Date: Tue, 23 Oct 2001 13:31:59 -0500 From: Alfred Perlstein <bright@mu.org> To: Luigi Rizzo <rizzo@aciri.org> Cc: net@freebsd.org Subject: Re: performance issues with M_PREPEND on clusters Message-ID: <20011023133159.J15052@elvis.mu.org> In-Reply-To: <20011023132813.I15052@elvis.mu.org>; from bright@mu.org on Tue, Oct 23, 2001 at 01:28:13PM -0500 References: <20011023110307.A34494@iguana.aciri.org> <20011023132813.I15052@elvis.mu.org>
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* Alfred Perlstein <bright@mu.org> [011023 13:28] wrote:
> * Luigi Rizzo <rizzo@aciri.org> [011023 13:06] wrote:
> >
> > Hi,
> > don't know what is the right forum to discuss this, but this is
> > certainly one. I have also Bcc-ed some people who might need to be
> > involved.
> >
> > In the mbuf code, M_LEADINGSPACE always returns 0 when M_EXT is
> > set, instead of calling the second part of M_WRITABLE to check
> > whether there is a chance of writing into the cluster. This means
> > that both M_PREPEND() and m_pullup() will have to allocate a second
> > mbuf when they are invoked on a cluster, which these days is
> > basically true for most of the incoming traffic (several network
> > drivers pass up the full packet in a cluster).
>
> I've seen this brought up before, this is not what you want to
> do otherwise you risk corrupting EXT data. The right thing to
> do is have the people allocating writable EXT bufs mark them
> as such. Other EXT type buffers such as sendfile bufs can not
> be written to no matter what the sharecount is.
>
> I hope this helps.
Oh yeah, using this method you could "fix" MCLGET in -stable to
from this:
#define MCLGET(m, how) do { \
struct mbuf *_mm = (m); \
\
MCLALLOC(_mm->m_ext.ext_buf, (how)); \
if (_mm->m_ext.ext_buf != NULL) { \
_mm->m_data = _mm->m_ext.ext_buf; \
_mm->m_flags |= M_EXT; \
_mm->m_ext.ext_free = NULL; \
_mm->m_ext.ext_ref = NULL; \
_mm->m_ext.ext_size = MCLBYTES; \
} \
} while (0)
to:
#define MCLGET(m, how) do { \
struct mbuf *_mm = (m); \
\
MCLALLOC(_mm->m_ext.ext_buf, (how)); \
if (_mm->m_ext.ext_buf != NULL) { \
_mm->m_data = _mm->m_ext.ext_buf; \
_mm->m_flags |= M_EXT | M_EXTWRITEABLE; \
_mm->m_ext.ext_free = NULL; \
_mm->m_ext.ext_ref = NULL; \
_mm->m_ext.ext_size = MCLBYTES; \
} \
} while (0)
--
-Alfred Perlstein [alfred@freebsd.org]
'Instead of asking why a piece of software is using "1970s technology,"
start asking why software is ignoring 30 years of accumulated wisdom.'
http://www.morons.org/rants/gpl-harmful.php3
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