From: Junio C Hamano <gitster@pobox.com>
To: David Turner <dturner@twopensource.com>
Cc: git@vger.kernel.org, pclouds@gmail.com
Subject: Re: [PATCH 04/19] index-helper: new daemon for caching index and related stuff
Date: Wed, 09 Mar 2016 15:09:15 -0800 [thread overview]
Message-ID: <xmqqtwkffdus.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: <1457548582-28302-5-git-send-email-dturner@twopensource.com> (David Turner's message of "Wed, 9 Mar 2016 13:36:07 -0500")
David Turner <dturner@twopensource.com> writes:
> From: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
>
> Instead of reading the index from disk and worrying about disk
> corruption, the index is cached in memory (memory bit-flips happen
> too, but hopefully less often). The result is faster read. Read time
> is reduced by 70%.
>
> The biggest gain is not having to verify the trailing SHA-1, which
> takes lots of time especially on large index files. But this also
> opens doors for further optimiztions:
>
> - we could create an in-memory format that's essentially the memory
> dump of the index to eliminate most of parsing/allocation
> overhead. The mmap'd memory can be used straight away. Experiment
> [1] shows we could reduce read time by 88%.
>
> - we could cache non-index info such as name hash
>
> The shared memory's name folows the template "git-<object>-<SHA1>"
> where <SHA1> is the trailing SHA-1 of the index file. <object> is
> "index" for cached index files (and may be "name-hash" for name-hash
> cache). If such shared memory exists, it contains the same index
> content as on disk. The content is already validated by the daemon and
> git won't validate it again (except comparing the trailing SHA-1s).
This indeed is an interesting approach; what is not explained but
must be is when the on-disk index is updated to reflect the reality
(if I am reading the explanation and the code right, while the
daemon is running, its in-core cache becomes the source of truth by
forcing everybody's read-index-from() to go to the daemon). The
explanation could be "this is only for read side, and updating the
index happens via the traditional 'write a new file and rename it to
the final place' codepath, at which time the daemon must be told to
re-read it."
next prev parent reply other threads:[~2016-03-09 23:09 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-09 18:36 [PATCH 00/19] index-helper, watchman David Turner
2016-03-09 18:36 ` [PATCH 01/19] trace.c: add GIT_TRACE_PACK_STATS for pack usage statistics David Turner
2016-03-09 22:58 ` Junio C Hamano
2016-03-10 0:05 ` David Turner
2016-03-10 10:59 ` Duy Nguyen
2016-03-09 18:36 ` [PATCH 02/19] read-cache.c: fix constness of verify_hdr() David Turner
2016-03-09 18:36 ` [PATCH 03/19] read-cache: allow to keep mmap'd memory after reading David Turner
2016-03-09 23:02 ` Junio C Hamano
2016-03-10 0:09 ` David Turner
2016-03-09 18:36 ` [PATCH 04/19] index-helper: new daemon for caching index and related stuff David Turner
2016-03-09 23:09 ` Junio C Hamano [this message]
2016-03-09 23:21 ` Junio C Hamano
2016-03-10 0:01 ` David Turner
2016-03-10 11:17 ` Duy Nguyen
2016-03-10 20:22 ` David Turner
2016-03-11 1:11 ` Duy Nguyen
2016-03-10 0:18 ` David Turner
2016-03-15 11:56 ` Duy Nguyen
2016-03-15 15:56 ` Junio C Hamano
2016-03-15 11:52 ` Duy Nguyen
2016-03-09 18:36 ` [PATCH 05/19] trace.c: add GIT_TRACE_INDEX_STATS for index statistics David Turner
2016-03-09 18:36 ` [PATCH 06/19] index-helper: add --strict David Turner
2016-03-09 18:36 ` [PATCH 07/19] daemonize(): set a flag before exiting the main process David Turner
2016-03-09 18:36 ` [PATCH 08/19] index-helper: add --detach David Turner
2016-03-09 18:36 ` [PATCH 09/19] index-helper: add Windows support David Turner
2016-03-16 11:42 ` Duy Nguyen
2016-03-17 12:18 ` Johannes Schindelin
2016-03-17 12:59 ` Duy Nguyen
2016-03-09 18:36 ` [PATCH 10/19] read-cache: add watchman 'WAMA' extension David Turner
2016-03-09 18:36 ` [PATCH 11/19] Add watchman support to reduce index refresh cost David Turner
2016-03-09 18:36 ` [PATCH 12/19] read-cache: allow index-helper to prepare shm before git reads it David Turner
2016-03-09 18:36 ` [PATCH 13/19] index-helper: use watchman to avoid refreshing index with lstat() David Turner
2016-03-09 18:36 ` [PATCH 14/19] update-index: enable/disable watchman support David Turner
2016-03-09 18:36 ` [PATCH 15/19] unpack-trees: preserve index extensions David Turner
2016-03-09 18:36 ` [PATCH 16/19] index-helper: rewrite pidfile after daemonizing David Turner
2016-03-09 18:36 ` [PATCH 17/19] index-helper: process management David Turner
2016-03-09 18:36 ` [PATCH 18/19] index-helper: autorun David Turner
2016-03-15 12:12 ` Duy Nguyen
2016-03-15 14:26 ` Johannes Schindelin
2016-03-16 11:37 ` Duy Nguyen
2016-03-16 18:11 ` David Turner
2016-03-16 18:27 ` Johannes Schindelin
2016-03-17 13:02 ` Duy Nguyen
2016-03-17 14:43 ` Johannes Schindelin
2016-03-17 18:31 ` David Turner
2016-03-18 0:50 ` Duy Nguyen
2016-03-18 7:14 ` Johannes Schindelin
2016-03-18 7:44 ` Duy Nguyen
2016-03-18 17:22 ` David Turner
2016-03-18 23:09 ` Duy Nguyen
2016-03-18 7:17 ` Johannes Schindelin
2016-03-18 7:34 ` Duy Nguyen
2016-03-18 15:57 ` Johannes Schindelin
2016-03-09 18:36 ` [PATCH 19/19] hack: watchman/untracked cache mashup David Turner
2016-03-15 12:31 ` Duy Nguyen
2016-03-17 0:56 ` David Turner
2016-03-17 13:06 ` Duy Nguyen
2016-03-17 18:08 ` David Turner
2016-03-29 17:09 ` [PATCH 00/19] index-helper, watchman Torsten Bögershausen
2016-03-29 21:51 ` David Turner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=xmqqtwkffdus.fsf@gitster.mtv.corp.google.com \
--to=gitster@pobox.com \
--cc=dturner@twopensource.com \
--cc=git@vger.kernel.org \
--cc=pclouds@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).