Re: gdth / SCSI read performance issues (2.2.19 and 2.4.10)

[Andrew Morton]

"Marinos J. Yannikos" wrote:
> 
> Andrew Morton wrote:
> 
> > Well that's pretty bad, isn't it?
> 
> We could have bought a much cheaper controller and slower disks ;-)

It's Linux's way of saving you money :)

> > - Disable CONFIG_HIGHMEM
> 
> That seems to have no effect on performance. Btw., the 64GB support in
> 2.4.10 seemed to be buggy ("0-order allocation failed", then the DB
> crashed), so we were using the 4GB setting.

There have been many reports of this happening, but they
are tapering off now.

> > - Try linux-2.4.13
> 
> This helped - now performance is up to par with 2.2.19 (~ 85MB/s) - thanks!

It's surprising that 2.4.10->2.4.13 actually doubled throughput.
 
> > - Profile the kernel. [...]  With something like:
> >       ~/kern-prof.sh cp some_huge_file /dev/null
> 
> I tried this, but with
>   dd if=/dev/sda of=/dev/null bs=1024k count=3000
> 
> (the fs is too slow - so "cp" peaks out at 17MB/s!)

Really?  Are you saying that on a controller which can
do 85 megs/second, you can't read files through the filesystem
at greater than 17?  Which filesystem?

> The result (last 4 lines):
> c01388fc try_to_free_buffers                          55   0.1511
> c0128b10 file_read_actor                            1179  14.0357
> c01053b0 default_idle                               6784 130.4615
> 00000000 total                                      8695   0.0065

OK, that's normal and proper.  Almost all the kernel time
is spent copying data.
 
> Does this suggest that the kernel isn't the bottleneck?

Well...   We seem to have three issues here:

1: Why isn't the controller achieving the manufacturer's
   claimed throughput?

   Don't know.  Maybe it's the software copy.  Maybe it's the
   device driver.  Maybe they lied :).  It'd be interesting
   to test it on the same machine with the vendor's drivers
   and win2k.

2: 0-order allocation failures.

3: Poor `cp' throughput.  This one is strange.  Perhaps
   `cp' is using a small transfer-size and the kernel's
   readhead isn't working properly.  Could you experiment
   with this some more?  For example, what happens with

	dd if=large_file of=/dev/null bs=4096k

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