Subject: Re: [RFC] Introducing different handling for small/large transactions

From: Jeff King <[email-masked]>
Date: Thu, 15 Jan 2015 17:46:05 -0500

On Thu, Jan 15, 2015 at 02:36:11PM -0800, Stefan Beller wrote:

> So for here is my proposal for small transactions:
> (just one ref [and/or reflog] touched):

The implication being that a "large" transaction is any with more than
one update.

I think performance may suffer if you do not also take into account the
size of the packed-refs file. If you are updating 5 refs and there are
10 in the packed-refs file, rewriting the extra 5 is probably not a big
deal. If there are 400,000 in the packed-refs file, it probably is. I'm
not sure where the cutoff is (certainly the per-ref cost is less for
packed-refs once you have started writing the file, so there is probably
some crossover percentage that you could measure).

> 	* detect if we transition to a large transaction
> 	  (by having more than one entry in transaction->updates)
> 	  if so:
> 		* Pack all currently existing refs into the packed
> 		  refs file, commit the packed refs file and delete
> 		  all loose refs. This will avoid (d/f) conflicts.
> 
> 		* Keep the packed-refs file locked and move the first
> 		  transaction update into the packed-refs.lock file

This increases lock contention, as now independent ref updates all need
to take the same packed-refs.lock. This can be a problem on a busy
repository, especially because we never retry the packed-refs lock.
We already see this problem somewhat on GitHub. Ref deletions need the
packed-refs.lock file, which can conflict with another deletion, or with
running `git pack-refs`.

-Peff