A.P. Lawrence

performance tuning sar nhbufs memory

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From: "James R. Sullivan" <jim@tarantella.com>
Newsgroups: comp.unix.sco.misc
Subject: Re: Performance question
Date: Thu, 27 Sep 2001 08:52:06 -0700
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Adrian wrote:
> 
> I was going to modify NBUF at boot up with the command:
> defbootstr nbuf=100000

I can't remember if the NHBUFS get automatically adjusted when
you change NBUF, but you should make sure that they are appropriately
sized.  In the past, you wanted a 4:1 ratio between NBUF and NHBUFS,
with NHBUFS being a power of 2.  This later changed to a 1:2 ratio
on MP systems.  Either way, make sure that NHBUFS is the right size
for NBUF=100000, probably around 65536 or 32768.
> 
> Currently, SDSKOUT = 4, and I am not sure what to increase that to.
> Is there a way of determinign a good first guess, similar to basing NBUF of the amount of
> free memory?
> I will probably set this at boot time, too, rather than rebuilding the kernel.

I'd set it as high as I could, generally 256, based on the mtune entries.  The higher
the number, the harder the SCSI bus will be working.  I have seen instances where
increasing this number caused the system to crash, due to the quality of the
SCSI bus/termination.  Go neutral, bump it to 128 and see what happens.

Unix and Linux Support

> Is there a performance boost building these settings into the kernel or will the setting
> at boot up be similar?

No idea.

> > Get a better disk subsystem :-)
> >
> 
> We have a DPT Century SCSI 3-channel Controller, with on-board memory.
> We have RAID1 and one of the RAID5 disks on one channel.
> We have the other three disks of the RAID5 array on the second channel.
> All CD_ROMs and tape drives are on their own channel.
> 
> Each disk is Cheetah ST39103LW (10,000rpm, blah blah blah)..
> 
> Will these settings (SDSKOUT and NBUF) interact detrimentally with the DPT's on-board
> cache?

Got me beat.  Haven't done OSR5 performance tuning for 3 years, at least :-)

> 
> Thanks for this advise.  The concensus is to fix the disk problem.

No problem.  Since the disk system seems pretty beefy, I wonder if the program
is performing all it's writes synchronously, which would cause these delays given
the number of writes that are happening.  This may be a setting within the program
that can be changed.  I suspect that by examining the file table for the program
you could determine if this was true.  There's probably an easier way, but I can't
remember it :-)

--
Jim Sullivan
Director, North American System Engineers
Tarantella!  http://www.tarantella.com
831 427 7384 - jim@tarantella.com

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