getconf CLK_TCK and CONFIG_HZ

[Mulyadi Santosa]

Dear Jim...

Allow me to help you by sharing what I know so far....

On Sun, Mar 13, 2011 at 05:37, Jim Cromie <jim.cromie at gmail.com> wrote:
> This agrees with sysconf granularity :
> $ getconf CLK_TCK
> 100
> but not with linux kernel HZ:
> $ grep _HZ /boot/config-`uname -r`
> CONFIG_RCU_FAST_NO_HZ=y
> CONFIG_NO_HZ=y
> # CONFIG_HZ_100 is not set
> # CONFIG_HZ_250 is not set
> # CONFIG_HZ_300 is not set
> CONFIG_HZ_1000=y
> CONFIG_HZ=1000

Not a surprise. I read somewhere that Linux ABI so far declares that
time clock granularity is always shown as 100 HZ or 10 ms. So it is
more due to "following the rule" and application in user space follow
this assumption, even though we know that most distros now shift into
250 or 1000 HZ

> is it possible to compile kernel with different HZ options to get the 1ms
> resolution ?

yep, use HZ=1000.... and to save some ticks  when there are nothing to
do, enable CONFIG_NO_HZ

> Why doesnt times() also count IO-wait states for a process (and children) ?

AFAIK, if you look closely into timing accounting and the way I/O code
path behaves, you shall see that most of the times I/O is done in
async style...and more over, you can't tell for sure that is it only
single I/O code path that runs or more than one that are currently
running (by either interleaving between them or executing them
simultaneously).

That's why IMO, it's quite hard (but still possible) to account for
per process I/O wait...but still quite easy to do it system widely.

> Could process specific IO-wait numbers reveal anything about cache
> performance ?

I personally think,the main perfomance indicator of cache perfomance
is cache hit utilization.

> Do cache misses contribute to IO-wait, or do they get counted in other ways
> ?

we're talking about page cache, right?

then sure, cache misses would trigger hard/major page fault, assuming
it is file backed page.

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com