Re: [rfc] lockless pagecache

[Hirokazu Takahashi <taka@valinux.co.jp>]

Hi,

> > Your patches improve the performance if lots of processes are
> > accessing the same file at the same time, right?
> > 
> 
> Yes.
> 
> > 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?
> > 
> 
> Sure it is a possibility.
> 
> I don't think you could call it effective like a completely
> lockless version is effective. You might take more locks during
> gang lookups, you may have a lot of ugly and not-always-working
> heuristics (hey, my app goes really fast if it spreads accesses
> over a 1GB file, but falls on its face with a 10MB one). You
> might get increased cache footprints for common operations.

I guess it would be enough to split a huge file into the same
size pieces simply and put each of them in its associated radix-tree
in most cases for practical use.

And I also feel your approach is interesting.

> I mainly did the patches for a bit of fun rather than to address
> a particular problem with a real workload and as such I won't be
> pushing to get them in the kernel for the time being.

I see.

I propose another idea if you don't mind, seqlock seems to make
your code much simpler though I'm not sure whether it works well
under heavy load. It would become stable without the tricks,
which makes VM hard to be enhanced in the future.

Thanks,
Hirokazu Takahashi.
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