Re: Low file-system performance for 2.6.11 compared to 2.4.26

["Richard B. Johnson" <linux-os@analogic.com>]

On Thu, 31 Mar 2005, Nick Piggin wrote:

> linux-os wrote:
> > For those interested, some file-system tests and a test-tools
> > are attached.
>
> I'll give it a run when I get a chance. Thanks.
> In the meantime, can you try with different io schedulers?

I was trying to emulate some old servers that had new
kernels installed. These servers are used to send medical
images around. One used to get a 512x512x16-bit image to
a work-station in a few hundred milliseconds. It takes
seconds with the newer kernels. I found out that the
SCSI disk(s) were running continuously and sampled
their I/O patterns. The 'C' code comes very close
to emulating that.

The installation involved an "ugrade" to linux-2.6.5-1.358
that came with a RedHat Fedora distibution. The provided
code will even HALT that distibution. Everything goes
into the 'D' state and waits forever (at least overnight).

Later versions like linux-2.6.8 will run to completion
but with verrry slow through-put.

Never versions, like linux-2.6.11 will run, but with a strange
slowness, everything in 'D' and the file-system can end up with
missing files.

The SCSI controller is AIC7XXX, and the fs is ext3 with jbd, just
as it comes from Red Hat.

> > Also, my .signature disappeared during the file-system tests.
> > There were no errors and e2fsck thinks everything is fine!
>
> You seem to be constantly plagued by gremlins. I don't
> know whether to laugh or cry.

I test many, many (too many) systems as part of my job. When
somebody writes a hardware device-driver and I get to check it,
sometimes it blows up or doesn't otherwise work. I then test the
bare kernel(s) and I often find some really strange things going
on.

For instance, there was a recent change that make the BKL be
held during an ioctl(). This has devistating performance
consequences with a lot of drivers. For instance, the stuff
that writes CD/ROMs. It does a lot of the work using ioctl(),
the firewire drivers also use ioctl() for I/O.

>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
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