[PATCH STABLE] revlog: add an experimental option to mitigated delta issues (issue5480)

[Gregory Szorc]

On Mon, Jul 3, 2017 at 12:03 PM, Augie Fackler <raf at durin42.com> wrote:

> On Sat, Jul 01, 2017 at 03:40:18PM -0700, Sean Farley wrote:
> >
> > Gregory Szorc <gregory.szorc at gmail.com> writes:
> >
> > > On Fri, Jun 30, 2017 at 4:43 PM, Sean Farley <sean at farley.io> wrote:
> > >
> > >>
> > >> Pierre-Yves David <pierre-yves.david at ens-lyon.org> writes:
> > >>
> > >> > On 06/27/2017 01:16 PM, Pierre-Yves David wrote:
> > >> >> # HG changeset patch
> > >> >> # User Pierre-Yves David <pierre-yves.david at octobus.net>
> > >> >> # Date 1498218574 -7200
> > >> >> #      Fri Jun 23 13:49:34 2017 +0200
> > >> >> # Branch stable
> > >> >> # Node ID 33998dea4a10b09502bf458e458daca273a3f29a
> > >> >> # Parent  231690dba9b4d31b5ad2c93284e454135f2763ca
> > >> >> # EXP-Topic manifest
> > >> >> # Available At https://www.mercurial-scm.org/
> > >> repo/users/marmoute/mercurial/
> > >> >> #              hg pull https://www.mercurial-scm.org/
> > >> repo/users/marmoute/mercurial/ -r 33998dea4a10
> > >> >> revlog: add an experimental option to mitigated delta issues
> (issue5480)
> > >> >
> > >> > Any news on this? 4.2.2 is tomorrow and I think it is really
> important
> > >> > to have it available for all people. This issue is really serious.
> > >> >
> > >> > (small extra number: the config shrink the pypy repo manifest by an
> > >> > extra half compared to just using aggressivemergedeltas (50MB →
> 25MB))
> > >>
> > >> I agree that this makes some repos basically unusable (which we've
> seen
> > >> here sometimes). Since it's behind a flag, I'll go ahead and queue
> this
> > >> for stable in a bit. If someone else in crew disagrees, we can dequeue
> > >> it.
> > >>
> > >> Queued (for the patchwork bot)
> > >>
> > >
> > > I don't believe this patch is appropriate for stable because it is a
> new
> > > feature. Introducing excessively long delta chain segments will result
> in
> > > high memory usage and clients doing uncompressed clones will inherit
> that
> > > excessive memory usage. I'd rather we land this on default along with a
> > > refactor to revlog reading so we don't incur excessive read I/O for
> long
> > > spans. I'd also prefer to stick the option directly in [format] and
> mark it
> > > as experimental via a comment. That's what we do for all the other
> revlog
> > > options in _applyopenerreqs().
> >
> > That's fair. I was on the fence about this anyways. Though, I did like
> > that _something_ would make these repos usable. I just noticed that the
> > commit has been marked public; don't know how strongly you feel about
> > this so up to you on how to proceed.
>
> I agree this is unsuitable for stable. How do we want to proceed?
>
> (I'll hold the 4.2.2 release until we get consensus here.)
>

I think we should back out from stable and re-land on default. The feature
is appropriate for default, even with the noted pitfalls.
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