netdev - Re: [PATCH net-next 4/9] net: sctp: sctp_outq: consolidate chars into bitfield

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Date:	Wed, 17 Apr 2013 11:27:57 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	David Laight <David.Laight@...LAB.COM>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-sctp@...r.kernel.org
Subject: Re: [PATCH net-next 4/9] net: sctp: sctp_outq: consolidate chars
 into bitfield

On Wed, Apr 17, 2013 at 11:26:21AM +0200, Daniel Borkmann wrote:
> On 04/17/2013 10:43 AM, David Laight wrote:
> >>  	__u32 outstanding_bytes;
> >>
> >>-	/* Are we doing fast-rtx on this queue */
> >>-	char fast_rtx;
> >>-
> >>-	/* Corked? */
> >>-	char cork;
> >>-
> >>-	/* Is this structure empty?  */
> >>-	char empty;
> >>+	__u8 fast_rtx:1,	/* Are we doing fast-rtx on this queue */
> >>+	     cork:1,		/* Corked? */
> >>+	     empty:1;		/* Is this structure empty?  */
> >>  };
> >
> >Use of bitfields just makes the code slower.
> >The only real excuse for using them is to reduce the size
> >of a structure that is allocated a lot.
> 
> sctp_outq is _embedded_ into an sctp_association structure, which
> has [size: 2280, cachelines: 36, members: 76]! A next step would be
> to try to reorder its elements carefully and see if we can reduce
> the size by filling some holes.
Unrelated note: How did you get pahole to give you that information?  For some
reason pahole can never glean any useful information about struct
sctp_association for me, despite having dwarf info embedded into the cu.

Neil

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