Re: [PATCH] shrinks dentry struct

[Eric Dumazet]

Paul E. McKenney a écrit :
> On Tue, Nov 29, 2005 at 04:22:00PM +0100, Eric Dumazet wrote:
>
> > Hi Andrew
> >
> > Could you add this patch to mm ?
> >
> > Thank you
> >
> > [PATCH] shrinks dentry struct
> >
> > Some long time ago, dentry struct was carefully tuned so that on 32 bits 
> > UP, sizeof(struct dentry) was exactly 128, ie a power of 2, and a multiple 
> > of memory cache lines.
> >
> > Then RCU was added and dentry struct enlarged by two pointers, with nice 
> > results for SMP, but not so good on UP, because breaking the above tuning 
> > (128 + 8 = 136 bytes)
> >
> > This patch reverts this unwanted side effect, by using an union (d_u), 
> > where d_rcu and d_child are placed so that these two fields can share their 
> > memory needs.
> >
> > At the time d_free() is called (and d_rcu is really used), d_child is known 
> > to be empty and not touched by the dentry freeing.
> >
> > Lockless lookups only access d_name, d_parent, d_lock, d_op, d_flags (so 
> > the previous content of d_child is not needed if said dentry was unhashed 
> > but still accessed by a CPU because of RCU constraints)
> >
> > As dentry cache easily contains millions of entries, a size reduction is 
> > worth the extra complexity of the ugly C union.
>
>
> Looks sound to me!  Some opportunities for simplification below.
>
> (Please accept my apologies for the delay -- some diversions turned out
> to be more consuming than I had expected.)
>
> 							Thanx, Paul

Hi Paul

My patch only address the layout of dentry structure, basically a 'global 
substitute' on various places.

Adding some 'optimizations' or simplifications was not the goal, so please 
submit a patch if you feel the need for it :)

Thank you

Eric
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