Re: [gentoo-user] SATA tuning ?

[Richard Fish]

Moving this back to gentoo-user, as I accidentally replied off list.

Meino, please don't CC me directly on replies.  I'll read them on the list...

On 8/15/06, Meino Christian Cramer <Meino.Cramer@×××.de> wrote:
> > >  WHen doing things, which mixes higher CPU-loads with  massive hd
> > >  utilization, things are going slow (compilation of Blender for
> > >  example).
> >
> > Ok, let's try to test that.  We'll start by saturating your CPU(s).
> > On one terminal start "bzip2 -9 < /dev/urandom >/dev/null".
>
>   This commandline puts a BIG SMILE onto my face ! Yes, this is as
>   simple as it is genious!!! Great! Really a nice CPU barbeque !
>
> > (If you
> > have multiple processors, start one of these bzip2 commands on one
> > terminal for each processor you have).
> >
> > Then on another, repeat the "hdparm -Tt /dev/sda"
>
>   These are the results __without__ the CPU roaster:
>
>     solfire:Mail/vim>sudo hdparm -tT /dev/sda
>     /dev/sda:
>     Timing cached reads:   2996 MB in  2.00 seconds = 1499.13 MB/sec
>     Timing buffered disk reads:  174 MB in  3.01 seconds =  57.79 MB/sec
>
>   and this are the results __with__ the CPU roaster:
>
> solfire:/home/mccramer>sudo hdparm -Tt /dev/sda
>     solfire:/home/mccramer>sudo hdparm -Tt /dev/sda
>     /dev/sda:
>     Timing cached reads:   2160 MB in  2.10 seconds = 1030.12 MB/sec
>     Timing buffered disk reads:  174 MB in  3.03 seconds =  57.41 MB/sec
>
>
> The chached reads dropped by ~469MByte/s. The buffered reads are
> nearly the same.

The buffered reads are all we care about.  They are the actual reads
from the disk to RAM.  The cached reads is just a repeated read of the
same sector of the disk, so today is really just a test of your memory
bandwidth.  Since we are loading memory and the CPU pretty heavily for
this test, and significant drop is to be expected.

So, it is not CPU utilization that is hurting your performance.

You mentioned problems compiling.  The most likely case I can think of
is that you do not have enough memory, and are inducing the system to
swap.  Indeed when compiling most programs, you should see very little
if any disk activity.  This is particularly suspect if you have
something like MAKEOPTS=-j4.

Regards,
-Richard
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