[Nix-dev] Locking breaks distributed builds with a common /nix/store

[Tony White]

On 1 April 2010 11:31, David Soergel <dev at davidsoergel.com> wrote:
> Oops, sorry for the double post.
>
> Anyhow I solved my problem by releasing the lock on the master node after the build hook accepts the job.  Then the compute node acquires a new lock and performs the build.
>
> I just added "outputLocks.unlock();" at build.cc line 1036.  It's possible that another goal will try to lock the path again between the time that the master releases the lock and the compute node acquires it.  In that case, I think, the original builder will remain blocked and we'll generate a new (redundant) build hook request.  Eventually one of the build processes should actually get the lock and build the derivation.  When it finishes, the extra builders should find the path valid and just use it at build.cc line 997.
>
> I realize that this is a hack that only applies to the situation of a common /nix/store.  In the standard model where each node has its own /nix/store, this patch could easily cause the same derivation to be built many times on different nodes.  Perhaps something like this could be a configuration option, though?
>
> Also, there may be some performance issues (e.g., lots of redundant processes waiting around for the same lock, then all acquiring and releasing it in turn).  But, in practice, my jobs seem to be running well now.
>
> -ds
>
>
> _______________________________________________________
> David Soergel                            (650) 303-5324
> dev at davidsoergel.com        http://www.davidsoergel.com
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Hi David,
Wow! Hats off to you sir! I mean, you've been able to build a hydra cluster?
If so, you might be the first person to do it unless hydra.nixos.org
is a cluster? (Correct me if I'm wrong guys.)

What kind of hardware are you using and how many machines are you running?
I know you've only just got it to work but will you be able to post
any statistics in the future, say for instance how long it takes to
build the desktop CD? Just to see how much power you can draw out of
it.
Are you writing a paper on it or is it just for fun?

Thanks,
Tony