Re: Warning - running *really* short on DMA buffers while doing file transfers

Jens Axboe
2002-09-27T06:08:26Z

On Thu, Sep 26 2002, Justin T. Gibbs wrote:
> > I reported this same problem some weeks ago -
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=103069116227685&w=2 .
> > 2.4.20pre kernels solved the error messages flooding the console, and
> > improved things a bit, but system load still got very high and disk read
> > and write performance was lousy. Adding more memory and using a
> > completely different machine didn't help. What did? Changing the Adaptec
> > scsi driver to aic7xxx_old . The performance was up 50% for writes and
> > 90% for reads, and the system load was acceptable. And i didn't even had
> > to change the RedHat kernel (2.4.18-10) for a custom one. The storage was
> > two external Arena raid boxes, btw.
>
> I would be interested in knowing if reducing the maximum tag depth for
> the driver improves things for you. There is a large difference in the
> defaults between the two drivers. It has only reacently come to my
> attention that the SCSI layer per-transaction overhead is so high that
> you can completely starve the kernel of resources if this setting is too
> high. For example, a 4GB system installing RedHat 7.3 could not even
> complete an install on a 20 drive system with the default of 253 commands.
> The latest version of the aic7xxx driver already sent to Marcelo drops the
> default to 32.

2.4 layer is most horrible there, 2.5 at least gets rid of the old
scsi_dma crap. That said, 253 default depth is a bit over the top, no?

--
Jens Axboe, who always uses 4