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Date: Tue, 4 Nov 2014 13:41:41 +0000
From: Andrew Turner <andrew@fubar.geek.nz>
To: Mateusz Guzik <mjguzik@gmail.com>
Subject: Re: svn commit: r274088 - head/sys/kern
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 <20141104114041.GA21297@dft-labs.eu>
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
 src-committers@freebsd.org, Hans Petter Selasky <hselasky@FreeBSD.org>
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On Tue, 4 Nov 2014 12:40:41 +0100
Mateusz Guzik <mjguzik@gmail.com> wrote:

> On Tue, Nov 04, 2014 at 11:29:49AM +0000, Hans Petter Selasky wrote:
> > Author: hselasky
> > Date: Tue Nov  4 11:29:49 2014
> > New Revision: 274088
> > URL: https://svnweb.freebsd.org/changeset/base/274088
> > 
> > Log:
> >   Simplify logic a bit. Ensure data buffer is properly aligned,
> >   especially for platforms where unaligned access is not allowed.
> > Make it possible to override the small buffer size.
> >   
> >   A simple continuous read string test using libusb showed a
> > reduction in CPU usage from roughly 10% to less than 1% using a
> > dual-core GHz CPU, when the malloc() operation was skipped for
> > small buffers. 
> >   MFC after:	2 weeks
> > 
> > Modified:
> >   head/sys/kern/sys_generic.c
> > 
> > Modified: head/sys/kern/sys_generic.c
> > ==============================================================================
> > --- head/sys/kern/sys_generic.c	Tue Nov  4 10:25:52
> > 2014	(r274087) +++ head/sys/kern/sys_generic.c	Tue
> > Nov  4 11:29:49 2014	(r274088) @@ -646,10 +646,13 @@ struct
> > ioctl_args { int
> >  sys_ioctl(struct thread *td, struct ioctl_args *uap)
> >  {
> > +#ifndef SYS_IOCTL_SMALL_SIZE
> > +#define	SYS_IOCTL_SMALL_SIZE 128
> > +#endif
> > +	u_char smalldata[SYS_IOCTL_SMALL_SIZE] __aligned(8);
> 
> Should not you align to word size instead?

On ARM it needs to be at least 8-byte aligned as this is the largest
alignment of any types.

Andrew