linux-kernel - Re: [PATCH] EPOLL_CTL_SET operatin for epoll

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Date:	Thu, 24 Sep 2009 17:34:28 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Carlo Marcelo Arenas Belon <carenas@...inet.com.pe>
cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] EPOLL_CTL_SET operatin for epoll_ctl when using
 EPOLLONESHOT

On Fri, 25 Sep 2009, Carlo Marcelo Arenas Belon wrote:

> On Thu, Sep 24, 2009 at 04:46:20PM -0700, Davide Libenzi wrote:
> > On Thu, 24 Sep 2009, Carlo Marcelo Arenas Belon wrote:
> > 
> > > Simplify userspace by eliminating the need to track whether a specific
> > > filehandle was added to epoll already and only needs to be re-arm
> > > after being internally disabled by EPOLLONESHOT.
> > > 
> > > EPOLL_CLT_SET can be used to add or modify a filehandle in epoll
> > > using the kernel's internal tracking to apply the right equivalent
> > > operation to EPOLL_CTL_ADD or EPOLL_CTL_MOD.
> > 
> > Applications (except poorly written ones) usually know what's in the 
> > epoll set and what's not, in their bookkeeping.
> 
> agree, but since the kernel is also doing that bookkeeping using
> ep_find it is possible by using this new operation to avoid doing
> that in userspace, saving some cycles (and probably some contention in
> multithreaded applications if the epoll set is global) in the hot path.
> 
> > I'm sorry, but I see no use of this code.
> 
> a very simple, probe of concept echoserver using this (once #defined)
> is available from :
> 
>   http://sajino.sajinet.com.pe/echoserver.c

As I said, a poorly designed code. You could easily have done an ADD in 
one place, and then have your multishot function handle only MOD.



- Davide


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