Re: [PATCH 5/8] read-cache: try index data from shared memory

[Erik Faye-Lund <kusmabite@gmail.com>]

On Tue, May 13, 2014 at 1:15 PM, Nguyễn Thái Ngọc Duy <pclouds@gmail.com> wrote:
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  Documentation/config.txt |  4 ++++
>  cache.h                  |  1 +
>  config.c                 | 12 ++++++++++++
>  environment.c            |  1 +
>  read-cache.c             | 43 +++++++++++++++++++++++++++++++++++++++++++
>  submodule.c              |  1 +
>  6 files changed, 62 insertions(+)
>
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index d8b6cc9..ccbe00b 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -617,6 +617,10 @@ relatively high IO latencies.  With this set to 'true', Git will do the
>  index comparison to the filesystem data in parallel, allowing
>  overlapping IO's.
>
> +core.useReadCacheDaemon::
> +       Use `git read-cache--daemon` to speed up index reading. See
> +       linkgit:git-read-cache--daemon for more information.
> +
>  core.createObject::
>         You can set this to 'link', in which case a hardlink followed by
>         a delete of the source are used to make sure that object creation
> diff --git a/cache.h b/cache.h
> index d0ff11c..fb29c7e 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -603,6 +603,7 @@ extern size_t packed_git_limit;
>  extern size_t delta_base_cache_limit;
>  extern unsigned long big_file_threshold;
>  extern unsigned long pack_size_limit_cfg;
> +extern int use_read_cache_daemon;
>
>  /*
>   * Do replace refs need to be checked this run?  This variable is
> diff --git a/config.c b/config.c
> index a30cb5c..5c832ad 100644
> --- a/config.c
> +++ b/config.c
> @@ -874,6 +874,18 @@ static int git_default_core_config(const char *var, const char *value)
>                 return 0;
>         }
>
> +#ifdef HAVE_SHM
> +       /*
> +        * Currently git-read-cache--daemon is only built when
> +        * HAVE_SHM is set. Ignore user settings if HAVE_SHM is not
> +        * defined.
> +        */
> +       if (!strcmp(var, "core.usereadcachedaemon")) {
> +               use_read_cache_daemon = git_config_bool(var, value);
> +               return 0;
> +       }
> +#endif
> +
>         /* Add other config variables here and to Documentation/config.txt. */
>         return 0;
>  }
> diff --git a/environment.c b/environment.c
> index 5c4815d..b76a414 100644
> --- a/environment.c
> +++ b/environment.c
> @@ -63,6 +63,7 @@ int merge_log_config = -1;
>  int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
>  struct startup_info *startup_info;
>  unsigned long pack_size_limit_cfg;
> +int use_read_cache_daemon;
>
>  /*
>   * The character that begins a commented line in user-editable file
> diff --git a/read-cache.c b/read-cache.c
> index a5031f3..0e46523 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -1462,6 +1462,48 @@ static struct cache_entry *create_from_disk(struct ondisk_cache_entry *ondisk,
>         return ce;
>  }
>
> +static void *try_shm(void *mmap, size_t *mmap_size)
> +{
> +       struct strbuf sb = STRBUF_INIT;
> +       size_t old_size = *mmap_size;
> +       void *new_mmap;
> +       struct stat st;
> +       int fd;
> +
> +       if (old_size <= 20 || !use_read_cache_daemon)
> +               return mmap;
> +
> +       strbuf_addf(&sb, "/git-index-%s.lock",
> +                   sha1_to_hex((unsigned char *)mmap + old_size - 20));
> +       fd = shm_open(sb.buf, O_RDONLY, 0777);

OK, so *here* you do an unguarded use of shm_open, but the code never
gets executed because of the HAVE_SHM guard in
git_default_core_config. Perhaps not introduce the compatibility shim
until this patch, then?