Re: question about: Facebook makes Mercurial faster than Git

From: David Lang <david@lang.hm>
Date: Mar 10, 2014, 10:13 UTC
Message-ID: <alpine.DEB.2.02.1403100310080.25193@nftneq.ynat.uz>
In-Reply-To: <531D8ED9.7040305@gmx.net>

On Mon, 10 Mar 2014, Dennis Luehring wrote:

Show 9 quoted lines

> according to these blog posts
>
> http://www.infoq.com/news/2014/01/facebook-scaling-hg
> https://code.facebook.com/posts/218678814984400/scaling-mercurial-at-facebook/
>
> mercurial "can" be faster then git
>
> but i don't found any reply from the git community if it is a real problem
> or if there a ongoing (maybe git 2.0) changes to compete better in this case

As I understand this, the biggest part of what happened is that Facebook made a tweak to mercurial so that when it needs to know what files have changed in their massive tree, their version asks their special storage array, while git would have to look at it through the filesystem interface (by doing stat calls on the directories and files to see if anything has changed)

In other words, unless you have a very high end storage system that can keep track of such things for you, the Facebook 'fix' won't help you. And even if it does have such a capability, unless you use the same storage system that Facebook uses, you would have to port it to your class of device.

Now, in addition to this, they did some other tweaks and changes, but compared to this status change, everything else is minor.

David Lang