Re: [PATCH] Cpuset: rcu optimization of page alloc hook

[Eric Dumazet <dada1@cosmosbay.com>]

Christoph Lameter a écrit :
> On Tue, 13 Dec 2005, Eric Dumazet wrote:
> 
> 
>>Say you move to read mostly most of struct kmem_cache *, they are guaranteed
>>to stay in 'mostly read'.
> 
> 
> True but then this variable is not frequently read. So false sharing would 
> not have much of an impact.
> 

If this variable is not frequently used, why then define its own cache ?

Ie why not use kmalloc() and let kernel use a general cache ?

On a 32 CPUS machine, a kmem_create() costs a *lot* of ram.

This overhead is acceptable if and only if :

- A lot of objects can be allocated in some cases (and using a special cache 
can save ram because of power of two general caches alignement)

- A lot of objects are created/deleted per unit of time (performance is critical)

For example pipe() could use its own cache to speedup some benchmarks, but the 
general kmalloc()/kfree() was chosen instead.

Eric