Re: [PATCH] : struct dentry : place d_hash close to d_parent and d_name to speedup lookups

Dipankar Sarma
2005-09-01T17:41:49Z

On Thu, Sep 01, 2005 at 05:48:28PM +0200, Eric Dumazet wrote:
> dentry cache uses sophisticated RCU technology (and prefetching if
> available) but touches 2 cache lines per dentry during hlist lookup.
>
> This patch moves d_hash in the same cache line than d_parent and d_name
> fields so that :
>
> 1) One cache line is needed instead of two.
> 2) the hlist_for_each_rcu() prefetching has a chance to bring all the
> needed data in advance, not only the part that includes d_hash.next.
>
> I also changed one old comment that was wrong for 64bits.
>
> A further optimisation would be to separate dentry in two parts, one that
> is mostly read, and one writen (d_count/d_lock) to avoid false sharing on
> SMP/NUMA but this would need different field placement depending on 32bits
> or 64bits platform.

Do you have performance numbers that show the benefits ? In the
past, I did try some optimizations like this but found no demonstrable
benefits. If it ain't broken .....

Thanks
Dipankar