kernel activity

[Per Engelbrecht]

Hi Bogdan
You don't have to give up device polling because of SMP. I run several
fbsd 4.9-stable (dual PIII) with device polling.
Go to /sur/src/sys/kern/kern_poll.c , outcomment the SMP part, add
device polling, hz=  et al to kernel, recompile, set sysctl variables,
reboot and you're done. Works like a charm.

respectfully
/per
per at xterm.dk

>
> 	Hi Peter and all,
>
> Thanks for the mails & advices... The box is a dual xeon @3GHz,
> with 4GB of ram and raid 5 on board (scsi HDDs), with a 4.9 on it.
> The 'tuning' includes removing all the unnecessary stuff from the
> kernel, activating the ACCEPT_FILTERS and tuning some sysctl
> values,
> especially the ones dealing with network (net.inet.tcp.msl etc.),
> and raising the values for maxfiles and somaxconn, etc. The box has
> two NICs, one of them is a fxp with link0 activated (cannot use
> polling because I don't want to give up SMP -- the userland
> activity is
> already 40%, so giving up one CPU as to reduce sys load it's just
> gonna leave the bottleneck where it is -- CPU, that is), and the
> other one is an em, but cannot use it since i don't have a gb
> switch. Before activating link0 on fxp, the level of interrups/sec
> on this interface peaked 6k, but after activating link0 it was
> reduced to 2k. Still, a lot of sys activity...
>
> As a webserver, I run apache, stripped down from the modules that I
> don't need, and compiled in php and some other modules
> (statically). Most of the content that I serve is static, there are
> only a few php scripts and they don't get much hits.
>
> I don't run any other 'intensive computing' application on the
> server, no firewall software or so. And no, I haven't tried turning
> HTT on and off, should I do that?
>
> I am also considering trussing one of the apaches, to see what
> system calls it's doing...
>
> Anyways, thanks for your help,
> bogdan
>
>
> On Tue, Mar 16, 2004 at 06:06:01PM +1100, Peter Jeremy wrote:
>> On Mon, Mar 15, 2004 at 04:04:38PM +0100, Bogdan TARU wrote:
>> > I'm running a pretty busy webserver, and right now I can see
>> > it's CPU-bound:
>>
>> A few more details would be useful:
>> What version of FreeBSD?
>> What hardware are you using (CPU and NIC in particular)?
>> What application(s) are you running?  You mention a webserver -
>> which one?
>>   Is it just serving static pages or is there lots of dynamic
>>   content (CGI or servlet etc)?  Any other applications?
>> Do you have firewall software running?  If so, which?  Is it
>> simple (a dozen
>>   or so rules) or complex (thousands of rules)?
>> What FreeBSD tuning have you done?
>> How does your kernel config compare to GENERIC?
>
>> If the CPU supports HTT, have you tried turning HTT on and off?
>>
>> >84 2 0 1135836 142692   82   0   0   0 405   0  35   0 3144
>> >246325 1739 42 58  0
>>
>> You have very high interrupt and system call rate - both of these
>> contribute to system time.
>>
>> I presume most of the interrupts are from your NIC.  Have you
>> considered using polling mode (see polling(4))?  Some NICs are
>> more efficient than others - fxp(4) is one of the best, rl(4) is
>> probably the worst.
>>
>> It's difficult to say whether the syscall rate is excessive or
>> not. The number of system calls is generally up to the application
>> - you need to tune or redesign it to reduce the number of system
>> calls it makes per unit of work.  That said, I've noticed that
>> threading on -STABLE adds quite a significant overhead - via high
>> system call rate and system time.  In one case, I improved the
>> throughput of a graphics manipulation process by about 10% by
>> removing the threading.
>>
>> It's difficult to get much visibility on where system time is
>> going (though "systat -v" will split out interrupt time).  In
>> theory, you could build a profiling kernel but this is non-trivial
>> and may or may not be functional at present.
>>
>> Peter
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