Re: [Lse-tech] Re: [PATCH] New dcache / inode hash tuning patch

[Paul Menage]

>
>It would be possible to cache line optimize the layout of struct dentry
>in addition. May be an interesting add-on project for someone...

I played with this a few months ago, and sent a preliminary patch to
linux-kernel, but in my (fairly brief) testing on a 4-way system I
wasn't able to produce a measurable benefit from it. I think part of
this may have been due to contention on dcache_lock, so maybe dcache_rcu
will help there.

The main changes were to bring together all the data needed for checking
a non-matching dcache hash entry (modulo hash collisions) into one
cacheline, and to separate out the mostly-read-only fields from the 
change-frequently fields.

The original patch is at 

http://marc.theaimsgroup.com/?l=linux-kernel&m=102650654002932&w=2

>But for lookup walking even one cache line - the one containing d_hash -
>should be needed. Unless d_hash is unlucky enough to cross a cache
>line for its two members ... but I doubt that.

No, but on a 32-byte cache line system, d_parent, d_hash and d_name are
all on different cache lines, and they're used when checking each entry.
On 64-byte systems, d_parent and d_hash will be on the same line, but
d_name is still on a separate line and d_name.hash gets checked before
d_parent. So bringing these three fields on to the same cacheline
would theoretically be a win.

Paul