On Wed, Mar 02, 2016 at 12:31:16AM -0800, Junio C Hamano wrote:
> Josh Triplett <josh@xxxxxxxxxxxxxxxx> writes:
> > I think several simpler optimizations seem
> > preferable, such as binary object names, and abbreviating complete
> > object sets ("I have these commits/trees and everything they need
> > recursively; I also have this stack of random objects.").
>
> Given the way pack stream is organized (i.e. commits first and then
> trees and blobs that belong to the same delta chain together), and
> our assumed goal being to salvage objects from an interrupted
> transfer of a packfile, you are unlikely to ever see "I have these
> commits/trees and everything they need" that are salvaged from such
> a failed transfer. So I doubt such an optimization is worth doing.
True for the resumable clone case. For that optimization, I was
thinking of the "pull during the merge window" case that Al Viro was
also interested in optimizing.
> Besides it is very expensive to compute (the computation is done on
> the client side, so the cycles burned and the time the user has to
> wait is of much less concern, though); you'd essentially be doing
> "git fsck" to find the "dangling" objects.
Trading client-side computation for bandwidth can potentially be
worthwhile if you have plenty of local compute but a slow and metered
link.
> The list of what would be transferred needs to come in full from the
> server end, as the list names objects that the receiving end may not
> have seen, but the response by the client could be encoded much
> tightly. For the full list of N objects from the server, we can
> think of your response to be a bitstream of N bits, each on-bit in
> which signals an unwanted object in the list. You can optimize this
> transfer by RLE compressing the bitstream, for example.
>
> As git-over-HTTP is stateless, however, you cannot assume that the
> server side remembers what it sent to the client (instead, the
> client side needs to re-post what it heard from the server in the
> previous exchange to allow the server side to use it after
> validating). So "objects at these indices in your list" kind of
> optimization may not work very well in that environment. I'd
> imagine that an exchange of "Here are the list of objects", "Give me
> these objects" done naively in full 40-hex object names would work
> OK there, though.
Good point. Between statelessness and Duy's point about the client list
usually being smaller than the server list, perhaps it would make sense
to not have the server send a list at all, and just have the client send
its own list.
- Josh Triplett
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