Re: Limit hash table size

[Andrew Morton]

Andi Kleen <ak@xxxxxxx> wrote:
>
> Andrew Morton <akpm@xxxxxxxx> 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.

> I cannot imagine a workload where it would make sense to ever cache 
> 500 million dentries. It just risks to keep the whole file system
> after an updatedb in memory on a big box, which is not necessarily
> good use of the memory.

A decent approach to the updatedb problem is an application hint which says
"reclaim i/dcache harder".  Just turn it on during the updatedb run -
crude, but it's a start.

But I've been telling poeple for a year that they should set
/proc/sys/vm/swappiness to zero during the updatedb run and afaik nobody has
bothered to try it...

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