Date: Thu, 27 Mar 2003 15:33:52 -0800 From: Luigi Rizzo <rizzo@icir.org> To: Sam Leffler <sam@errno.com> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/conf options src/sys/netinet ip_output.c Message-ID: <20030327153352.A66323@xorpc.icir.org> In-Reply-To: <031301c2f49b$09d2bfb0$52557f42@errno.com>; from sam@errno.com on Thu, Mar 27, 2003 at 11:56:47AM -0800 References: <200303260452.h2Q4quap015364@www.ambrisko.com> <20030326114030.U2075@odysseus.silby.com> <20030326183351.GJ57674@elvis.mu.org> <20030326130903.G2075@odysseus.silby.com> <20030327013224.P7674@odysseus.silby.com> <031301c2f49b$09d2bfb0$52557f42@errno.com>
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[about coalescing long chains of mbufs...]
On Thu, Mar 27, 2003 at 11:56:47AM -0800, Sam Leffler wrote:
[have read the rest of the thread too]
> I thought about this some more. If the purpose of m_defrag* is to handle
...
> driver unless you make a second pass. Given that this problem should happen
> infrequently and that you're just trying to avoid discarding the packet I
> think you're best off doing a best effort job where you coalesce only mbufs
> and leave clusters/ext's alone. Then if the subsequent bus_dma_load_mbuf
> call fails you discard the packet. Other than the read/write requirements
the problem is that the code path leading to these situation behave
sistematically like this -- ie. you are likely to keep dropping the packet
again and again, so dropping may not be an option.
Anyways, one path where this came out was
tcp_usr_send() --> sbappend() --> sbcompress()
and i think if the code decides to call sbcompress(),
then the latter should try and do its job as good as it can,
including allocating a cluster when the caller does a
pathological number of short writes, or merging segments
trying to reduce the waste factor to something reasonable
E.g. note that sbcompress() does the following:
while (m) {
...
if (n && (n->m_flags & M_EOR) == 0 &&
M_WRITABLE(n) &&
m->m_len <= MCLBYTES / 4 && /* XXX: Don't copy too much */
m->m_len <= M_TRAILINGSPACE(n) &&
n->m_type == m->m_type) {
... do the copy and free m
continue;
}
...
}
so individual writes of 513+ bytes will result in wasting up to 75%
of the socket buffer space. At the very least, i would drop the
'm->m_len <= MCLBYTES / 4' check to reduce the waste.
cheers
luigi
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