Re: [PATCH] mktree: learn about promised objects

[Derrick Stolee <derrickstolee@github.com>]

On 6/14/2022 12:33 PM, Richard Oliver wrote:
> On 14/06/2022 15:14, Derrick Stolee wrote:
>> On 6/14/2022 9:36 AM, Richard Oliver wrote:
>>> Do not use oid_object_info() to determine object type in mktree_line()
>>> as this can cause promised objects to be dynamically faulted-in one at a
>>> time which has poor performance. Instead, use a combination of
>>> oid_object_info_extended() (with OBJECT_INFO_SKIP_FETCH_OBJECT option),
>>> and the newly introduced promisor_object_type() to determine object type
>>> before defaulting to fetch from remote.
>>
>> Have you run some performance tests on this? It seems like this will
>> scan all packed objects, which is probably much slower than just asking
>> the remote for the object in most cases.
>>
>> Thanks,
>> -Stolee
> 
> 
> Hi Stolee,
> 
> I've put together a synthetic experiment below (adding a new blob to anexisting tree) to show you the behaviour that we've been seeing.  Our
> actual use-case (where we first encountered this behaviour) is updating
> submodules to a known hash. As you can see, the round-trip time of fetching
> objects one-by-one is very expensive.
> 
> Before, using system git (git version 2.32.0 (Apple Git-132)):
> 
>> $ git init
>> # Fetch a recent tree
>> $ git fetch --filter=tree:0 --depth 1 https://github.com/git/git cdb48006b0ec7fe19794daf7b5363ab42d9d9371
>> remote: Enumerating objects: 1, done.
>> remote: Counting objects: 100% (1/1), done.
>> remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
>> Receiving objects: 100% (1/1), 13.12 KiB | 13.12 MiB/s, done.
>> From https://github.com/git/git
>>  * branch            cdb48006b0ec7fe19794daf7b5363ab42d9d9371 -> FETCH_HEAD
>>
>> $ NEW_BLOB=$(echo zzz | git hash-object --stdin -w)
>>
>> $ cat <(git ls-tree FETCH_HEAD) <(printf "100644 blob ${NEW_BLOB}\tzzz") | time git mktree
>> remote: Enumerating objects: 1, done.
>> remote: Counting objects: 100% (1/1), done.
>> remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
>> Receiving objects: 100% (1/1), 334 bytes | 334.00 KiB/s, done.
>> remote: Enumerating objects: 1, done.
>> remote: Counting objects: 100% (1/1), done.
>> remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
>> Receiving objects: 100% (1/1), 2.01 KiB | 2.01 MiB/s, done.
>> remote: Enumerating objects: 1, done.
>> remote: Counting objects: 100% (1/1), done.
>> remote: Total 1 (delta 0), reused 0 (delta 0), pack-reused 0
>> Receiving objects: 100% (1/1), 256 bytes | 256.00 KiB/s, done.
>> # ...
>> # SOME TIME LATER
>> # ...
>> e26c7ce7357b1649da7b4200d4e80d0b668db8d4
>>       286.49 real        15.66 user        15.59 sys

I see. The problem here is that we are making _many requests_ for the same
tree, so maybe it would be better to introduce a batched download for the
list of missing objects. This would require iterating over the objects for
the tree to check existence (in quick mode) and adding the missing ones in
a list, then requesting that set altogether in a single request.

That probably won't be as fast as your modified mktree experiment below,
but would match my expectations of "probably faster assuming the repo is
big enough".

> Repeated experiment, but using modified mktree:
> 
>> $ cat <(git ls-tree FETCH_HEAD) <(printf "100644 blob ${NEW_BLOB}\tzzz") | time git mktree
>> e26c7ce7357b1649da7b4200d4e80d0b668db8d4
>>         0.06 real         0.01 user         0.03 sys
> 
> Did you have any other sort of performance test in mind? The remotes we
> typically deal with are geographically far away and deal with a high volume
> of traffic so we're keen to move behaviour to the client where it makes sense
> to do so.

I guess I wonder how large your promisor pack-files are in this test,
since your implementation depends on for_each_packed_object(), which
should be really inefficient if you're actually dealing with a large
partial clone.

Thanks,
-Stolee