Re: [PATCH 1/2] Add support for multi threaded checkout

["James Pickens" <jepicken@gmail.com>]

On Thu, Dec 18, 2008, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> So instead of doing all the unpacking etc in parallel (with locking around
> it to serialize it), I'd suggest doing ll the unpacking serially since
> that isn't the problem anyway (and since you have to protect it with a
> lock anyway), and just have a "write out and free the buffer" phase that
> is done in the threads.

That's certainly a more elegant way to do it, but unless I'm missing
something, it requires rewriting a good bit of code.  The main reason I
went with the locking was to keep the patch as simple and non-intrusive
as possible.

> The alternative would be to actually do what your patch suggests, but
> actually try to make the code git SHA1 object handling be thread-safe. At
> that point, the ugly locking in write_entry() would go away, and you might
> actually improve performance on SMP thanks to doing the CPU part in
> parallel.

I started down that path at one point, and quickly got in over my head.
Making all that code thread safe looks like a big task to me.  From my
perspective, I get a ~350% speedup from this easy patch, and I might get
an additional 25% (blind guess) from a much more difficult patch.  It
didn't seem worth the effort.

James