Date: Thu, 11 Jul 2002 19:31:17 -0400 (EDT) From: Kelly Yancey <kbyanc@posi.net> To: Bosko Milekic <bmilekic@unixdaemons.com> Cc: Luigi Rizzo <rizzo@icir.org>, freebsd-net@FreeBSD.ORG Subject: Re: mbuf external buffer reference counters Message-ID: <Pine.BSF.4.05.10207111913160.46046-100000@kronos.networkrichmond.com> In-Reply-To: <20020711171255.A19014@unixdaemons.com>
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On Thu, 11 Jul 2002, Bosko Milekic wrote:
>
> On Thu, Jul 11, 2002 at 01:56:08PM -0700, Luigi Rizzo wrote:
> > example: userland does an 8KB write, in the old case this requires
> > 4 clusters, with the new one you end up using 4 clusters and stuff
> > the remaining 16 bytes in a regular mbuf, then depending on the
> > relative producer-consumer speed the next write will try to fill
> > the mbuf and attach a new cluster, and so on... and when TCP hits
> > these data-in-mbuf blocks will have to copy rather than reference
> > the data blocks...
> >
> > Maybe it is irrelevant for performance, maybe it is not,
> > i am not sure.
>
> I see what you're saying. I think that what this means is simply that
> the `optimal' chunk of data to send is just a different size, so
> instead of it being 8192 bytes, it'll be something like 8180 bytes or
> something (to account for the counters). So, in other words, it
> really depends on the frequency of exact 8192 sized sends in userland
> applications.
>
...or exactly 2k or 4k or 6k or 10k...
> This is a good observation if we're going to be doing benchmarking,
> but I'm not sure whether the repercussions are that important (unless,
> as I said, there's a lot of applications that send exactly 8192
> byte chunks?). Basically, what we're doing is shifting the optimal
> send size when using exactly 4 clusters, in this case, to (8192 - 16)
> bytes. We can still send with exactly 4 clusters, it's just that the
> optimal send size is a little different, that's all (this produces a
> small shift in block send benchmark curves, usually).
>
Are you kidding? Benchmarks, presumably like every other piece of
software produced by someone trying to get the most performance out of
the system, are more likely to have power-of-two write buffers. Are you
willing to risk that they didn't also just happen to pick a multiple of
2^11?
Yes, it seems elegant to put the counters in the space that is normally
unused for receive mbuf clusters, but you can't just blow off Luigi's
point regarding the send side.
Kelly
--
Kelly Yancey -- kbyanc@{posi.net,FreeBSD.org}
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