Re: inotify to minimize stat() calls

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> I checked read-cache.c and preload-index.c code.  To get the
> discussion rolling, I think something like the outline below may be
> a good starting point and a feasible weekend hack for somebody
> competent:
>
>  * At the beginning of preload_index(), instead of spawning the
>    worker thread and doing the lstat() check ourselves, we open a
>    socket to our daemon (see below) that watches this repository and
>    make a request for lstat update.  The request will contain:
>
>     - The SHA1 checksum of the index file we just read (to ensure
>       that we and our daemon share the same baseline to
>       communicate); and
>
>     - the pathspec data.
>
>    Our daemon, if it already has a fresh data available, will give
>    us a list of <path, lstat result>.  Our main process runs a loop
>    that is equivalent to what preload_thread() runs but uses the
>    lstat() data we obtained from the daemon.  If our daemon says it
>    does not have a fresh data (or somehow our daemon is dead), we do
>    the work ourselves.
>
>  * Our daemon watches the index file and the working tree, and
>    waits for the above consumer.  First it reads the index (and
>    remembers what it read), and whenever an inotify event comes,
>    does the lstat() and remembers the result.  It never writes
>    to the index, and does not hold the index lock.  Whenever the
>    index file changes, it needs to reload the index, and discard
>    lstat() data it already has for paths that are lost from the
>    updated index.

I left the details unsaid in thee above because I thought it was
fairly obvious from the nature of the "outline", but let me spend a
few more lines to avoid confusion.

 - The way the daemon "watches" the changes to the working tree and
   the index may well be very platform dependent.  I said "inotify"
   above, but the mechanism does not have to be inotify.

 - The channel the daemon and the client communicates would also be
   system dependent.  UNIX domain socket in $GIT_DIR/ with a
   well-known name would be one possibility but it does not have to
   be the only option.

 - The data given from the daemon to the client does not have to
   include full lstat() information.  They start from the same index
   info, and the only thing preload_index() wants to know is for
   which paths it should call ce_mark_uptodate(ce), so the answer
   given by our daemon can be a list of paths.