Low perf with smp

[Lucas Holt]

As i recall, mysql under FreeBSD 4.x used a userland threads  
library.  That means its only capable of being scheduled on 1 cpu at  
a time and each thread is part of one process.  I don't think SMP  
will help you much in this case.  There have been a number of threads  
on some of the lists (hacker@ ?) in the last few months about MySQL  
performance indicating it might have something to do with the timer,  
etc.  It might be worth looking into.

Some people have stated that FreeBSD 5 and 6 are slower with disk io,  
etc.  Plus you've got a raid 1 mirror which will slow down writes.   
6.1 also has a reported issue with the 4bsd scheduler performance.  
Maybe thats part of the problem?

On Mar 30, 2006, at 11:41 AM, Eric wrote:

> Mail NETvigieHi all,
>
> I buy a new DELL PowerEdge 1850 with Bi-Xeon 2.8GHZ/2*2MB Dual Core  
> 800FSB processor, 2Go DDR2 RAM, PERC4di RAID controller with 2 SCSI  
> U320  15.000tpm 36Go hdd on RAID1.
>
> I install on it Freebsd 6.0, apache 2.2.0 and mysql 4.1.18 and  
> compare performance with :
> OLD PowerEDge with only simple Bi-Xeon 3.0 GHZ and freebsd 4.11.
>
> The test is simple mysql query done by php script.
>
> Result is that test is two time slower with freebsd 6.0 and dual- 
> core than with old server.
> I try with and without smp and  threaded kernel and results are same.
> I came back to Generic kernel to see if it come from my kernel  
> configuration but result was same.
>
> I then though it come from server and try to upgrade an other one  
> and have same result.
> I then think it come from 6.0 and try with 6.1 beta4 and see same  
> result
> I then downgrad to 4.11 without smp and then find good perfs.
> So I just add to 4.11 generic kernel :
> options         SMP                     # Symmetric MultiProcessor  
> Kernel
> options         APIC_IO                 # Symmetric (APIC) I/O
>
> And then perf go back down from 30secondes to 54 sec.
> I search a lot on internet and try with this : sysctl  
> machdep.cpu_idle_hlt=0
> But result is same.
>
> When doing whith smp top it's just as mysql have to wait to have  
> cpu. During 10,20 sec it just go to 2,4% then grow. Without smp it  
> grow very quickly.
>
> Does anyone know what's go wrong and what do I have to do to have  
> same perf with smp than without?
>
> Thanks,
>
> Eric.
>
>
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Lucas Holt
Luke at FoolishGames.com
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