[PATCH 1 of 5 V2] treemanifest: create treemanifest class

[Matt Mackall]

On Mon, 2015-03-16 at 16:27 -0700, Martin von Zweigbergk wrote:
> # HG changeset patch
> # User Martin von Zweigbergk <martinvonz at google.com>
> # Date 1426435855 25200
> #      Sun Mar 15 09:10:55 2015 -0700
> # Node ID 0d969d6efeef08935c93afc416bf406def6b8e59
> # Parent  567ae53657544744155897ada91f16f8af61ad8a
> treemanifest: create treemanifest class
> 
> There are a number of problems with large and flat manifests. Copying
> from http://mercurial.selenic.com/wiki/ManifestShardingPlan:
> 
>  * manifest too large for RAM
> 
>  * manifest resolution too much CPU (long delta chains)
> 
>  * committing is slow because entire manifest has to be hashed
> 
>  * impossible for narrow clone to leave out part of manifest as all is
>    needed to calculate new hash
> 
>  * diffing two revisions involves traversing entire subdirectories
>    even if identical
> 
> This is a first step in a series introducing a manifest revlog per
> directory.
> 
> This change adds boolean configuration option
> experimental.treemanifest. When the option is enabled, manifests are
> parsed into a new tree data structure with one tree node per
> directory. At this point, it is just a different data structure in
> memory; there is still just a single manifest revlog on disk.

I think the right way to do this is:

- at repo create time, check the flag
   - add treemanifest to requires file
- at startup, check requires file
   - unconditionally use treemanifest if set

..and that wants to be staged as a couple patches later in the series,
not here.

-- 
Mathematics is the supreme nostalgia of our time.