Re: [GSoC] How to protect cached_objects

[Matheus Tavares Bernardino <matheus.bernardino@usp.br>]

On Fri, May 24, 2019 at 3:13 AM Jeff King <peff@peff.net> wrote:
>
> On Thu, May 23, 2019 at 01:51:47PM -0300, Matheus Tavares Bernardino wrote:
>
> > 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.
>
> It seems like a lot of the sha1-reading code is 99% read-only, but very
> occasionally will require a write (e.g., refreshing the packed_git list
> when we fail a lookup, or manipulating the set of cached mmap windows).
>
> I think pthreads has read/write locks, where many readers can hold the
> lock simultaneously but a writer blocks readers (and other writers).
> Then in the common case we'd only pay the price to take the lock, and
> not deal with contention. I don't know how expensive it is to take such
> a read lock; it's presumably just a few instructions but implies a
> memory barrier. Maybe it's worth timing?

The pthread_rwlock_t, right? Nice! I didn't know about this type of
lock. It will be very handy in other situations as well, thanks for
the suggestion.

> -Peff