Re: [PATCH] shrinks dentry struct

[Eric Dumazet <dada1@cosmosbay.com>]

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