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

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Date:	Tue, 3 Feb 2009 15:08:16 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Eric Dumazet <dada1@...mosbay.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	andi@...stfloor.org, oleg@...hat.com, viro@...IV.linux.org.uk,
	David Miller <davem@...emloft.net>, hch@....de,
	linux-api@...r.kernel.org, mpm@...enic.com,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH 2/4] Convert epoll to a bitlock

On Tue, 3 Feb 2009, Eric Dumazet wrote:

> Andrew Morton a écrit :
> > On Mon,  2 Feb 2009 11:20:09 -0700
> > Jonathan Corbet <corbet@....net> 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