Re: Another proposal for DAX fault locking

[Cedric Blancher]

On 9 February 2016 at 18:24, Jan Kara <jack@xxxxxxx> wrote:
> Hello,
>
> I was thinking about current issues with DAX fault locking [1] (data
> corruption due to racing faults allocating blocks) and also races which
> currently don't allow us to clear dirty tags in the radix tree due to races
> between faults and cache flushing [2]. Both of these exist because we don't
> have an equivalent of page lock available for DAX. While we have a
> reasonable solution available for problem [1], so far I'm not aware of a
> decent solution for [2]. After briefly discussing the issue with Mel he had
> a bright idea that we could used hashed locks to deal with [2] (and I think
> we can solve [1] with them as well). So my proposal looks as follows:
>
> DAX will have an array of mutexes

One folly here: Arrays of mutexes NEVER work unless you manage to
align them to occupy one complete L2/L3 cache line each. Otherwise the
CPUS will fight over cache lines each time they touch (read or write)
a mutex, and it then becomes a O^n-like scalability problem if
multiple mutexes occupy one cache line. It becomes WORSE as more
mutexes fit into a single cache line and even more worse with the
number of CPUS accessing such contested lines.

Ced
-- 
Cedric Blancher <cedric.blancher@xxxxxxxxx>
Institute Pasteur