Oliver Rauch wrote:
>
> msitkows@au1.ibm.com wrote:
>
> >
> > Add to that, the fact that different versions of Unix (like AIX or Solaris)
> > use slightly different implementations of pthreads with each release, and
> > my advice would be, forget it. I think portability is more important, for a
> > public software package.
> >
>
> Hi,
>
> I think the same.
Most scsi scanners do not support disconnect or tagged queuing
so you are not going to get much benefit from async io. Abel
Deuring has done some work in this area and did get some
timing improvements on Linux. I think scsi commands were
probably queued up at the HBA and thus got out to the
scanner quicker after it finished the previous command
(compared with having to go up through the scsi layers,
out to the app and back down again). Abel used non blocking
techniques rather than POSIX threads, I think. [I have used
POSIX threads to do async IO on high end disks with good
results (compared with the standard dd command).]
There seems to be a non-linear effect with scanners. You have
a time window to get the next "read" out to the device after
the previous one has finished. If the window is missed the
scan stops, the mechanism backs up a little and recommences.
A small change in the speed the next command gets out to the
scanner can mean the difference between a slow, jerky scan
and one continuous pass.
Doug Gilbert
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