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SubjectRe: Adaptec vs Symbios performance


On Sat, 3 Nov 2001, Justin T. Gibbs wrote:

[...]

> >I can see _both_ comparing aic with symbios.
>
> I'm not sure that you would see much of a difference if you set the
> symbios driver to use 253 commands per-device. I haven't looked at

This is discouraged. :)
Better, IMO, to compare behaviours with realistic queue depths. As you
know, more than 64 for hard disks does not make sense (yet).
Personnaly, I use 64 under FreeBSD and 16 under Linux. Guess why ? :-)

> the sym driver for some time, but last I remember it does not use
> a bottom half handler and handles queue throttling internally. It

There is no BH in the driver. The stock sym53c8xx even uses scsi_obsolete
that requires more load in interrupt context for command completion.
SYM-2 that comes back from FreeBSD uses the EH threaded stuff that just
queues to a BH on completion. Stephan may want to give SYM-2 a try, IMO.

> may perform less work at interrupt time than the aic7xxx driver if
> locally queued I/O is compiled into a format suitable for controller
> consumption rather than queue the ScsiCmnd structure provided by
> the mid-layer. The aic7xxx driver has to convert a ScsiCmnd into a
> controller data structure to service an internal queue and this can
> take a bit of time.

The sym* drivers also uses an internal data structure to handle I/Os. The
SCSI script does not know about any O/S specific data structure.

> I would be interresting if there is a disparity in the TPS numbers
> and tag depths in your comparisons. Higher tag depth usually means
> higher TPS which may also mean less interactive response from the
> system. All things being equal, I would expect the sym and aic7xxx
> drivers to perform about the same.

Agreed.

Gérard.

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