On Wed 10-02-16 08:19:22, Mel Gorman wrote: > On Tue, Feb 09, 2016 at 07:46:05PM +0100, Cedric Blancher wrote: > > 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. > > > > That is a *potential* performance concern although I agree with you in that > mutex's false sharing a cache line would be a problem. However, it is a > performance concern that potentially is alleviated by alternative hashing > where as AFAIK the issues being faced currently are data corruption and > functional issues. I'd take a performance issue over a data corruption > issue any day of the week. Exactly. We have to add *some* locking to fix the data corruption. Cache aliasing of hashed mutexes may be an issue but I believe the result will be still better than a single mutex. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
