view doc/README @ 39865:31b7e8e7132e

changegroup: port to emitrevisions() (issue5976)

We now have a unified API for emitting revision data from a storage
backend. It handles sorting nodes and the complicated delta versus
revision decisions for us.

This commit ports changegroup to that API.

There should be no behavior changes for changegroups not using
ellipsis. And lack of test changes seems to confirm that.

There are some changes for ellipsis mode, however.

Before, when sending an ellipsis revision, we would always send a
fulltext revision (as opposed to a delta). There was a TODO tracking
this open item.

One of the things the emitrevisions() API does for us is figure out
whether we can safely emit a delta. So, it is now possible for
ellipsis revisions to be sent as deltas! (It does this by not
assuming parent/ancestor revisions are available and tracking which
revisions have been sent out.)

Because we eliminated the list of revision delta request objects,
performance has improved substantially:

$ hg perfchangegroupchangelog
before: ! wall 24.348077 comb 24.330000 user 24.140000 sys 0.190000 (best of 3)
after:  ! wall 18.245911 comb 18.240000 user 18.100000 sys 0.140000 (best of 3)

That's a lot of overhead for creating a few hundred thousand Python
objects!

This is still a little slower than 4.7. Probably due to 23d582ca
introducing a type for the revision/delta results. There is
potentially room to optimize. But at some point we need to abstract
storage in order to support alternate storage backends. Unfortunately
that means using a Python data structure to represent results. And
unfortunately there is overhead with every new Python object created.

Differential Revision: https://phab.mercurial-scm.org/D4725

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