From: David Mosberger <davidm@napali.hpl.hp.com>
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Message-ID: <16491.2184.253165.545651@napali.hpl.hp.com>
Date: Wed, 31 Mar 2004 10:06:00 -0800
To: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
Cc: ulrich.windl@rz.uni-regensburg.de, linux-kernel@vger.kernel.org
Subject: Re: 2.4.21 on Itanium2: floating-point assist fault at ip 400000000062ada1, isr 0000020000000008
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>>>>> On Wed, 31 Mar 2004 19:00:17 +0200, Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua> said:

  Denis> kernel says that you have them too frequently, which probably
  Denis> impairs efficiency. It's a hint to programmer.

Close: the kernel limits the frequency of the printing to avoid
flooding the log files.  Even if you do get the faults frequently, it
won't print more than 5 warning messages every 5 seconds.  The
floating-point software-assist (fpswa) faults are harmless in the
sense that they don't affect correctness of the program, but if you do
get them _very_ frequently (which is quite rare), they could impair
performance.  FPSWA faults occur only for corner-cases of
floating-point arithmetic, such as operations on denormals or
non-finite numbers.  Many programs don't need denormal support at all
and for those, you can link the program with -ffast-math (GCC) or -ftz
(Intel compiler).  This will turn on "flush-to-zero" mode and avoid
any FPSWA-faults due to denormals (in x86-speak, this is equivalent to
the "flush-to-zero" mode that SSE offers).

If the messages appear with a frequency of less than 5 messages/5
seconds, then there is certainly no performance issue and you may want
to just turn off the messages.  This can be done via the prctl(2)
system-call or with the prctl command.  With the latter:

	prctl --fp-emul=silent

will fork a new shell and disable the printing of the FPSWA-messages
in the shell and all its children.  There is also "--unaligned=silent"
which will turn off a similar message for unaligned access emulation
done by the kernel.

	--david
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