2 x quad-core system is slower that 2 x dual core on FreeBSD

Alexey Popov lol at chistydom.ru
Tue Nov 20 09:45:34 PST 2007


Tom Evans wrote:
>> After that I rebuilt with SMP GENERIC kernel and put on that server 2 
>> times more requests that UP could handle. For the first time it worked 
>> good. Then I increased load to 2.5 times more than UP. Immediately 
>> Apache child count increased to MaxClients (24), most of them in RUN 
>> state, and %sys became greater than %user (see attach). I think after 
>> some threshold of load FreeBSD is paying more CPU time to the management 
>> of running processes than to run them.
> MaxClients of 24 seems very low for a 8 cpu box, running prefork MPM. On
> our quad CPU boxes, running custom apache modules, we use 
>   MaxClients 70
>   MinSpareServers 5
>   MaxSpareServers 15
>   StartServers 20
> Perhaps you are seeing high system load because the system is having to
> maintain a lot of queued connections. Certainly, our load remains
> in-between comfortable margins, except when heavily stressed.
I believe 8-core FreeBSD server is able to maintain 1024 waiting TCP 
connections without measurable CPU load.

As of this problem: increasing MaxClients leads to growing %sys part of 
CPU load. Generally large MaxClients value is useful when most Apache 
children are waiting for I/O or something else but CPU.

With best regards,
Alexey Popov

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