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freetds - RE: [freetds] FreeBSD 5.0 performance problems with TCP_NOPUSH

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  • From: "Lowden, James K" <LowdenJK AT bernstein.com>
  • To: <ml AT freetds.org>
  • Cc:
  • Subject: RE: [freetds] FreeBSD 5.0 performance problems with TCP_NOPUSH
  • Date: Tue, 15 Mar 2005 10:37:19 -0500

> From: ZIGLIO, Frediano, VF-IT
> Sent: Tuesday, March 15, 2005 7:14 AM
>
> > > You tested that TCP_NOPUSH under FreeBSD 5.0 do not work
> > correctly so
> > > would be so kind to try with
> >
> > > /* Try to use TCP_CORK on *BSD */
> > > #elif (defined(__FreeBSD__) || defined(__GNU_FreeBSD__) ||
> > > defined(__OpenBSD__)) && defined(TCP_CORK)
> > > #define USE_CORK 1
> >
> > Hi Freddy,
> >
> > That works fine on my FreeBSD 5.0 system.
> >
> > FWIW here are the avaliable options from tcp.h
> >
> > #define TCP_NODELAY 0x01 /* don't delay send to
> > coalesce packets */
> > #if __BSD_VISIBLE
> > #define TCP_MAXSEG 0x02 /* set maximum segment size */
> > #define TCP_NOPUSH 0x04 /* don't push last block of write */
> > #define TCP_NOOPT 0x08 /* don't use TCP options */
> > #endif
> >
>
> So I think is working using NODELAY option... do you know a way to
> "emulate" linux CORK behavior, that is coalesce packets and flush them
> when needed ??

This might be helpful: http://dotat.at/writing/nopush.html.
http://www.kegel.com/c10k.html#writev recommends writev(2) over
TCP_CORK.

I have a basic question: Why are we doing this? It's the TCP stack's
job to decide when to flush packets. Why should we worry about such
low-level, implementation-dependent, nonstandard details? Unless the
timeout is very long, the advantage of coalescing frames ISTM is very
small, particularly in light of portability issues.

I can see the advantage of TCP_NODELAY, given how often TDS sends small
packets. I don't see the advantage in using less widely implemented
options.

Is there a measurable difference with tsql or freebcp?

--jkl



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