From owner-freebsd-net  Sat Jun 29 14:53: 8 2002
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Date: Sat, 29 Jun 2002 14:53:03 -0700
From: Luigi Rizzo <rizzo@icir.org>
To: net@freebsd.org
Subject: Should we keep a cache of mbuf+cluster ready for use ?
Message-ID: <20020629145303.A75543@iguana.icir.org>
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Hi,
during some experiments i was doing recently, i noticed that there
is a significant improvement in the forwarding speed (especially
at very high speeds) if we keep a small pool of mbuf+cluster
ready for use. This is because most network drivers do something
like this

                MGETHDR(m_new, M_DONTWAIT, MT_DATA);
                if (m_new == NULL)
                        return(ENOBUFS);

                MCLGET(m_new, M_DONTWAIT);
                if (!(m_new->m_flags & M_EXT)) {
                        m_freem(m_new);
                        return(ENOBUFS);
                }

when replenishing the receive buffers, and both macros are quite
long even if there are available blocks in the free lists. We can
store buffers of this form when/if they are released with some code
like this:

	if (my_pool_count < my_pool_max && m->m_next == NULL &&
			(m->m_flags & M_EXT) && M_EXT_WRITABLE(m) ) {
		m->m_nextpkt = my_pool;
		m->m_data = ->m_ext.ext_buf;
		m->m_len = m->m_pkthdr.len = MCLBYTES;
		my_pool = m;
		my_pool_now++;
	} else {
		... rest of m_freem() ...
	}

and save a lot of overhead (we just need to reset m_data and
m_len and m_pkthdr.len) when someone wants to allocate them.

Is there interest in committing some code like this to
mbuf.h and maybe uipc_mbuf*.c ?

	cheers
	luigi


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