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

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 15, 2017, 01:12 UTC
Message-ID: <xmqqwp2s1h1x.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <73fd93cd-91f4-1286-732c-cd8185fe2027@gmail.com>

Ben Peart <peartben@gmail.com> writes:

> I have no thoughts or plans for changes in the future of IEOT (which I
> plan to rename ITOC).  At this point in time, I can't even imagine
> what else we'd want as the index only contains cache entries, ...

Yeah, but the thing is that this is open source, and the imagination of the originator of the initial idea does not limit how the code and data structure evolves.

Back when I added the index extensions to the system, I didn't have perfect foresight, and I didn't have specific plans to add anything beyond "TREE" to optimize "write-tree" and "diff-index --cached".

In hindsight, I got one thing right and one thing wrong.

Even though I didn't have any plan to add a mandatory extension, I made the code to ignore optional ones and error out on mandatory ones if an index extension section is not understood. It turns out that later (in fact much later---the "TREE" extension dates back to April 2006, while "link" came in June 2014) we could add the split-index mode without having to worry about older versions of Git doing random wrong things when they see this new extension, thanks to that design decision. That went well.

On the other hand, I didn't think things through to realize that some operations may want to peek only the extensions without ever using the main table, that some other operations may want to read some extensions first before reading the main table, or more importantly, that these limitations would start mattering once Git becomes popular enough and starts housing a project tree with very many paths in the main table.

I really wish somebody had brought it up as a potential issue during the review---I would have defined the extended index format to have the simple extension at the end that points back to the tail end of the main table back then, and we wouldn't be having this back and forth now. But I was just too happy and excited that I have found a room to squeeze extension sections into the index file format without breaking existing implementations of Git (which checked the trailer checksum matches to the result of hashing the whole thing, and read the recorded number of entries from the main table, without even noticing that there is a gap in between), and that excitement blinded me.

Show 5 quoted lines

> I understand the risk but the list of offsets into the cache entries
> is pretty simple as well. I prefer the simplicity of a single TOC
> extension that gives us random access to the entire index rather than
> having to piece one together using multiple extensions.  That model
> has its own set of risks and tradeoffs.

I thought that you are not using the "where does the series of extensions begin" information in the first place, no? That piece of information is useful independent of the usefulness of "index into the main table to list entries where the prefix-compression is reset". So if anything, I'd prefer the simplicity of introducing that "where does the series of extensions begin" that does not say anything else, and build on other things like ITOC as mere users of the mechanism.