run-time settting of PIPE_BUF

[zdenko tomasic]

In article <320 at crcaus.cactus.org> john at crcaus.UUCP (John R. Miller) writes:
>
> .... good points on PIPE_BUF ...
> ...
>If you are seeing too many context switches, I don't think it's caused
                   ^^^^^^^^^^^^^^^^
>by pipe buffers filling up.

Actually, I should mention that the whole thing started when I just
let the shell set up the pipe for 2 user filters. The context
switches were excessive (in millions) and the cpu time was way up
just for the input part of the receiving end of the pipe (I did
not measure the sending filter timing).

After playing around with pipes set up explicitly through a C
program and changing the pipe buffer sized with setbuffer (Chuck
Karsh was kind to point that in his e-mail to me), I could push
the number of context switches up by reducing the pipe buffer size.

Since the pipe size buffer of less than 80 bytes roughly gives the
number of context switches in the right ballpark, I conclude that
the (csh,ksh) shells use line buffering for pipes, i.e. pipe buffers
are then rather small --> too many context switches. I guess somebody
assumed that stdin and stdout will allways involve terminal io and
left everything line buffered at all times.

The work-around is to make your own pipes and avoid shell pipes.

I initially assumed (mislead by comments in the /usr/include/sys/limits.h)
that 512 was the default PIPE_BUF and I am convinced now that it
is not so (and even if it were it would not produce the observed
number of context switches).

I hope somebody could verify the above and change shell pipe
buffering as other systems (e.g. MIPS) do not have such problems.
There is no reason to make piping through shell as expensive as it
is now, is there?



>
>disclaimer: I've been wrong before.
             me,  too!
>
>-- 
>John R. Miller   13102 Briar Hollow Dr.   Austin, Texas  78729
>hm: 512/331-0155 john at crcaus.cactus.org  or ..cs.utexas.edu!bigtex!crcaus!john
>wk: 512/823-3867 john at glasnost.austin.ibm.com


--
___________________________________________________________________
Zdenko Tomasic, Rice U., Chem. Dept., P.O. Box 1892, Houston, Tx 77251
INTERNET: zdenko at katzo.rice.edu
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