view CONTRIBUTORS @ 25402:0c2ded041d10

exchange: support transferring .hgtags fnodes mapping

On Mozilla's mozilla-beta repository .hgtags fnodes resolution takes
~18s from a clean cache on my machine. This means that the first time
a user runs `hg tags`, `hg log`, or any other command that displays or
accesses tags data, a ~18s pause will occur. There is no output during
this pause. This results in a poor user experience and perception
that Mercurial is slow.

The .hgtags changeset to filenode mapping is deterministic. This
patch takes advantage of that property by implementing support
for transferring .hgtags filenodes mappings in a dedicated bundle2
part. When a client advertising support for the "hgtagsfnodes"
capability requests a bundle, a mapping of changesets to .hgtags
filenodes will be sent to the client.

Only mappings of head changesets included in the bundle will be sent. The
transfer of this mapping effectively eliminates one time tags cache related
pauses after initial clone.

The mappings are sent as binary data. So, 40 bytes per pair of
SHA-1s. On the aforementioned mozilla-beta repository,
659 * 40 = 26,360 raw bytes of mappings are sent over the wire
(in addition to the bundle part headers). Assuming 18s to populate
the cache, we only need to transfer this extra data faster than
1.5 KB/s for overall clone + tags cache population time to be shorter.
Put into perspective, the mozilla-beta repository is ~1 GB in size.
So, this additional data constitutes <0.01% of the cloned data.
The marginal overhead for a multi-second performance win on clones
in my opinion justifies an on-by-default behavior.

[This file is here for historical purposes, all recent contributors
should appear in the changelog directly]

Andrea Arcangeli <andrea at suse.de>
Thomas Arendsen Hein <thomas at intevation.de>
Goffredo Baroncelli <kreijack at libero.it>
Muli Ben-Yehuda <mulix at mulix.org>
Mikael Berthe <mikael at lilotux.net>
Benoit Boissinot <bboissin at gmail.com>
Brendan Cully <brendan at kublai.com>
Vincent Danjean <vdanjean.ml at free.fr>
Jake Edge <jake at edge2.net>
Michael Fetterman <michael.fetterman at intel.com>
Edouard Gomez <ed.gomez at free.fr>
Eric Hopper <hopper at omnifarious.org>
Alecs King <alecsk at gmail.com>
Volker Kleinfeld <Volker.Kleinfeld at gmx.de>
Vadim Lebedev <vadim at mbdsys.com>
Christopher Li <hg at chrisli.org>
Chris Mason <mason at suse.com>
Colin McMillen <mcmillen at cs.cmu.edu>
Wojciech Milkowski <wmilkowski at interia.pl>
Chad Netzer <chad.netzer at gmail.com>
Bryan O'Sullivan <bos at serpentine.com>
Vicent Seguí Pascual <vseguip at gmail.com>
Sean Perry <shaleh at speakeasy.net>
Nguyen Anh Quynh <aquynh at gmail.com>
Ollivier Robert <roberto at keltia.freenix.fr>
Alexander Schremmer <alex at alexanderweb.de>
Arun Sharma <arun at sharma-home.net>
Josef "Jeff" Sipek <jeffpc at optonline.net>
Kevin Smith <yarcs at qualitycode.com>
TK Soh <teekaysoh at yahoo.com>
Radoslaw Szkodzinski <astralstorm at gorzow.mm.pl>
Samuel Tardieu <sam at rfc1149.net>
K Thananchayan <thananck at yahoo.com>
Andrew Thompson <andrewkt at aktzero.com>
Michael S. Tsirkin <mst at mellanox.co.il>
Rafael Villar Burke <pachi at mmn-arquitectos.com>
Tristan Wibberley <tristan at wibberley.org>
Mark Williamson <mark.williamson at cl.cam.ac.uk>