Re: worktrees vs. alternates

[Martin Fick <mfick@codeaurora.org>]

On Wednesday, May 16, 2018 03:11:47 PM Konstantin Ryabitsev 
wrote:
> On 05/16/18 15:03, Martin Fick wrote:
> >> I'm undecided about that. On the one hand this does
> >> create lots of small files and inevitably causes
> >> (some) performance degradation. On the other hand, I
> >> don't want to keep useless objects in the pack,
> >> because that would also cause performance degradation
> >> for people cloning the "mother repo." If my
> >> assumptions on any of that are incorrect, I'm happy to
> >> learn more.
> > 
> > My suggestion is to use science, not logic or hearsay.
> > :)
> > i.e. test it!
> 
> I think the answer will be "it depends." In many of our
> cases the repos that need those loose objects are rarely
> accessed -- usually because they are forks with older
> data (hence why they need objects that are no longer used
> by the mother repo). Therefore, performance impacts of
> occasionally touching a handful of loose objects will be
> fairly negligible. This is especially true on
> non-spinning media where seek times are low anyway.
> Having slimmer packs for the mother repo would be more
> beneficial in this case.
> 
> On the other hand, if the "child repo" is frequently used,
> then the impact of needing a bunch of loose objects would
> be greater. For the sake of simplicity, I think I'll
> leave things as they are -- it's cheaper to fix this via
> reducing seek times than by applying complicated logic
> trying to optimize on a per-repo basis.

I think a major performance issue with loose objects is not 
just the seek time, but also the fact that they are not 
delta compressed.  This means that sending them over the 
wire will likely have a significant cost before sending it. 
Unlike the seek time, this cost is not mitigated across 
concurrent fetches by the FS (or jgit if you were to use it) 
caching,

-Martin

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