Re: [PATCH v13 11/20] index-helper: use watchman to avoid refreshing index with lstat()

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On Thu, Jun 30, 2016 at 7:55 PM, Ben Peart <peartben@xxxxxxxxx> wrote:
> David Turner <novalis <at> novalis.org> writes:
>
>>
>> Hiding watchman behind index-helper means you need both daemons. You
>> can't run watchman alone. Not so good. But on the other hand, 'git'
>> binary is not linked to watchman/json libraries, which is good for
>> packaging. Core git package will run fine without watchman-related
>> packages. If they need watchman, they can install git-index-helper and
>> dependencies.
>>
>
> Have you considered splitting index-helper and watchman apart?  Using
> Watchman to not lstat unchanged entries is a huge perf win with very
> large repos.

On large repos (i.e. lots of files/dirs on worktree), the cost of
reading index will increase proportionally. Yes lstat costs, but I
suspect index reading (integrity verification actually) may cost more,
especially on platforms with cheap lstat like linux. On these repos
you really want to enable all four: index-helper (with watchman),
split-index (I still need to work out pruning on split-index) and
untracked cache. There's still a lot more to make git run fast on
large repos though.

> It would also be interesting to make the Watchman backend replaceable by
> using an extensible API.  This has the benefit of not having to link the
> 'git' binary to the watchman/json libraries.

'git' binary is not linked to watchman libraries. git-index-helper is
a separate binary, by design. In theory you can create a
'git-index-helper' replacement binary with something other than
watchman. I think David documented the protocol well (it may change in
the future though and we are not prepared for capability progression)

> Is there any pattern already in git for accomplishing this?
-- 
Duy
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