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

[Sean Farley]

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.
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