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Re: [GSoC] How to protect cached_objects

On Thu, May 23, 2019 at 11:51 PM Matheus Tavares Bernardino
<matheus.bernardino@xxxxxx> wrote:

> Hi, everyone
> As one of my first tasks in GSoC, I'm looking to protect the global
> states at sha1-file.c for future parallelizations. Currently, I'm
> analyzing how to deal with the cached_objects array, which is a small
> set of in-memory objects that read_object_file() is able to return
> although they don't really exist on disk. The only current user of
> this set is git-blame, which adds a fake commit containing
> non-committed changes.
> As it is now, if we start parallelizing blame, cached_objects won't be
> a problem since it is written to only once, at the beginning, and read
> from a couple times latter, with no possible race conditions.
> But should we make these operations thread safe for future uses that
> could involve potential parallel writes and reads too?
> If so, we have two options:
> - Make the array thread local, which would oblige us to replicate data, or
> - Protect it with locks, which could impact the sequential
> performance. We could have a macro here, to skip looking on
> single-threaded use cases. But we don't know, a priori, the number of
> threads that would want to use the pack access code.
> Any thought on this?

I would go with "that's the problem of the future me". I'll go with a
simple global (I mean per-object store) mutex. After we have a
complete picture how many locks we need, and can run some tests to see
the amount of lock contention we have (or even cache missess if we
have so many locks), then we can start thinking of an optimal

I mean, this is an implementation detail and can't affect object
access API right? That gives us some breathing room to change stuff
without preparing for something that we don't need right now (like
multiple cached_objects writers)

> Thanks,
> Matheus Tavares