Re: [PATCH v1 1/4] fastindex: speed up index load through parallelization

From: Ben Peart <peartben@gmail.com>
Date: Nov 14, 2017, 14:31 UTC
Message-ID: <7428e41e-b705-f377-1951-b11af851c4d5@gmail.com>
In-Reply-To: <xmqq7eut8y36.fsf@gitster.mtv.corp.google.com>

On 11/13/2017 8:10 PM, Junio C Hamano wrote:

Show 35 quoted lines

> Ben Peart <peartben@gmail.com> writes:
> 
>> The proposed format for extensions (ie having both a header and a
>> footer with name and size) in the current patch already enables having
>> multiple extensions that can be parsed forward or backward.  Any
>> extensions that would want to be parse-able in reverse would just all
>> need to be written one after the other after right before the trailing
>> SHA1 (and of course, include the proper footer).
> 
> In other words, anything that wants to be scannable from the tail is
> welcome to reimplement the same validation structure used by IEOT to
> check the section specific sanity constraint, and this series is not
> interested in introducing a more general infrastructure to make it
> easy for code that want to find where each extension section in the
> file begins without pasing the body of the index.
> 
> I find it somewhat unsatisfactory in that it is a fundamental change
> to allow jumping to the start of an extension section from the tail
> that can benefit any future codepath, and have expected a feature
> neutral extension whose sole purpose is to do so [*1*].
> 
> That way, extension sections whose names are all-caps can stay to be
> optional, even if they allow locating from the tail of the file.  If
> you require them to implement the same validation struture as IEOT
> to perform section specific sanity constraint and also require them
> to be placed consecutively at the end, the reader MUST know about
> all such extensions---otherwise they cannot scan backwards and find
> ones that appear before an unknown but optional one.  If you keep an
> extension section at the end whose sole purpose is to point at the
> beginning of extension sections, the reader can then scan forward as
> usual, skipping over unknown but optional ones, and reading your
> IEOT can merely be an user (and the first user) of that more generic
> feature that is futureproof, no?
> 
>

How about I add the logic to write out a special extension right before the SHA1 that contains an offset to the beginning of the extensions section. I will also add the logic in do_read_index() to search for and load this special extension if it exists.

This will provide a common framework for any future extension to take advantage of if it wants to be loaded/processed before or in parallel with the cache entries or other extensions.

For all existing extensions that assume they are loaded _after_ the cache entries, in do_read_index() I'll add the logic to use the offset (if it exists) to adjust the src_offset and then load them normally.

Given the IEOT extension is just another list of offsets into the index to enable out of order processing, I'll add those offsets into the same extension so that it is a more generic "table of contents" for the entire index. This enables us to have common/reusable way to have random access to _all_ sections in the index while maintaining backwards comparability with the existing index formats and code.

These additional offsets will initially only be used to parallelize the loading of cache entries and only if the user explicitly enables that option but I can think of other interesting uses for them in the future.