question about frequency timer

[Wojciech Ziniewicz]

Hello,

I would like to know what is the cause the stock debian kernel is
compiled with frequency timer of 250Hz while i find 1000Hz ( + dynamic
ticks ) more reliable , faster and more realtime , using the same
power for the CPU's ?
My understanding of frequency timer is not so bright so please let me
know if i've gone wrong.

regards
WZ

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Wojciech Ziniewicz
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[Giacomo Catenazzi]

Wojciech Ziniewicz wrote:
> Hello,
>
> I would like to know what is the cause the stock debian kernel is
> compiled with frequency timer of 250Hz while i find 1000Hz ( + dynamic
> ticks ) more reliable , faster and more realtime , using the same
> power for the CPU's ?

250Hz is the default value of linux kernel, and it is a trade-off
between faster and "realtime".

On desktops it seems faster with 1000Hz setting, because
it is more interactive, but there are 10 time more overhead
in comparison of server setting (100hz). This is due
to 10 time more scheduling on kernel (which take
CPU), and more cache misses, due to reloading processes.

So 250 is a good compromise between server and desktop
use.

Anyway, why do you poste in debian-isp list?

ciao
        cate




> My understanding of frequency timer is not so bright so please let me
> know if i've gone wrong.
>
> regards
> WZ
>


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[Wojciech Ziniewicz]

2008/6/5 Giacomo A. Catenazzi <cate@...>:
> Anyway, why do you poste in debian-isp list?

apparently question is concerning debian stock kernel, it came to me
while developing hosting system for internet multiplayer games like
counter strike etc. With timer of 1000Hz and dynamic ticks , servers
are about 50% more stress capable.

Moreover with this setting in kernel, servers are more reliable and
less "lagging" (still talking bout those css server) .

And the most interesting part : I've tried this setting on www server
and bgp peer routing about 40-100 mbits and everytime the latency is
better (i mean lower) , ram usage become minimally lower (why ? ) and
tottaly enigmatic for me - entropy is about hmm... 10-100 times bigger
(depends on server) ?

Generally recompiling the kernel for those machines in the manner i've
mentioned change almost every parameter in the better way. So what
problems can I expect ?

regards.

p.s. every statistics were taken on 32bit  machine from munin-graphs



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Wojciech Ziniewicz
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ex;unzip;head;tail; mount;workbone;fsck;yes;gasp;fsck;more;yes;yes;eje
ct;umount;makeclean; zip;split;done;exit:xargs!!;)}


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[Giacomo Catenazzi]

Wojciech Ziniewicz wrote:
> 2008/6/5 Giacomo A. Catenazzi <cate@...>:
>> Anyway, why do you poste in debian-isp list?
>
> apparently question is concerning debian stock kernel, it came to me
> while developing hosting system for internet multiplayer games like
> counter strike etc. With timer of 1000Hz and dynamic ticks , servers
> are about 50% more stress capable.

Ok. Game servers use a lot more CPU then normal servers.
The "normal" servers, could give answer in few millisecond,
so in a server with 100Hz or 250Hz, you don't have scheduler
and cache overhead: the process go in a "waiting" state
before kernel will call the scheduler.
This is optimal and fast "normal" server (web, dns, ssh, ...).
On some dynamic script (but if the process doesn't
stop before, because disk, net or SQL queries),
the scheduler could be called few time.

With 1000Hz, the processes is rescheduled a lot more time,
which is not optimal on web/dns/... servers.

This is the main reason of 250Hz: a good trade-off between
normal server use and desktop interactiveness.
On desktop the optimal value is 1000Hz, because
there are a lot more process running, but
user still want interactivity.
Take as example: 100Hz, and 5 process running,
it would take 1/20 of second before an application
could run, but 1/20 of second could be noticeable.
250Hz is enough good, if user doesn't have
a lot more process running.


You have a special case, with a lot of CPU usage, and
possibly less "waiting state" from SQL or other sources,
so a special setting could be better.
But you have a special case, so I don't think should
be addressed by Debian kernel.

Maybe you should tweak other parameters of
scheduler.

If you have time and resources, you could
benchmark some of latest vanilla kernel
and send data to Linux kernel mailing list.
They are tweaking/changing scheduler, and
they kernel people are happy to see
strange behavior (an 50% more output
on stress case are very interessant).

ciao
        cate

>
> Moreover with this setting in kernel, servers are more reliable and
> less "lagging" (still talking bout those css server) .
>
> And the most interesting part : I've tried this setting on www server
> and bgp peer routing about 40-100 mbits and everytime the latency is
> better (i mean lower) , ram usage become minimally lower (why ? ) and
> tottaly enigmatic for me - entropy is about hmm... 10-100 times bigger
> (depends on server) ?
>
> Generally recompiling the kernel for those machines in the manner i've
> mentioned change almost every parameter in the better way. So what
> problems can I expect ?
>
> regards.
>
> p.s. every statistics were taken on 32bit  machine from munin-graphs
>
>
>

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[Wojciech Ziniewicz]

2008/6/5 Giacomo A. Catenazzi <cate@...>:
[...]
> If you have time and resources, you could
> benchmark some of latest vanilla kernel
> and send data to Linux kernel mailing list.
> They are tweaking/changing scheduler, and
> they kernel people are happy to see
> strange behavior (an 50% more output
> on stress case are very interessant).
>
thank You for your support.

WZ

--
Wojciech Ziniewicz
Unix SEX :{look;gawk;find;sed;talk;grep;touch;finger;find;fl
ex;unzip;head;tail; mount;workbone;fsck;yes;gasp;fsck;more;yes;yes;eje
ct;umount;makeclean; zip;split;done;exit:xargs!!;)}


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[Jim Popovitch]

On Thu, Jun 5, 2008 at 11:22 AM, Giacomo A. Catenazzi <cate@...> wrote:
> Ok. Game servers use a lot more CPU then normal servers.
> The "normal" servers, could give answer in few millisecond,

Where did you get that data from?

-Jim P.


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