Re: Inode Lock Scalability V4

[Nick Piggin]

On Sun, Oct 17, 2010 at 01:55:33PM +1100, Nick Piggin wrote:
> On Sun, Oct 17, 2010 at 01:47:59PM +1100, Dave Chinner wrote:
> > On Sun, Oct 17, 2010 at 04:55:15AM +1100, Nick Piggin wrote:
> > > On Sat, Oct 16, 2010 at 07:13:54PM +1100, Dave Chinner wrote:
> > > > This patch set is just the basic inode_lock breakup patches plus a
> > > > few more simple changes to the inode code. It stops short of
> > > > introducing RCU inode freeing because those changes are not
> > > > completely baked yet.
> > > 
> > > It also doesn't contain per-zone locking and lrus, or scalability of
> > > superblock list locking.
> > 
> > Sure - that's all explained in the description of what the series
> > actually contains later on. 
> > 
> > > And while the rcu-walk path walking is not fully baked, it has been
> > > reviewed by Linus and is in pretty good shape. So I prefer to utilise
> > > RCU locking here too, seeing as we know it will go in.
> > 
> > I deliberately left out the RCU changes as we know that the version
> > that is in your tree causes siginificant performance regressions for
> > single threaded and some parallel workloads on small (<=8p)
> > machines.
> 
> The worst-case microbenchmark is not a "significant performance
> regression". It is a worst case demonstration. With the parallel
> workloads, are you referring to your postmark xfs workload? It was
> actually due to lazy LRU, IIRC. I didn't think RCU overhead was
> noticable there actually.
> 
> Anyway, I've already gone over this couple of months ago when we
> were discussing it. We know it could cause some small regressions,
> if they are small it is considered acceptable and outweighed
> greatly by fastpath speedup. And I have a design to do slab RCU
> which can be used if regressions are large. Linus signed off on
> this, in fact. Why weren't you debating it then?

It goes to the heart of the locking model, and I think it is silly
just to do this and then go and rewrite locking a release or two later
when rcu is introduced. And change it again when you start listening
to the people who want per-zone lrus.

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