Re: Limit hash table size

[Matt Mackall <mpm@selenic.com>]

On Thu, Feb 05, 2004 at 07:09:04PM -0800, Andrew Morton wrote:
> Andi Kleen <ak@suse.de> wrote:
> >
> > Andrew Morton <akpm@osdl.org> writes:
> > 
> > > Ken, I remain unhappy with this patch.  If a big box has 500 million
> > > dentries or inodes in cache (is possible), those hash chains will be more
> > > than 200 entries long on average.  It will be very slow.
> > 
> > How about limiting the global size of the dcache in this case ? 
> 
> But to what size?
> 
> The thing is, any workload which touches a huge number of dentries/inodes
> will, if it touches them again, touch them again in exactly the same order.
> This triggers the worst-case LRU behaviour.
> 
> So if you limit dcache to 100MB and you happen to have a workload which
> touches 101MB's worth, you get a 100% miss rate.  You suffer a 100000%
> slowdown on the second pass, which is unhappy.  It doesn't seem worth
> crippling such workloads just because of the updatedb thing.

A less strict approach to LRU might serve. Probabilistically dropping
something in the first half of the LRU rather than the head would go a
long way to gracefully degrading the "working set slightly larger than
cache" effect. There are a couple different ways to do this that are
reasonably efficient.

-- 
Matt Mackall : http://www.selenic.com : Linux development and consulting