Re: [rfc] lockless pagecache

[Hirokazu Takahashi]

Hi Nick,

Your patches improve the performance if lots of processes are
accessing the same file at the same time, right?

If so, I think we can introduce multiple radix-trees instead,
which enhance each inode to be able to have two or more radix-trees
in it to avoid the race condition traversing the trees.
Some decision mechanism is needed which radix-tree each page
should be in, how many radix-tree should be prepared.

It seems to be simple and effective.

What do you think?

> Now the tree_lock was recently(ish) converted to an rwlock, precisely
> for such a workload and that was apparently very successful. However
> an rwlock is significantly heavier, and as machines get faster and
> bigger, rwlocks (and any locks) will tend to use more and more of Paul
> McKenney's toilet paper due to cacheline bouncing.
> 
> So in the interest of saving some trees, let's try it without any locks.
> 
> First I'll put up some numbers to get you interested - of a 64-way Altix
> with 64 processes each read-faulting in their own 512MB part of a 32GB
> file that is preloaded in pagecache (with the proper NUMA memory
> allocation).

Thanks,
Hirokazu Takahashi.

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