire protocol honors it when determining whether
to compress the wire protocol response.

The added test demonstrates that the raw result from the wire
protocol is not compressed. It also demonstrates that the server
will serve stream responses when the feature isn't enabled. We'll
address that in another commit.

The effect of this change is that server-side CPU usage for bundle2
stream clones is significantly reduced by removing zstd compression.
For the mozilla-unified repository:

before: 37.69 user 8.01 system
after:  27.38 user 7.34 system

Assuming things are CPU bound, that ~10s reduction would translate to
faster clones on the client. zstd can decompress at >1 GB/s. So the
overhead from decompression on the client is small in the grand scheme
of things. But if zlib compression were being used, the overhead would
be much greater.

Differential Revision: https://phab.mercurial-scm.org/D1926

# scmutil.py - Mercurial core utility functions
#
#  Copyright Matt Mackall <[e-mail]> and other
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
from __future__ import absolute_import

from . import repoview

def cachetocopy(srcrepo):
    """return the list of cache file valuable to copy during a clone"""
    # In local clones we're copying all nodes, not just served
    # ones. Therefore copy all branch caches over.
    cachefiles = ['branch2']
    cachefiles += ['branch2-%s' % f for f in repoview.filtertable]
    cachefiles += ['rbc-names-v1', 'rbc-revs-v1']
    cachefiles += ['tags2']
    cachefiles += ['tags2-%s' % f for f in repoview.filtertable]
    cachefiles += ['hgtagsfnodes1']
    return cachefiles