Re: [PATCH 2/4] Convert epoll to a bitlock

[Davide Libenzi]

On Tue, 3 Feb 2009, Eric Dumazet wrote:

> Andrew Morton a écrit :
> > On Mon,  2 Feb 2009 11:20:09 -0700
> > Jonathan Corbet <corbet@xxxxxxx> wrote:
> > 
> >> Matt Mackall suggested converting epoll's ep_lock to a bitlock as a way of
> >> saving space in struct file.  This patch makes that change.
> > 
> > hrm.  bit_spin_lock() makes people upset (large penguiny people).  iirc
> > it doesn't have all the correct/well-understood memory/compiler
> > ordering semantics which spinlocks have.  And lockdep doesn't know about
> > it.
> > 
> 
> In a previous attempt (2005), I suggested using a single global lock.
> 
> http://search.luky.org/linux-kernel.2005/msg50862.html
> 
> Probably an array of hashed spinlocks would be more than enough.

That could be done, although I'm not sure it's worth going that way to 
save 4 bytes. The effective saving rate is not even 4/sizeof(struct file) 
since struct file never comes alone, and when you allocate a struct file 
you always carry more allocations behind (at least for the cases where you 
tend to have a lot of them around, so size would matter).
The add/remove path in epoll is not a super-hot one, so it could be done. 
I dunno how this change matter with the patchset though.



- Davide