Re: [rfc][patch] dynamic resizing dentry hash using RCU

From: Nick Piggin
Date: Fri Feb 23 2007 - 20:26:27 EST

On Fri, Feb 23, 2007 at 09:25:28AM -0800, Zach Brown wrote:
> On Feb 23, 2007, at 7:37 AM, Nick Piggin wrote:
> >
> >The dentry hash uses up 8MB for 1 million entries on my 4GB system
> >is one
> >of the biggest wasters of memory for me. Because I rarely have more
> >than one or
> >two hundred thousand dentries. And that's with several kernel trees
> >worth of
> >entries. Most desktop and probably even many types of servers will
> >only use a
> >fraction of that.
> >
> >So I introduce a new method for resizing hash tables with RCU, and
> >apply
> >that to the dentry hash.
> Can you compare what you've done to the design that Paul and David
> talked about a year ago?

Thanks for the link, I wasn't aware of Paul's algorithm before.

I guess most variants are going to have a double pointer scheme,
so they are similar in that regard.

I think Paul's is quite complex in moving entries to the new table,
and it looks like it requires a lot of grace periods. I avoid all
that by using the seqlock.

It wasn't clear to me how Paul handled the case where an item is
present in the not_current table, but the lookup misses it when it
gets moved between the tables. It is a little tricky to follow
the find details because it is not in code or pseudo code format.

> I'd love to see a generic implementation of RCU hashing that
> subsystems can then take advantage of. It's long been on the fun
> side of my todo list. The side I never get to :/.

Yeah if this is to be used anywhere else, I think it definitely
needs to be made into a generic library if possible. Main thing
I wanted was to get something working and see what people think
about it.

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