[PATCH 4 of 4] revlog: add an aggressivemergedelta option

[Durham Goode]

On 8/31/15 3:48 PM, Matt Mackall wrote:
> On Sun, 2015-08-30 at 15:09 -0700, Durham Goode wrote:
>> # HG changeset patch
>> # User Durham Goode <durham at fb.com>
>> # Date 1440968612 25200
>> #      Sun Aug 30 14:03:32 2015 -0700
>> # Node ID ffaccbf9565ec5f73ded2ae3f9f611c943baad17
>> # Parent  03338124de4c33848974b5a512d4e6ef71b4424b
>> revlog: add an aggressivemergedelta option
>>
>> This adds an option for delta'ing against both p1 and p2 when applying merge
>> revisions and picking whichever is smallest.
> I've queued all four of these, thanks.
>
>> Some before and after stats on manifest.d size:
>>
>> internal large repo:
>> before: 1.2 GB
>> after: 930 MB
>>
>> mozilla-central:
>> before: 261 MB
>> after: 92 MB
> This is big enough that I'd probably consider just enabling it all the
> time. Two considerations here:
>
> - bandwidth is typically scarcer than CPU
> - it's a one-time cost at commit, rather than an on-going cost
>
It's not actually a one-time cost though.  A user pulling the merge 
commit will build a delta against p1 (hopefully the downloaded delta 
provides this, so it's free), but then he will also compute the delta 
for p2, which involves decompressing the manifest, etc.  So pulling N 
merge commits will incur at least N extra manifest decompressions.

We could probably make it smarter by saying 'if the server gave me a p1 
delta, don't even bother with p2 unless we're going to result in a 
fulltext'.