Re: [RFC PATCH 00/18] Multi-pack index (MIDX)

On Sunday, January 07, 2018 01:14:41 PM Derrick Stolee wrote:

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My interest in this feature would be to speed up fetches when there is more than one large pack-file with many of the same objects that are in other pack-files. What does your MIDX design do when it encounters multiple copies of the same object in different pack files? Does it index them all, or does it keep a single copy?

In our Gerrit instance (Gerrit uses jgit), we have multiple copies of the linux kernel repos linked together via the alternatives file mechanism. These repos have many different references (mostly Gerrit change references), but they share most of the common objects from the mainline. I have found that during a large fetch such as a clone, jgit spends a significant amount of extra time by having the extra large pack-files from the other repos visible to it, usually around an extra minute per instance of these (without them, the clone takes around 7mins). This adds up easily with a few repos extra repos, it can almost double the time.

My investigations have shown that this is due to jgit searching each of these pack files to decide which version of each object to send. I don't fully understand its selection criteria, however if I shortcut it to just pick the first copy of an object that it finds, I regain my lost time. I don't know if git suffers from a similar problem? If git doesn't suffer from this then it likely just uses the first copy of an object it finds (which may not be the best object to send?)

It would be nice if this use case could be improved with MIDX. To do so, it seems that it would either require that MIDX either only put "the best" version of an object (i.e. pre-select which one to use), or include the extra information to help make the selection process of which copy to use (perhaps based on the operation being performed) fast.

This also leads me to ask, what other additional information (bitmaps?) for other operations, besides object location, might suddenly be valuable in an index that potentially points to multiple copies of objects? Would such information be appropriate in MIDX, or would it be better in another index?

Thanks,

-Martin