On Thu, 8 May 2014, Sebastian Schuberth wrote:

On 03.05.2014 05:40, Felipe Contreras wrote:

That's very interesting. Do you get similar improvements when doing
something similar in Merurial (watchman vs . no watchman).

I have not tried it.  My understanding is that this is why Facebook
wrote Watchman and added support for it to Mercurial, so I would assume
that the improvements are at least this good.

Yeah, my bet is that they are actually much better (because Mercurial
can't be so optimized as Git).

I'm interested in this number because if watchman in Git is improving it
by 30%, but in Mercurial it's improving it by 100% (made up number),
therefore it makes sens that you might want it more if you are using hg,
but not so much if you are using git.

Also, if similar repositories with Mercurial+watchman are actually
faster than Git+watchman, that means that there's room for improvement
in your implementation. This is not a big issue at this point of the
process, just something nice to know.

The article at [1] has some details, they claim "For our repository, enabling 
Watchman integration has made Mercurial's status command more than 5x faster than Git's 
status command".


a lot of that speed comparison is going to depend on your storage system and the size of your repository.

if you have a high-end enterprise storage system that tracks metadata very differently from the file contents (I've seen some that have rackes worth of SATA drives for contents and then 'small' arrays of a few dozen flash drives for the metadata), and then you have very large repositories (Facebook has everything in a single repo), then you have a perfect storm where something like watchman that talks the proprietary protocol of the storage array can be FAR faster than anything that needs to operate with the standard POSIX calls.

That can easily account for the difference between the facebook announcement and the results presented for normal disks that show an improvement, but with even stock git being faster than improved mercurial.

David Lang
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